volcanic eruption essay introduction

Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (2017)

Chapter: summary.

Volcanoes are a key part of the Earth system, and open a window into the inner workings of the planet. More than a dozen volcanoes are usually erupting on Earth at any given time. Some of these eruptions are devastating, killing people, damaging homes and infrastructure, altering landscapes, and even disrupting climate. Fortunately, many eruptions are preceded by signs of unrest (precursors) that can be used to anticipate eruptions and support disaster planning.

Accurate forecasts of the likelihood and magnitude of an eruption in a specified timeframe are rooted in a scientific understanding of the processes that govern the storage, ascent, and eruption of magma. Yet our understanding of volcanic systems is incomplete and biased by the limited number of volcanoes and eruption styles observed with advanced instrumentation. Eruption behaviors are diverse (e.g., violently explosive or gently effusive, intermittent or sustained, last hours or decades) and may change over time at a volcano. More accurate and societally useful forecasts of eruptions and their hazards are possible by using new observations and models of volcanic processes.

At the request of managers at the National Aeronautics and Space Administration, the National Science Foundation (NSF), and the U.S. Geological Survey (USGS), the National Academies of Sciences, Engineering, and Medicine established a committee to undertake the following tasks:

• Summarize current understanding of how magma is stored, ascends, and erupts.
• Discuss new disciplinary and interdisciplinary research on volcanic processes and precursors that could lead to forecasts of the type, size, and timing of volcanic eruptions.
• Describe new observations or instrument deployment strategies that could improve quantification of volcanic eruption processes and precursors.
• Identify priority research and observations needed to improve understanding of volcanic eruptions and to inform monitoring and early warning efforts.

These four tasks are closely related. Improved understanding of volcanic processes guides monitoring efforts and improves forecasts. In turn, improved monitoring provides the insights and constraints to better understand volcanic processes. This report identifies key science questions, research and observation priorities, and approaches for building a volcano science community capable of tackling them. The discussion below first summarizes common themes among these science questions and priorities, and then describes ambitious goals (grand challenges) for making major advances in volcano science.

KEY QUESTIONS AND RESEARCH AND OBSERVATION PRIORITIES

Many fundamental aspects of volcanoes are understood conceptually and often quantitatively. Plate tectonics and mantle convection explain where volcanoes occur. We understand how magma is initially created in Earth’s mantle, how it rises toward the surface, that it can be stored and evolve in magma chambers within the crust, and that a number of processes initiate eruptions. We understand in general terms why some magmas erupt explosively and others do not, and why some volcanoes erupt more often than others. High-resolution observations and models combined provide a detailed and quantitative picture of eruptions once they begin.

Our understanding is incomplete, however, especially those aspects of volcano behavior that define the timing, duration, style, size, and consequences of eruptions. Additional questions relate to our ability to forecast eruptions. What processes produce commonly observed geophysical and geochemical precursors? What factors determine if and when unrest will be followed by eruption? How rapidly do magmas mobilize prior to eruption? Which volcanoes are most likely to erupt in coming years and decades? And we are only beginning to decipher the impacts of large volcanic eruptions on Earth’s climate and biosphere.

Our understanding of the entire life cycle and diversity of volcanoes—from their conception in the mantle to their periods of repose, unrest, and eruption to their eventual demise—is poised for major advances over the next decades. Exciting advances in our ability to observe volcanoes—including satellite measurements of ground deformation and gas emissions, drone observations, advanced seismic monitoring, and real-time, high-speed acquisition of data during eruptions—await broad application to volcanic systems. Parallel advances in analytical capabilities to decipher the history of magmas, and in conceptual, experimental, and numerical models of magmatic and volcanic phenomena, both below and above ground, will provide new insights on the processes that govern the generation and eruption of magma and greatly improve the quality of short-term, months to minutes, forecasts. The time is ripe to test these models with observations from new instrumentation, data collected on fine temporal and spatial scales, and multidisciplinary synthesis.

Four common themes emerged from the research priorities detailed in the following chapters:

• Develop multiscale models that capture critical processes, feedbacks, and thresholds to advance understanding of volcanic processes and the consequences of eruptions on Earth systems.

Advances will come from measurements of physical and chemical properties of magmas and erupted materials, deciphering the history of magmas (before and during eruption) recorded in their crystals and bubbles, and developing new models that account for the numerous interacting processes and vast range of scales, from microscopic ash particles and crystals, to eruption columns that extend to the stratosphere.

• Collect high-resolution measurements at more volcanoes and throughout their life cycle to overcome observational bias.

Few volcanoes have a long record of monitoring data. New and expanded networks of ground, submarine, airborne, and satellite sensors that characterize deformation, gases, and fluids are needed to document volcanic processes during decade-long periods of repose and unrest. High-rate, near-real-time measurements are needed to capture eruptions as they occur, and efficient dissemination of information is needed to formulate a response. Both rapid response and sustained monitoring are required to document the life cycle of volcanoes. Monitoring and understanding volcanic processes go hand-in-hand: Different types of volcanoes have different life cycles and behaviors, and hence merit different monitoring strategies.

• Synthesize a broad range of observations, from the subsurface to space, to interpret unrest and forecast eruption size, style, and duration.

Physics-based models promise to improve forecasts by assimilating monitoring data and observations. Progress in forecasting also requires theoretical and experimental advances in understanding eruption processes, characterization of the thermal and mechanical properties of magmas and their host rocks, and model validation and verification. Critical to eruption forecast-

ing is reproducing with models and documenting with measurements the emergent precursory phenomena in the run-up to eruption.

• Obtain better chronologies and rates of volcanic processes.

Long-term forecasts rely on understanding the geologic record of eruptions preserved in volcanic deposits on land, in marine and lake sediments, and in ice cores. Secondary hazards that are not part of the eruption itself, such as mud flows and floods, need to be better studied, as they can have more devastating consequences than the eruption. Understanding the effects of eruptions on other Earth systems, including climate, the oceans, and landscapes, will take coordinated efforts across disciplines. Progress in long-term forecasts, years to decades, requires open-access databases that document the full life cycle of volcanoes.

GRAND CHALLENGES

The key science questions, research and observation priorities, and new approaches highlighted in this report can be summarized by three overarching grand challenges. These challenges are grand because they are large in scope and would substantially advance the field, and they are challenges because great effort will be needed. Figure S.1 illustrates these challenges using the example of the 2016 eruption of Pavlof volcano, Alaska. The volcanic hazards and eruption history of Pavlof are summarized by Waythomas et al. (2006) .

A principal goal of volcano science is to reduce the adverse impacts of volcanism on humanity, which requires accurate forecasts. Most current eruption forecasts use pattern recognition in monitoring and geologic data. Such approaches have led to notable forecasts in some cases, but their use is limited because volcanoes evolve over time, there is a great diversity of volcano behavior, and we have no experience with many of the potentially most dangerous volcanoes. A major challenge is to develop forecasting models based instead on physical and chemical processes, informed by monitoring. This approach is used in weather forecasting. Addressing this challenge requires an understanding of the basic processes of magma storage and ascent as well as thresholds of eruption initiation. This understanding and new discoveries will emerge from new observations, experimental measurements, and modeling approaches. Models are important because they capture our conceptual and quantitative understanding. Experiments test our understanding. Relating models to observations requires multiple types of complementary data collected over an extended period of time.

Determining the life cycle of volcanoes is key for interpreting precursors and unrest, revealing the processes that govern the initiation and duration of eruptions, and understanding how volcanoes evolve between eruptions. Our understanding is biased by an emphasis over the last few decades of observation with modern instruments, and most of these well-studied eruptions have been small events that may not scale to the largest and most devastating eruptions. Strategic deployment of instruments on volcanoes with different characteristics would help build the requisite knowledge and confidence to make useful forecasts. For every volcano in the United States, a realistic goal is to have at least one seismometer to record the small earthquakes that accompany magma movement. Even in the United States, less than half of potentially active volcanoes have a seismometer, and less than 2 percent have continuous gas measurements. Global and daily satellite images of deformation, and the ability to measure passive CO 2 degassing from space would fill critical observational gaps. Geologic and geophysical studies are required to extend understanding of the life cycle of volcanoes to longer periods of time. On shorter time scales, satellite measurements, emerging technologies such as drones, and expansion of ground-based monitoring networks promise to document processes that remain poorly understood.

The volcano science community needs to be prepared to capitalize on the data and insights gained from eruptions as they happen. This will come from effective integration of the complementary research and monitoring roles by universities, the USGS, and other government agencies. Volcano science is fundamentally interdisciplinary and the necessary expertise is spread across these institutions. The science is also international, because every volcano provides insights on processes that drive eruptions. Volcanic eruptions can have global impacts and so demand international collaboration and cooperation. New vehicles are needed to support interdisciplinary research and training, including community collaboration and education at all levels. Examples of similar successful programs in other fields include NSF’s Cooperative Studies of the Earth’s Deep Interior program for interdisciplinary research and National Earthquake Hazards Reduction

Program for federal government agency–academic partnerships.

Results of the above investments in science will be most evident to the public in improved planning and warning and, ideally, a deeper appreciation of this amazing natural phenomenon.

Volcanic eruptions are common, with more than 50 volcanic eruptions in the United States alone in the past 31 years. These eruptions can have devastating economic and social consequences, even at great distances from the volcano. Fortunately many eruptions are preceded by unrest that can be detected using ground, airborne, and spaceborne instruments. Data from these instruments, combined with basic understanding of how volcanoes work, form the basis for forecasting eruptions—where, when, how big, how long, and the consequences.

Accurate forecasts of the likelihood and magnitude of an eruption in a specified timeframe are rooted in a scientific understanding of the processes that govern the storage, ascent, and eruption of magma. Yet our understanding of volcanic systems is incomplete and biased by the limited number of volcanoes and eruption styles observed with advanced instrumentation. Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing identifies key science questions, research and observation priorities, and approaches for building a volcano science community capable of tackling them. This report presents goals for making major advances in volcano science.

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Essay on volcanoes | geology.

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After reading this article you will learn about:- 1. Introduction to Volcanoes 2. Volcano Formation 3. Volcanic Landforms 4. Major Gases Emitted by Volcanoes 5. Lightning and Whirlwinds 6. Features Produced by the Escape of Gases from Volcanic Lavas 7. Volcanic Products 8. Source of the Explosive Energy 9. Classification of Pyroclastics 10. Lahars-Mudflows on Active and Inactive Cones and Other Details.

Essay Contents:

• Essay on the Volcanoes and Atmospheric Pollution

Essay # 1. Introduction to Volcanoes :

A volcano is a cone shaped hill or mountain which is built-up around an opening in the earth’s surface through which hot gases, rock fragments and lavas are ejected.

Due to the accumulation of the solid fragments around the conduit a conical mass is built which increases in size to become a large volcanic mountain. The conical mass so built-up is called a volcano. However the term volcano is taken to include not only the central vent in the earth but also the mountain or hill built around it.

Volcanoes are in varying sizes, varying from small conical hills to loftiest mountains on the earth’s surface. The volcanoes of the Hawaiian Islands are nearly 4300 metres above sea level since they are built over the floor of the Pacific ocean which at the site is 4300 to 5500 metres deep, the total height of the volcano may be about 9000 m or more.

The very high peaks in the Andes, in the Cascade Range of the Western United States, Mt. Baker, Mt. Adams, Mt. Hood etc. are all volcanoes which have now become extinct. Over 8000 independent eruptions have been identified from earth’s volcanoes. There are many inaccessible regions and ocean floors where volcanoes have occurred undocumented or unnoticed.

The eruption of a volcano is generally preceded by earthquakes and by loud rumblings like thunder which may continue on a very high scale during the eruption. The loud rumblings are due to explosive movement of gases and molten rock which are held under very high pressure. Before eruption of a volcano fissures are likely to be opened, nearby lakes likely to be drained and hot springs may appear at places.

The eruptive activity of volcanoes is mostly named after the well-known volcanoes, which are known for particular type of behaviour, like Strambolian, Vulcanian, Vesuvian, Hawaiian types of eruption. Volcanoes may erupt in one distinct way or may erupt in many ways, but, the reality is, these eruptions provide a magical view inside the earth’s molten interior.

The nature of a volcanic eruption is determined largely by the type of materials ejected from the vent of the volcano. Volcanic eruptions may be effusive (fluid lavas) or dangerous and explosive with blasts of rock, gas, ash and other pyroclasts.

Some volcanoes erupt for just a few minutes while some volcanoes spew their products for a decade or more. Between these two main types viz. effusive and explosive eruptions, there are many subdivisions like, eruption of gases mixed with gritty pulverised rock forming tall dark ash clouds seen for many kilometres, flank fissure eruptions with lava oozing from long horizontal cracks on the side of a volcano.

There is also the ground hugging lethally hot avalanches of volcanic debris called pyroclastic flows. When magma rises, it may encounter groundwater causing enormous phreatic, i.e., steam eruptions. Eruptions may also release suffocating gases into the atmosphere. Eruptions may produce tsunamis and floods and may trigger earthquakes. They may unleash ravaging rockslides and mudflows.

Volcanoes which have had no eruptions during historic times, but may still show fairly fresh signs of activity and have been active in geologically recent times are said to be dormant. There are also volcanoes which were formerly active but are of declining activity a few of which may be emitting only steam and other gases.

Geysers are hot springs from which water is expelled vigorously at intervals and are characteristics of regions of declining volcanic activity. Geysers are situated in Iceland, the Yellowstone park in USA and in New Zealand.

In contrast to the explosive type of volcanoes, there exist eruptions of great lava flows quietly pouring out of fissures developed on the earth’s surface. These eruptions are not accompanied by explosive outbursts. These are fissure eruptions.

Ex: Deccan Trap formations in India. The lavas in these cases are mostly readily mobile and flow over low slopes. The individual flows are seldom over a few meters in thickness; the average thickness may be less than 15 meters. If the fissure eruptions have taken place in valleys however, the thickness may be much greater.

A noteworthy type of volcano is part of the world encircling mid-ocean ridge (MOR) visible in Iceland. The MOR is really a single, extremely long, active, linear volcano, connecting all spreading plate boundaries through all oceans. Along its length small, separate volcanoes occur. The MOR exudes low-silica, highly fluid basalt producing the entire ocean floor and constituting the largest single structure on the face of the earth.

Essay # 2. Location of Volcanoes:

Volcanoes are widely distributed over the earth, but they are more abundant in certain belts. One such belt encircles the Pacific ocean and includes many of the islands in it. Other volcanic areas are the island of West Indies, those of the West coast of Africa, the Mediterranean region and Iceland.

Most volcanoes occur around or near the margins of the continents and so these areas re regarded as weak zones of the earth’s crust where lavas can readily work their way upward. There are over 400 active volcanoes and many more inactive ones. Numerous submarine volcanoes also exist.

Since it is not possible to examine the magma reservoir which fees a volcano our information must be obtained by studying the material ejected by the volcano. This material consists of three kinds of products, viz. liquid lava, fragmented pyroclasts and gases. There may exist a special problem in studying the gases, both in collecting them under hazardous conditions or impossible conditions.

It may also be difficult to ascertain that the gases collected are true volcanic gases and are not contaminated with atmospheric gases. Investigation of the composition of extruded rock leads to a general, although not very detailed, correlation between composition and intensity of volcanic eruption.

In general, the quite eruptions are characteristic of those volcanoes which emit basic or basaltic lavas, whereas the violent eruptions are characteristic of volcanoes emitting more silicic rocks.

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Essay # 3 . formation of volcanoes :.

The term volcano is used to mean both the opening in the earth’s crust, i.e. the vent through which the eruption of magma occurs as well as the hill built- up by the erupted material. Volcanoes occur where the cracks in the earth’s crust lead to the magma chamber.

The liquid magma which is lighter than the surrounding rocks is under high pressure is pushed up towards the surface through these cracks. In this process the gases dissolved in the magma which expand are released providing an upward push to the magma.

As the magma gets closer to the surface, due to the reducing confining pressure to overcome, the magma and the gases flow faster. The magma, depending on its viscosity may quietly pour to the surface in the form of a flood of molten rock or it may explosively spurt out the molten rock to considerable heights as showers on the surrounding region with solid rock fragments and globs of molten rock. The liquid magma discharged to the surface is called lava.

Essay # 4 . Volcanic Landforms :

Many surface features of volcanic origin are created. These features range from towering peaks and huge lava sheets to small and low craters. The features created by a volcano vary depending on the type of eruption, the material erupted and the effects of erosion.

Four types of volcanic landforms are formed:

i. Ash and Cinder Cones or Explosion Cones:

These appear where explosive eruptions take place. When very hot solid fragments from a central crater (or a subsidiary crater) are ejected. A concave cone of height not exceeding 300 m is formed.

ii. Lava Cones:

These are formed from slowly upwelling lava.

These are of two types:

(a) Steep Sided Volcanoes:

These are formed from sticky acid lava which gets hardened quickly. The highly viscous lava which is squeezed out makes spines like tower.

(b) Shield Volcanoes:

These show gently sloping dome features. These are formed from runny lava which flows long distances, before getting hardened.

iii. Composite Cones or Strato-Volcanoes or Strato Cones:

These volcanoes have concave cone shaped sides of alternating ash and lava layers. These are common in most very high volcanoes. In some cases solid lava may plug the main pipe to the crater. Then pent up gases may blast the top off.

When the magma chamber empties, the summit of the volcano collapses. As a consequence, the feature produced is a vast shallow cavity called a Caldera. Strato volcanoes are the accumulated products of many volcanoes. Chemically most of these products are andesite. Some are dacite and a few are basalt and rhyolite. Due to this chemical mix and characteristic interlayering of lava flows, this volcano is called strato volcano.

iv. Shield Volcanoes:

When a volcano vent produces many successive basaltic lava flows stacked one on top of another in eruptive order, the resulting landform is called a shield volcano. A cinder cone and its associated lava flow can be thought of as the initial building blocks of a shield volcano.

A cinder cone is monogenetic because it forms from a single short-lived eruption (of a few years to a decade or two in duration). In contrast, a shield volcano that is an accumulation of the products of many eruptions over a period of say thousands to hundreds of thousands of years is polygenic.

On land these volcanoes have low angle cones. When they form under water they start with a steeper shape because the lava freezes much faster and does not travel far. The shape fattens to the shield form as the cone builds above the sea level.

v. Plateau Basalts or Lava Plains:

These form the bulk of many volcanic fields. These are features which occur where successive flows of basic lava leaks through fissures, over land surface and then cools and hardens forming a blanket-like feature.

The surface appearance of a flow provides information on the composition and temperature of the magma before it solidified. Very hot low viscosity basalt flows far and fast and produces smooth ropy surfaces. Cooler and less-fluid basalt flows form irregular, jagged surfaces littered with blocks.

The lava flows have blanketed to about 2000 m thickness covering 6,50,000 sq.km. in the Indian Deccan Plateau. Such lava flows have also created the U.S. Columbia River Plateau, the Abyssinian Plateau, the Panama Plateau of South America and the Antrim Plateau of Northern Ireland.

Magmas like dacite and rhyolite that have high silica contents are cooler and more viscous than basalt and hence they do not flow far resulting in the features, lobes, pancakes and domes. Domes often plug up the vent from which they issued, sometimes creating catastrophic explosions and may create a crater.

Eroded volcanoes have their importance. They give us a glimpse of the interior plumbing along which the magma rose to the surface. At the end of an eruption, magma solidifies in the conduits along which it had been rising. The rock so formed is more resistant than the shattered rock forming the walls and hence these lava filled conduits are often left behind when the rest of the volcano has been eroded away.

The filling of the central vertical vent is somewhat circular in section and forms a spire called a neck. The filling of cracks along which lava rose forms nearly vertical tabular bodies called dikes. Sometimes magma works its way along cracks that are nearly horizontal, often along bedding planes of sedimentary rocks. This results in the formation of table-like bodies called sills.

Essay # 5 . Major Gases Emitted by Volcanoes :

Volcanic gases present within the magma are released as they reach the earth’s surface, escaping at the major volcanic opening or from fissures and vents along the side of the volcano. The most prevalent gases emitted are steam, carbon dioxide and hydrogen sulphide. Carbon dioxide is an invisible, odourless poisonous gas. The table below shows the gases emitted from volcanoes.

Essay # 6 . Lightning and Whirlwinds :

Lightning flashes accompany most volcanic eruptions, especially those involving dust. The cause of this lightning is believed to be either contact of sea water with magma or generation of static electricity by friction between colliding particles carried in the erupting gases. Lightning is characteristic of vulcanian eruptions and is common during glowing avalanches.

Whirlwinds are seen during many volcanic eruptions. They are seen above hot lavas. Sometimes they form inverted cones extending a little below the eruption cloud. Energy for the whirlwinds might be from the hot gases and lava, high velocity gas jets in the eruption, heat released into the atmosphere during falls of hot tephra or where lava flows into the sea creating steam.

Essay # 7 . Features Produced by the Escape of Gases from Volcanic Lavas :

The gases of volcanic lavas produce several interesting features while they escape. They expand in the lava of the flow and thus cause the formation of Scoriaceous and Pumiceous rocks. By their explosion, they blow the hardened lava above them in the conduit, into bits and thus produce pyroclastic material.

They form clouds above volcanoes, the rain from which assists in the production of mud flows. When the volcano becomes inactive, they escape aiding in the formation of jumaroles, geysers and hot springs. Scoriaceous rocks are extremely porous. They are formed by the expansion of the steam and other gases beneath the hardened crust of a lava. The final escape of the gases from the hardening lava leaves large rounded holes in the rock.

Pumice is a rock also formed by the expansion and escape of gases. In pumice, many of the holes are in the form of long, minute, closed tubes which make the rock so light that it will float on water.

These tubes are formed by the expansive force of large amounts of gases in an extremely viscous lava that cools very rapidly, forming a glassy rock. Pumice is the rock that is usually formed from the lava ejected from explosive volcanoes. It can be blown to kilometres by explosions.

Essay # 8 . Volcanic Products :

Volcanoes give out products in all the states of matter – gases, liquids and solids.

Steam, hydrogen, sulphur and carbon dioxide are discharged as gases by a volcano. The steam let out by a volcano condenses in the air forming clouds which shed heavy rains. Various gases interact and intensify the heat of the erupting lavas. Explosive eruptions cause burning clouds of gas with scraps of glowing lava called nuees ardentes.

The main volcanic product is liquid lava. Sticky acid lava on cooling, solidifies and hardens before flowing long distances. Such lava can also block a vent resulting in pressure build-up which was relieved by an explosion. Basic fluid lava of lesser viscosity flows to great distances before hardening.

Some lava forms are produced by varying conditions as follows. Clinkery block shaped features are produced when gas spurted from sluggish molten rock capped by cooling crust. These are called Aa.

Pahoehoe is a feature which has a wrinkled skin appearance caused by molten lava flowing below it.

Pillow lava is a feature resembling pillows. This feature piles up when fast cooling lava erupts under water.

Products in explosive outbursts are called Pyroclasts. These consist of either fresh material or ejected scraps of old hard lava and other rock. Volcanic bombs include pancake-flat scoria shaped on impacting the ground and spindle bombs which are twisted at ends as they whizzle through the air. Acid lava full of gas formed cavities produces a light volcanic rock.

Pumice which is so light it can float on water. The product Ignimbrite shows welded glassy fragments. Lapilli are hurled out cinder fragments. Vast clouds of dust or very tiny lava particles are called volcanic ash. Volcanic ash mixed with heavy rain creates mudflows.

Sometimes mudflows can bury large areas of land. Powerful explosions can smoother land for many kilometres around with ash and can hurl huge amount of dust into the higher atmosphere. Violent explosions destroy farms and towns, but volcanic ash provides rich soil for crops.

i. Hot springs:

The underground hot rocks heat the spring waters creating hot springs. The hot springs shed minerals dissolved in them resulting in crusts of calcium carbonate and quartz (geyserite).

ii. Smoker:

This is a submarine hot spring at an oceanic spreading ridge. This submarine spring emits sulphides and builds smoky clouds.

iii. Geyser:

Periodically steam and hot water are forced up from a vent by super-heated water in pipe like passage deep down. Famous geysers are present in Iceland and Yellowstone National Park.

iv. Mud volcano:

This is a low mud cone deposited by mud-rich water gushing out of a vent.

v. Solfatara:

This is a volcanic vent which emits steam and sulphurous gas.

vi. Fumarole:

This is a vent which emits steam jets as at Mt. Etna, Sicily and Valley of Ten Thousand smokes in Alaska.

vii. Mofette:

This is a small vent which emits gases including carbon dioxide. These occur in France, Italy and Java.

Various terms used while describing volcanic features are given below:

i. Magma Chamber:

Magma is created below the surface of the earth (at depth of about 60 km) and is held in the magma chamber until sufficient pressure is built-up to push the magma towards the surface.

This is a pipe like passage through which the magma is pushed up from the magma chamber.

This is the outlet end of the pipe. Magma exits out of the vent. If a vent erupts only gases, it is called fumarole.

iv. Crater:

Generally the vent opens out to a depression called crater at the top of the volcano. This is caused due to the collapse of the surface materials.

v. Caldera:

This is a very big crater formed when the top of an entire volcanic hill collapses inward.

When the erupted materials cover the vent, a volcanic dome is created covering the vent. Later as the pressure of gas and magma rises, another eruption occurs shattering the dome.

A mountain-like structure created over thousands of years as the volcanic lava, ash, rock fragments are poured out onto the surface. This feature is called volcanic cone.

viii. Pyroclastic Flow :

A pyroclastic flow (also known as nuee ardentes (French word) is a ground hugging, turbulent avalanche of hot ash. pumice, rock fragments, crystals, glass shards and volcanic gas. These flows can rush down the steep slopes of a volcano at 80 to 160 km/li, burning everything in their path.

Temperatures of these flows can reach over 500°C. A deposit of this mixture is also often referred to as pyroclastic flow. An even more energetic and dilute mixture of searing volcanic gases and rock-fragments is called a pyroclastic surge which can easily ride up and over ridges.

ix. Seamounts :

A spectacular underwater volcanic feature is a huge localized volcano called a seamount. These isolated underwater volcanic mountains rise from 900 m to 3000 m above the ocean floor, but typically are not high enough to poke above the water surface.

Seamounts are present in all the oceans of the world, with the Pacific ocean having the highest concentration. More than 2000 seamounts have been identified in this ocean. The Gulf of Alaska also has many seamounts. The Axial Seamount is an active volcano off the north coast of Oregon (currently rises about 1400 m above the ocean floor, but its peak is still about 1200 m below the water surface.

Essay # 9 . Source of the Explosive Energy :

The energy for the explosive violence comes from the expansion of the volatile constituents present in the magma, the gas content of which determines the degree of commination of the materials and the explosive violence of the eruption.

This energy is expanded in two ways, firstly in the expulsion of the materials into the atmosphere and secondly, due to expansion within the magma leading to the development of vesicles. The most important gas is steam, which may form between 60 to 90 per cent of the total gas content in a lava. Carbon dioxide, nitrogen and sulphur dioxide occur commonly and hydrogen, carbon monoxide, sulphur and chlorine are also present.

Essay # 10 . Classification of Pyroclastics :

Pyroclastics refer to fragmental material erupted by a volcano. The larger fragments consisting of pieces of crystal layers beneath the volcano or of older lavas broken from the walls of the conduit or from the surface of the crater are called blocks.

Volcanic bombs are masses of new lava blown from the crater and solidified during flight, becoming round or spindle shaped as they are hurled through the air. They may range in size from small pellets up to huge masses weighing many kilonewtons.

Sometimes they are still plastic when they strike the surface and are flattened or distorted as they roll down the side of the cone. Another type called bread crust bomb resembles a loaf of bread with large gaping cracks in the crust.

This cracking of the crust results from the continued expansion of the internal gases. Many fragments of lava and scoria solidified in flight drop back into the crater and are intermixed with the fluid lava and are again erupted.

In contrast to bombs, smaller broken fragments are lapilli (from Italian meaning, little stones) about the size of walnuts; then in decreasing size, cinders, ash and dust. The cinders and ash are pulverized lava, broken up by the force of rapidly expanding gases in them or by the grinding together of the fragments in the crater, as they are repeatedly blown out and dropped back into the crater after each explosion.

Pumice is a type of pyroclastic produced by acidic lavas if the gas content is so great as to cause the magma to froth as it rises in the chimney of the volcano. When the expansion occurs the rock from the froth is expelled as pumice. Pumice is of size ranging from the size of a marble to 30 cm or more in diameter. Pumice will float in water due to many air spaces formed by the expanding gases.

Lava fountains in which steam jets blow the lava into the air produce a material known as Pele’s hair which is identical with rock wool which is manufactured by blowing a jet of steam into a stream of molten rock (Rock wool is used for many types of insulation).

Coarse angular fragments become cemented to form a rock called volcanic breccia. The finer material like cinders and ash forms thick deposits which get consolidated through the percolation of ground water and is called tuff. Tuff is a building stone used in the volcanic regions. It is soft and easily quarried and can be shaped and has enough strength to be set into walls with mortar.

i. Agglomerate:

The debris in and around the vent contains the largest ejected masses of lava bombs which are embedded in dust and ash. A deposit of this kind is known as agglomerate. The layers of ash and dust which are formed for some distance around the volcano and which builds its cone, become hardened into rocks which are called tuffs.

Ash includes all materials with size less than 4 mm. It is pulverized lava, in which the fragments are often sharply angular and formed of volcanic glass; these angular and often curved fragments are called shards.

Since the gas content of ash on expulsion is high it has considerable mobility on reaching the surface; it is also hot and plastic, the result of these conditions being that the fragments often become welded together. The finest of ash is so light that wind can transport it for great distances.

The table below sets out a general classification of pyroclastic rocks based on the particle size of the fragments forming the rocks.

The chart in Fig. 15.3 summarizes the names of the common magmas and their associated ranges in silica. A very important property of magma that determines the eruption style and the eventual shape of the volcano it builds, is its resistance to flow, namely its viscosity.

Magma viscosity increases as its silica content increases. Eruptions of highly viscous magmas are violent. The highly viscous rhyolite magma piles up its ticky masses right over its eruptive vent to farm tall steep sided volcanoes.

On the contrary the basaltic magma flows great distances from its eruptive vent to from low, broad volcanic features. Magma in the intermediate viscosity spectrum say the andesite magma tends to form volcanoes of profile shapes between these two extremes.

An additional important ingredient of magma is water. Magmas also contain carbon dioxide and various sulphur-containing gases in solution. These substances are considered volatile since they tend to occur as gases at temperatures and pressures at the surface of the earth.

As basaltic magma changes composition toward rhyolite the volatiles become concentrated in the silica-rich magma. Presence of these volatiles (mainly water) in high concentration produces highly explosive volcanoes. It should be noted that these volatiles are held in magma by confining pressure. Within the earth, the confining pressure is provided by the load of the overlying rocks.

As the magma rises from the mantle to depths about 1.5 km or somewhat less, the rock load is reduced to that extent that the volatiles (mainly water) start to boil. Bubbles rising through highly viscous rhyolitic magma have such difficulty to escape their way, that many carry blobs of magma and fine bits of rock with them and they finally break free and jet violently upward resulting in a violent buoyant eruption column that can rise to kilometres above the earth.

The fine volcanic debris in such a powerful eruption gets dispersed within the upper atmosphere, hide the sunlight affecting the weather. The greater the original gas concentration in a magma and the greater the volume rate of magma leaving the vent, the taller is the eruption column produced.

The gases escaping from magma during eruption mix with the atmosphere and become part of the air humans, animals and plants breath and assimilate. However as magma cools and solidifies to rock during eruption, some of the gas remains trapped in bubbles creating vesicles. Generally all volcanic rocks contain some gas bubbles. A variety of vesicular rhyolite is pumice. Pumice is vesicular to such an extent, it floats in water.

Essay # 15. Classification of Volcanic Activity:

A classification of volcanic activity based on the type of product is shown in Fig. 15.4. The basic subdivision is based on the proportions of the gas, liquid and solid components, which can be represented on a triangular diagram. The four basic triangles represent the domain of four basic kinds of volcanic activity.

Essay # 16. Cone Topped and Flat Topped Volcanoes:

Generally rhyolite volcanoes are flat-topped because rhyolite magma which is extremely viscous, oozes out of the ground, piles up around the vent and then oozes away a bit to form a pancake shape. In contrast basalt volcanoes generally feed lava flows that flow far from the vent, building a cone.

Basaltic tephra (large particles of different size) is a spongy-looking black, rough material of pebble or cobble. Commercially this tephra is known as cinder and is used for gardening and rail-road beds. In some situations basaltic volcanoes develop flat top profile.

Flat topped volcanoes of basalt can form when there is an eruption under a glacier. Instead of getting ejected as tephra to form a cone, it forms a cauldron of lava surrounded by ice and water and eventually solidifying. When the ice melts, a steep-sided, table-shaped mountain known as a tuya remains. Volcanoes of this type are common in Iceland and British Columbia, where volcanoes have repeatedly erupted under glaciers.

Surprisingly, the Pacific ocean is a home to many flat-topped undersea basaltic mountains. These are called seamounts. How these seamounts were formed was a mystery for a long time. Surveying and dredging operations revealed that most seamounts were formerly conical volcanoes projecting above the water.

Geologists found that the conical volcanoes got lowered due to subsidence and the tops of the volcanoes came near the sea water level and the powerful waves mowed them flat. Continued subsidence caused them to drop below the water surface.

Essay # 17. Types of Volcanoes :

There are many types of volcanoes depending on the composition of magma especially on the relative proportion of water and silica contents. If the magma contains little of either of these, it is more liquid and it flows freely forming a shallow rounded hill.

Large water content with little silica permits the vapour to rapidly rise through the molten rock, throwing fountains of fire high into the air. More silica and less water in the magma make the magma more viscous. Such magma flows slowly and builds-up a high dome.

High content of both water and silica create another condition. In such a case the dense silica prevents the water from vaporizing until it is close to the surface and results in a highly explosive way. Such an eruption is called a Vulcan eruption.

Other types of eruption are named after people or regions associated with them. Vesuvian eruption named after Vesuvius is a highly explosive type occurring after a long period of dormancy. This type ejects a huge column of ash and rock to great heights upto 50 km.

A peleean eruption named after the eruption of Mt. Pelee in Martin que in 1902 is a highly violent eruption ejecting a hot cloud of ash mixed with considerable quantity of gas which flows down the sides of the volcano like a liquid. The cloud is termed nuee ardente meaning glowing cloud. Pyroclastic or ash flow refers to a flow of ash, solid rock pieces and gas. Hawaiian eruptions eject fire fountains.

Essay # 18. Violence of Volcanic Eruptions :

Volcanic activity may be classified by its violence, which in turn is generally related to rock type, the course of eruptive activity and the resulting landforms. We may in general distinguish between lava eruptions associated with basic and intermediate magmas and pumice eruptions associated with acid magmas.

The percentage of the fragmentary material in the total volcanic material produced can be used as a measure of explosiveness and if calculated for a volcanic region can be adopted as an Explosion Index (E), useful for comparing one volcanic region with others. Explosion Index for selected volcanic regions by Rittmann (1962) are shown in the table below.

Newhall and Self (1982) proposed a Volcanic Explosivity Index (VEI) which helps to summarize many aspects of eruption and is shown in the table below.

Essay # 19. Famous Volcanoes around the World :

Many volcanoes are present around the world. Some of the largest and well known volcanoes are listed in the table below.

Essay # 20. Volcanic Hazards :

Volcanic eruptions have caused destruction to life and property. In most cases volcanic hazards cannot be controlled, but their impacts can be mitigated by effective prediction methods.

Flows of lava, pyroclastic activity, emissions of gas and volcanic seismicity are major hazards. These are accompanied with movement of magma and eruptive products of the volcano. There are also other secondary effects of the eruptions which may have long term effects.

In most cases volcanoes let out lava which causes property damage rather than injuries or deaths. For instance, in Hawaii lava flows erupted from Kilauea for over a decade and as a consequence, homes, roads, forests, cars and other vehicles were buried in lavas and in some cases were burned by the resulting fires but no lives were lost. Sometimes it has become possible to control or divert the lava flow by constructing retaining walls or by some provision to chill the front of the lava flow with water.

Lava flows move slowly. But the pyroclastic flows move rapidly and these with lateral blasts may kill lives before they can run away. In 1902, on the island of Martinique the most destructive pyroclastic flow of the century occurred resulting in very large number of deaths.

A glowing avalanche rushed out of the flanks of Mount Pelee, running at a speed of over 160 km/h and killed about 29000 people. In A.D. 79 a large number of people of Pompeii and Herculaneum were buried under the hot pyroclastic material erupted by Mount Vesuvius.

The poisonous gas killed many of the victims and their bodies got later buried by pyroclastic material. In 1986, the eruption of the volcano at Lake Nyos, Cameroon killed over 1700 people and over 3000 cattle.

When magma moves towards the surface of the earth rocks may get fractured and this may result in swarms of earthquakes. The turbulent bubbling and boiling of magma below the earth can produce high frequency seismicity called volcanic tremor.

There are also secondary and tertiary hazards connected with volcanic eruptions. A powerful eruption in a coastal setting can cause a displacement of the seafloor leading to a tsunami. Hazardous effects are caused by pyroclastic material after a volcanic eruption has ceased.

Either melt water from snow or rain at the summit of the volcano can mix with the volcanic ash and start a deadly mud flow (called as lahar). Sometimes a volcanic debris avalanche in which various materials like pyroclastic matter, mud, shattered trees etc. is set out causing damage.

Volcanic eruptions produce other effects too. They can permanently change a landscape. They can block river channels causing flooding and diversion of water flow. Mountain terrains can be severely changed.

Volcanic eruptions can change the chemistry of the atmosphere. The effects of eruption on the atmosphere are precipitation of salty toxic or acidic matter. Spectacular sun set, extended period of darkness and stratospheric ozone depletion are all other effects of eruptions. Blockage of solar radiation by fine pyroclastic material can cause global cooling.

Apart from the above negative effects of volcanisms there are a few positive effects too. Periodic volcanic eruptions replenish the mineral contents of soils making it fertile. Geothermal energy is provided by volcanism. Volcanism is also linked with some type of mineral deposits. Magnificent scenery is provided by some volcanoes.

The study of volcanoes has great scientific as well as social interest. Widespread tephra layers inter-bedded with natural and artificial deposits have been used for deciphering and dating glacial and volcanic sequences, geomorphic features and archeological sites.

For example, ash from Mt. St. Helens Volcano in Washington travelled at least 900 km into Alberta. North American Indians fashioned tools and weapons out of volcanic glass, the origin of which is used to trace migratory and trading routes.

Volcanoes are windows through which the scientists look into the interiors of the earth. From volcanoes we learn the composition of the earth at great depths below the surface. We learn about the history of shifting layers of the earth’s crust. We learn about the processes which transform molten material into solid rock.

From the geological historical view point, volcanic activity was crucial in providing to the earth a unique habitat for life. The degassing of molten materials provided water for the oceans and gases for the atmosphere – indeed, the very ingredients for life and its sustenance.

Essay # 21. Volcanoes and Atmospheric Pollution :

During eruptions volcanoes inject solid particles and gases into the atmosphere. Particles may remain in the atmosphere for months to years and rain back on to the earth. Volcanoes also release chlorine and carbon dioxide.

The main products injected into the atmosphere from volcanic eruptions however are volcanic ash particles and small drops of sulphuric acid in the form of a fine spray known as aerosol. Most chlorine released from volcanoes is in the form of hydrochloric acid which is washed out in the troposphere. Volcanoes also emit carbon dioxide.

During the times of giant volcanic eruptions in the past the amount of carbon dioxide released may have been enough to affect the climate. In general global temperatures are cooler for a year or two after a major eruption.

A large magnitude pyroclastic eruption such as a caldera-forming event can be expected to eject huge volumes of fine ash high into the atmosphere where it may remain for several years, carried around the globe by strong air currents in the upper atmosphere.

The presence of this ash will increase the opacity of the atmosphere, that is, it will reduce the amount of sunlight reaching the earth’s surface. Accordingly, the earth’s surface and climate will become cooler. Various other atmospheric effects may be observed. Particularly noticeable is an increase in the intensity of sunsets.

i. Global Warming :

Besides blocking the rays of the sun, the vast clouds of dust and ash that result from a volcanic eruption can also trap ultraviolet radiation within the atmosphere causing global warming.

Volcanic eruptions usually include emissions of gases such as carbon dioxide which can further enhance this warming. Even if it lasted only for a relatively short time, a sudden increase in temperature could in turn have contributed to extinctions by creating an environment unsuitable for many animals.

ii. Geothermal Energy :

Geothermal energy is the heat energy trapped below the surface of the earth. In all volcanic regions, even thousands of years after activity has ceased the magma continues to cool at a slow rate. The temperature increases with depth below the surface of the earth. The average temperature gradient in the outer crust is about 0.56° C per 30 m of depth.

There are regions however, where the temperature gradient may be as much as 100 times the normal. This high heat flow is often sufficient to affect shallow strata containing water. When the water is so heated such surface manifestations like hot springs, fumaroles, geysers and related phenomena often occur.

It may be noted that over 10 per cent of the earth’s surface manifests very high heat flow and the hot springs and related features which are present in such areas have been used throughout the ages, for bathing, laundry and cooking.

In some places elaborate health spas and recreation areas have been developed around the hot-spring areas. The cooling of magma, even though it is relatively close to the surface is such a slow process that probably in terms of human history, it may be considered to supply a source of heat indefinitely.

Temperatures in the earth rise with increasing depth at about 0.56°C per 30 m depth. Thus if a well is drilled at a place where the average surface temperature is say 15.6°C a temperature of 100°C would be expected at about 4500 m depth. Many wells are drilled in excess of 6000 m and temperatures far above the boiling point of water are encountered.

Thermal energy is stored both in the solid rocks and in water and steam filling the pore spaces and fractures. The water and steam serve to transmit the heat from the rocks to a well and then to the surface.

In a geothermal system water also serves as the medium by which heat is transmitted from a deep igneous source to a geothermal reservoir at a depth shallow enough to be tapped by drilling. Geothermal reservoirs are located in the upward flowing part of a water – convective system. Rainwater percolates underground and reaches a depth where it is heated as it comes into contact with the hot rocks.

On getting heated, the water expands and moves upward in a convective system. If this upward movement is unrestricted the water will be dissipated at the surface as hot springs; but if such upward movement is prevented, trapped by an impervious layer the geothermal energy accumulates, and becomes a geothermal reservoir.

Until recently it was believed that the water in a geothermal system was derived mainly from water given off by the cooling of magma below the surface. Later studies have revealed that most of the water is from surface precipitation, with not more than 5 per cent from the cooling magma.

Production of electric power is the most important application of geothermal energy. A geothermal plant can provide a cheap and reliable supply of electrical energy. Geothermal power is nearly pollution free and there is little resource depletion.

Geothermal power is a significant source of electricity in New Zealand and has been furnishing electricity to parts of Italy. Geothermal installations at the Geysers in northern California have a capacity of 550 megawatts, enough to supply the power needs of the city of San Francisco.

Geothermal energy is versatile. It is being used for domestic heating in Italy, New Zealand and Iceland. Over 70 per cent of Iceland’s population live in houses heated by geothermal energy. Geothermal energy is being used for forced raising of vegetables and flowers in green houses in Iceland where the climate is too harsh to support normal growth. It is used for animal husbandry in Hungary and feeding in Iceland.

Geothermal energy can be used for simple heating processes, drying or distillation in every conceivable fashion, refrigeration, tempering in various mining and metal handling operations, sugar processing, production of boric acid, recovery of salts from seawater, pulp and paper production and wood processing.

Geothermal desalinization of sea water holds promise for abundant supply of fresh water. In some areas it is a real alternative to fossil fuels and hydroelectricity and in future may help meet the crisis of our insatiable appetite for energy.

iii. Phenomena Associated with Volcanism :

In some regions of current or past volcanic activity some phenomena related to volcanism are found. Fumaroles, hot springs and geysers are the widely known belonging to this group. During the process of consolidation of molten magma either at the surface or at some depths beneath the surface gaseous emanations may be given off.

These gas vents constitute the fumaroles. The Valley of Ten Thousand Smokes in Alaska is a well-known fumarole and is maintained as a national monument. This group of fumaroles was formed by the eruption of Mount Katmai in 1912. This valley of area of about 130 square kilometres contains thousands of vents discharging steam and gases.

These gases are of varied temperatures and the temperatures vary from that of ordinary steam to superheated steam coming out as dry gas. Many of the gases escaping from the vents may be poisonous, such as hydrogen sulphide and carbon monoxide which are suffocating and may settle at low places in the topography. For example, the fumaroles at the Poison Valley, Java discharge deadly poisonous gases.

Solfataras are fumaroles emitting sulphur gases. At some places, the hydrogen sulphide gases undergo oxidation on exposure to air to form sulphur. The sulphur accumulates in large amount so that the rocks close to the solfataras may contain commercial quantities of sulphur.

Hot springs are also phenomena associated with volcanic activity. Waters from the surface which penetrate into the ground can get heated either by contact with the rocks which are still hot or by gaseous emanations from the volcanic rocks. The water so heated may re-emerge at the surface giving rise to hot springs. In some situations the hot springs may be intermittently eruptive. Such intermittently hot springs are called geysers.

Related Articles:

• Lava: Types and Eruptions | Volcanoes
• Submarine and Sub Glacial Eruptions | Volcanoes

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Volcanic eruptions.

Photo by R.E. Stinson.

Introduction

Volcanic eruptions are among the most awesome of all natural phenomena on Earth. They may be strangely beautiful as fountains of glowing-red lava rise above a vent to feed a lava flow that spreads rapidly downhill. Or they may consist of terrifying explosions that send clouds of scorching hot ash high into the atmosphere or roaring down a volcano’s slopes and destroying everything in its path. While a great range in the type, style, and violence of volcanic eruptions exists, they all are part of one of the most fundamental geologic processes that builds and shapes Earth’s crust. The place to begin an exploration of the diversity of the different types and kinds of volcanic eruptions is with the definition. A volcanic eruption is the expulsion of gases, rock fragments, and/or molten lava from within the Earth through a vent onto the Earth’s surface or into the atmosphere.

USGS illustration.

Some volcanic eruptions consist of mostly gas emissions, others are relatively quiet discharges of fluid lava, and yet others are cataclysmic explosions. Different kinds of eruptions leave characteristic deposits that, in turn, build different types of volcanoes. A magma’s composition, viscosity, and gas content, the eruption rate, and the size of the magma reservoir determines many aspects of eruptions, including how explosive they are. National parks are great places to observe current volcanic activity.

• Kilauea in Hawai’i Volcanoes National Park was active from 1983 to 2018, and has had shorter periods of eruption since then.
• Katmai National Park and Preserve in Alaska is one of the world’s most active volcanic areas with 10 volcanoes that have had historic eruptions.
• The most recent eruption at the Lassen volcanic center in California occurred in 1917.

A number of other national parks contain volcanoes that have had prehistoric eruptions. Volcanoes in many other parks erupted in the even more distant past. Today, these mountains evoke peace and serenity that belies the violence in their history.

• Mount Mazama exploded about 7,550 years ago to form Crater Lake.
• Capulin Volcano erupted just over 54,000 years ago.
• The Valdez Caldera erupted 1.25 million years ago.

Whether active or ancient, volcanic landforms found in national parks result from their eruption dynamics, with the volcanoes themselves and the lava flows and other deposits they leave behind serving as tangible evidence of the volcanic processes that formed them.

Generation and Rise of Magma

The melted rock (magma) that is erupted in volcanoes does not come from the Earth’s core or even from deep within the mantle. There are also no permanent pools of melted rock found within the crust or mantle. Instead, magma produced from partial melting of the upper mantle. The heat that causes the partial melting comes from several sources; most importantly, from decay of radioactive elements, such as uranium, thorium, and potassium. Once magma is generated, it rises buoyantly because it is lighter than the surrounding rock. It moves in slow-moving balloons or diapirs; or in planar fractures (dikes). Sometimes it pools at the base of the crust, and other times it continues to rise to form magma chambers. Shallow magma chambers may form in regions of neutral buoyancy, i.e., areas where the pressure within the magma body equals the pressure outside it. Magma typically begins to crystallize while they are being stored in magma chambers, forming a liquid-crystal mush. Eruptions from a shallow magma body may be triggered by injection of more magma into the chamber, by overpressure from increased volatile (gas) content, or by some other factor.

Composition & Viscosity

Volcanic eruptions are inherently physical processes given that they are the emission of gas, magma, and rock from within the Earth. Yet many aspects of eruptions are actually controlled by magma chemistry. In fact, the composition of the magma, as well as its gas content, largely determines whether an eruption is explosive, and the magnitude of that explosivity. Magma composition impacts nearly everything about a volcano, with viscosity of the melt being one of the most important factors that determines eruption dynamics, and even the shape of a volcanic edifice. Viscosity is the internal friction, or resistance to flow—or how “thick and sticky” or “thin and runny” a lava is.

• Lavas with low viscosity such as those erupted at Kilauea in Hawai’i Volcanoes National Park flow easily, with flow fronts that move up to 6 miles (10 km) per hour. Speeds in channels or lava tubes on steep slopes can be as fast as 19 miles (30 km) per hour.
• Highly viscous lavas do not spread out to form wide lava flows, but instead form steep-sided domes immediately above a vent, such as the dome at Novarupta in Katmai National Park .

Modified from NASA illustration.

Silica (SiO2) content has the greatest impact on magma viscosity. Most igneous rocks are made predominately of silica with concentrations ranging from about 45 to 78% by weight. Specifically, silica is arranged in tetrahedrons (Si04 complexes). Silica tetrahedrons can share oxygen atoms to form chains or networks in a melt. Higher concentrations of silica leads to longer and more complex chains. The greater abundance of complex chains of silica tetrahedrons have a greater propensity to tangle with one another. This impedes their ability to flow past one another, somewhat akin to tangling of long strands of spaghetti.

Therefore, the general rule is that magmas with high silica content are highly viscous, and ones with low silica have low viscosity (e.g., are inviscid ). The presence of other elements, particularly sodium and potassium, can lower viscosity in rhyolitic magmas because they interfere with silica’s ability to form complex chains. Similarly, the presence of water in the melt (which is common) can also decrease viscosity. In general the viscosity of a low silica magma like basalt is thousands of times more viscous than liquid water. High silica melts can be many orders of magnitude more viscous than basalts. Their relatively low viscosities are why basalts are generally extruded in quiet (effusive) or mildly explosive eruptions. On the other hand, eruptions of high silica magmas are likely to be explosive (due to both high viscosity and higher gas content).

NPS photo by B. Seibert.

Photo by Lanny Simpson, Alaska High Mountain Images.

Gas content & exsolution

Courtesy of James St. John (Flickr)

Magmas typically contain small amounts of dissolved gas ( volatiles ). Water and carbon dioxide are the most common volatiles, although sulfur dioxide, hydrogen sulfide, and others may be present. Until a magma nears the Earth’s surface, the enormous pressure of the overlying rock keeps gases dissolved. Near the surface, the pressure decreases and they can exsolve from the melt, ultimately forming gas bubbles in a process called vesiculation . This exsolution of magmatic gases as a magma ascends towards the surface is one of the forces that propels volcanic eruptions. The release of pressure as a magma nears the Earth’s surface is similar to the release of pressure in a carbonated beverage when it is opened. The exsolution of carbon dioxide from soda pop due to the release of confining pressure when the can is opened is like the expansion and exsolution of gases that propels eruptions. Higher volatile contents increase the likelihood of explosive eruptions compared to eruptions of magma with lower concentrations of gases. Viscosity is also important because gases can escape more easily from thin fluids than thick ones, as can be observed in the spattering that can happen when making jam (a more viscous liquid) versus the simple boiling of water. In general, the higher the viscosity and the higher the gas content, the more explosive the eruption will be.

USGS photo.

• In the eruptions that build some cinder cones, the vesiculation of basaltic magma from expanding and exsolving gas throws blobs of magma perhaps tens to hundreds of feet into the air that then cool and fall around the vent as cinders.
• In highly explosive eruptions of silicic magmas, vesiculation can completely shatter the erupting material into tiny bits called volcanic ash in columns that rise tens of thousands of feet into the atmosphere. For example, the April 21, 1989 eruption of Redoubt Volcano in Lake Clark National Park and Preserve formed an eruption column that ascended to a height of 62,000 feet (19 km).

Rate of Eruption

The rate of eruption can also influence how explosive an eruption is. If magma ascends slowly from deep within the crust, it is possible for the dissolved gases to escape nonviolently over time. But when magma ascension and eruption rate is rapid, the dissolved gases must escape all at once and the eruption is more explosive. Likewise, a soda gently fizzes when the gases dissolved in it are slowly released. Yet it explodes violently when carbon dioxide exsolves rapidly, such as happens after a can is shaken. It is the sudden release of energy by gas under pressure by rapid exsolution that is one of the main drivers of explosive eruptions.

Size of Magma Reservoir

The size of the magma body beneath a volcano has a strong controlling factor in the magnitude of eruptions because the availability of magma can strongly constrain its size. Small magma bodies simply cannot sustain large eruptions because there is not enough material available. Cinder cones, even exceptionally large ones like Sunset Crater Volcano, only tap small magma sources. The volume of erupted rock material erupted In 1085 CE to form Sunset Crater Volcano about was 0.12 cubic miles (0.52 cubic km) in volume in contrast to the largest eruption at Yellowstone 2.1 million years ago that expelled nearly 600 cubic miles (2,450 cubic km) of material.

Volcanic Activity

An erupting volcano may include varying types of activity, along with a range of intensity. Volcanic activity includes earthquakes caused by magma movement, gas emissions, effusive emissions of lava, and cataclysmic eruptions. Most geologists use the term eruption to encompass a whole period of volcanic activity which is bracketed by quiet intervals. The Smithsonian Institute has set an arbitrary interval of three months of complete inactivity of a volcano to separate one eruption from another. Eruptions generally consist of eruptive pulses and eruptive phases.

• Eruptive pulses are single explosions that may last a few seconds to minutes.
• Eruptive phases consist of numerous eruptive pulses that generate a pulsating eruptive column or lava flow, and may last from a few hours to days.

An eruption may consist of many eruptive pulses and last a few days, months, or even years.

Active, Dormant & Extinct

Volcanologists describe volcanoes as being active, dormant, or extinct based on how recently they erupted and whether they are likely to do so again.

• Active : A volcano is considered potentially active if it has erupted during the last 10,000 years. Some volcanoes may have dormant periods between eruptions greater than 10,000 years, but 10,000 years is a convenient cut-off date for activity and is used by convention. An active volcano is currently erupting, or is one that has erupted in historic time. Even though Mount Rainier hasn’t had a significant eruption for a thousand years, it is considered to be an active volcano.
• Dormant : A volcano that is not erupting now, but is considered likely to erupt in the future. There is no precise distinction between active and dormant volcanoes. Sometimes dormant volcanoes are described as being potentially active. Mount Rainier and the El Malpais National Monument volcanic field are considered dormant.
• Extinct : An extinct volcano is one that is not expected to erupt again in the future. Sometimes the determination of whether a volcano is extinct is based on the amount of time since its last eruption. Alternatively, some types of volcanoes such as cinder cones typically only erupt once. For example, Capulin Volcano, a cinder cones, is extinct.

Last updated: July 18, 2022

Human and Environmental Impacts of Volcanic Ash

Volcanic ash is made of tiny fragments of jagged rock, minerals, and volcanic glass. Ash is a product of explosive volcanic eruptions.

Earth Science, Geology, Meteorology, Geography, Physical Geography

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Volcanic ash is made of tiny fragments of jagged rock , minerals , and volcanic glass . Unlike the soft ash created by burning wood , volcanic ash is hard, abrasive , and does not dissolve in water. Generally, particles of volcanic ash are two millimeters (0.08 inches) across or smaller. Coarse particles of volcanic ash look and feel like grains of sand , while very fine particles are powdery . Particles are sometimes called tephra —which actually refers to all solid material ejected by volcanoes .

Ash is a product of explosive volcanic eruptions . When gases inside a volcano's magma chamber expand , they violently push molten rock ( magma ) up and out of the volcano. The force of these explosions shatters and propels the liquid rock into the air. In the air, magma cools and solidifies into volcanic rock and glass fragments. Eruptions can also shatter the solid rock of the magma chamber and volcanic mountain itself. These rock fragments can mix with the solidified lava fragments in the air and create an ash cloud.

Wind can carry small volcanic ash particles great distances. Ash has been found thousands of kilometers away from an eruption site. The smaller the particle, the further the wind will carry it. The 2008 eruption of Chaitén in Chile produced an ash cloud that blew 1,000 kilometers (620 miles) across Patagonia to Argentina, reaching both the Atlantic and Pacific Coasts .

Volcanic ash deposits tend to be thicker and have larger particles closer to the eruption site. As distance from the volcano increases, the deposit tends to thin out. The 1994 double eruption of Vulcan and Tavurvur in Papua New Guinea covered the nearby city of Rabaul in a layer of ash 75 centimeters (about two feet) deep, while areas closer to the volcanoes were buried under 150-213 centimeters (five to seven feet) of ash.

In addition to shooting volcanic ash into the atmosphere , an explosive eruption can create an avalanche of ash, volcanic gases, and rock, called a pyroclastic flow . These incredibly fast avalanches of volcanic debris can be impossible for humans to outrun. Pyroclastic flows are capable of razing buildings and uprooting trees.

Volcanic Ash Impacts

Plumes of volcanic ash can spread over large areas of sky, turning daylight into complete darkness and drastically reducing visibility . These enormous and menacing clouds are often accompanied by thunder and lightning . Volcanic lightning is a unique phenomenon and scientists continue to debate the way it works. Many scientists think that the sheer energy of a volcanic explosion charges its ash particles with electricity . Positively charged particles meet up with negatively charged particles, either in the cooler atmosphere or in the volcanic debris itself. Lightning bolts then occur as a means of balancing these charge distributions.

Volcanic ash and gases can sometimes reach the stratosphere , the upper layer in Earth’s atmosphere. This volcanic debris can reflect incoming solar radiation and absorb outgoing land radiation, leading to a cooling of the Earth’s temperature . In extreme cases, these “ volcanic winters ” can affect weather patterns across the globe. The 1815 eruption of Mount Tambora, Indonesia, the largest eruption in recorded history, ejected an estimated 150 cubic kilometers (36 cubic miles) of debris into the air. The average global temperature cooled by as much as 3° Celsius (5.4° Fahrenheit), causing extreme weather around the world for a period of three years. As a result of Mount Tambora’s volcanic ash, North America and Europe experienced the “Year Without a Summer” in 1816. This year was characterized by widespread crop failure, deadly famine , and disease .

Airborne volcanic ash is especially dangerous to moving aircraft . The small, abrasive particles of rock and glass can melt inside an airplane engine and solidify on the turbine blades—causing the engine to stall . Air traffic controllers take special precautions when volcanic ash is present. The 2010 eruption of Eyjafjallajökull, Iceland, produced an ash cloud that forced the cancelation of roughly 100,000 flights and affected seven million passengers, costing the aviation industry an estimated $2.6 billion.

Volcanic ash can impact the infrastructure of entire communities and regions . Ash can enter and disrupt  the functioning of machinery found in power supply, water supply, sewage treatment , and communication facilities. Heavy ash fall can also inhibit road and rail traffic and damage vehicles. When mixed with rainfall, volcanic ash turns into a heavy, cement -like sludge that is able to collapse roofs. In 1991, Mount Pinatubo erupted in the Philippines at the same time that a massive tropical storm wreaked havoc in the area. Heavy rains mixed with the ash fall, collapsing the roofs of houses, schools, businesses, and hospitals in three different provinces.

Ash also poses a threat to ecosystems , including people and animals. Carbon dioxide and fluorine, gases that can be toxic to humans, can collect in volcanic ash. The resulting ash fall can lead to crop failure, animal death and deformity , and human illness . Ash’s abrasive particles can scratch the surface of the skin and eyes, causing discomfort and inflammation . If inhaled , volcanic ash can cause breathing problems and damage the lungs . Inhaling large amounts of ash and volcanic gases can cause a person to suffocate . Suffocation is the most common cause of death from a volcano.

Volcanic Ash Clean Up

Volcanic ash is very difficult to clean up. Its tiny, dust -sized particles can enter into practically everything—from car engines, to office building air vents, to personal computers . It can severely erode anything that it contacts, often causing machinery to fail. When dry, ash can be blown by the wind, spreading into and polluting previously unaffected areas. Meanwhile, wet ash binds to surfaces like cement and removing it often means stripping away what is found underneath.

Cleaning up volcanic ash is a costly and time-consuming procedure . Communities must make coordinated efforts to dispose of ash while ensuring the safety of their residents. The 1980 eruption of Mount St. Helens covered the city of Yakima, Washington, U.S.A., in tons of volcanic ash. Declaring a state of emergency , Yakima received donated maintenance equipment and workers, who were then dispatched throughout the city in a grid pattern. Citizens also helped with a block-by-block cleanup effort. Yakima removed 544,000 metric tons of ash and disposed of it in landfills and local fairgrounds. The city even filled in a wasteland to create a new city park . The process took seven around-the-clock days and cost the city $5.4 million, often cited as an efficient and cost-effective example of ash cleanup.

Organizations such as the International Volcanic Health Hazard Network, the USGS Volcano Hazards Program, and the Cities and Volcanoes Commission create and disseminate information to the public about preparing for and cleaning up volcanic ash fall. Their guidelines are used throughout the world by city and town governments and by the citizens they serve.

Andisol Andisol is a type of soil formed from volcanic ash. Andisols are generally very fertile, support extensive agricultural development, and exist mostly around the Ring of Fire.

Flying High Scientists recently discovered that the eruption of Mount Churchill in the U.S. state of Alaska roughly 1,200 years ago produced an ash fall that reached from Canada to Germany some 7,000 kilometers (4,350 miles) away. The discovery was especially surprising given that the volcano ejected a relatively small amount of ash of 50 cubic kilometers (12 cubic miles). As the ash spread, however, it transformed into microscopic shards called cryptotephra that had a unique compositional signature. Scientists were able to identify these distinct shards in the Canadian province of Nova Scotia, Greenland, and across Northern Europe, suggesting that the cryptotephra was so light that it travelled easily along the high-altitude winds of the Northern Hemisphere. 

Pompeii Preserved One of the most famous explosive volcanic eruptions occurred in 79 C.E., when Mount Vesuvius buried the Roman (now Italian) cities of Pompeii under 18 meters (60 feet) of ash. The ash buried the cities so completely that it preserved entire buildings, paintings, and artifacts. It also created very detailed molds around the bodies of people who were killed. Starting in the 18th century, archaeologists began excavating Pompeii. They discovered the hollow impressions left by bodies in the hardened ash and developed a way to inject them with plaster to create casts of the bodies. Today the excavated city and its gruesome models of dead and dying people and animals are popular tourist attractions.

Smoke Signal When Mount St. Helens, in the U.S. state of Washington, erupted in 1980, a column of ash from the volcano rose 19 kilometers (12 miles) into the air.

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Essay On The Volcano – 10 Lines, Short & Long Essay For Kids

Key Points To Remember When Writing An Essay On The Volcano For Lower Primary Classes

10 lines on the volcano for kids, a paragraph on the volcano for children, short essay on volcano in 200 words for kids, long essay on volcano for children, interesting facts about volcanoes for children, what will your child learn from this essay.

A volcano is a mountain formed through an opening on the Earth’s surface and pushes out lava and rock fragments through that. It is a conical mass that grows large and is found in different sizes. Volcanoes in Hawaiian islands are more than 4000 meters above sea level, and sometimes the total height of a volcano may exceed 9000 meters, depending on the region it is found. Here you will know and learn how to write an essay on a volcano for classes 1, 2 & 3 kids. We will cover writing tips for your essay on a volcano in English and some fun facts about volcanoes in general.

Volcanoes are formed as a result of natural phenomena on the Earth’s surface. There are several types of volcanoes, and each may emit multiple gases. Below are some key points to remember when writing an essay on a volcano:

• Start with an introduction about how volcanoes are formed. How they impact the Earth, what they produce, and things to watch out for.
• Discuss the different types of volcanoes and talk about the differences between them.
• Cover the consequences when volcanoes erupt and the extent of the damage on Earth.
• Write a conclusion paragraph for your essay and summarise it. 

When writing a few lines on a volcano, it’s crucial to state interesting facts that children will remember. Below are 10 lines on volcanoes for an essay for classes 1 & 2 kids.

• Some volcanoes erupt in explosions, and then some release magma quietly.
• Lava is hot and molten red in colour and cools down to become black in colour. 
• Hot gases trapped inside the Earth are released when a volcano erupts.
• A circle of volcanoes is referred to as the ‘Ring of Fire.’
• Volcano formations are known as seismic activities.
• Active volcanoes are spread all across the earth. 
• Volcanoes can remain inactive for thousands of years and suddenly erupt.
• Most volcanic eruptions occur underwater and result from plates diverging from the margins.
• Volcanic hazards happen in the form of ashes, lava flows, ballistics, etc.
• Volcanic regions have turned into tourist attractions such as the ones in Hawaii.

Volcanoes can be spotted at the meeting points of tectonic plates. Like this, there are tons of interesting facts your kids can learn about volcanoes. Here is a short paragraph on a volcano for children:

A volcano can be defined as an opening in a planet through which lava, gases, and molten rock come out. Earthquake activity around a volcano can give plenty of insight into when it will erupt. The liquid inside a volcano is called magma (lava), which can harden. The Roman word for the volcano is ‘vulcan,’ which means God of Fire. Earth is not the only planet in the solar system with volcanoes; there is one on Mars called the Olympus Mons. There are mainly three types of volcanoes: active, dormant, and extinct. Some eruptions are explosive, and some happen as slow-flowing lava.

Small changes occur in volcanoes, determining if the magma is rising or not flowing enough. One of the common ways to forecast eruptions is by analysing the summit and slopes of these formations. Below is a short essay for classes 1, 2, & 3:

As a student, I have always been curious about volcanoes, and I recently studied a lot about them. Do you know? Krakatoa is a volcano that made an enormous sound when it exploded. Maleo birds seek refuge in the soil found near volcanoes, and they also bury their eggs in these lands as it keeps the eggs warm. Lava salt is a popular condiment used for cooking and extracted from volcanic rocks. And it is famous for its health benefits and is considered superior to other forms of rock or sea salts. Changes in natural gas composition in volcanoes can predict how explosive an eruption can be. A volcano is labelled active if it constantly generates seismic activity and releases magma, and it is considered dormant if it has not exploded for a long time. Gas bubbles can form inside volcanoes and blow up to 1000 times their original size!

Volcanic eruptions can happen through small cracks on the Earth’s surface, fissures, and new landforms. Poisonous gases and debris get mixed with the lava released during these explosions. Here is a long essay for class 3 kids on volcanoes:

Lava can come in different forms, and this is what makes volcanoes unique. Volcanic eruptions can be dangerous and may lead to loss of life, damaging the environment. Lava ejected from a volcano can be fluid, viscous, and may take up different shapes. 

When pressure builds up below the Earth’s crust due to natural gases accumulating, that’s when a volcanic explosion happens. Lava and rocks are shot out from the surface to make room on the seafloor. Volcanic eruptions can lead to landslides, ash formations, and lava flows, called natural disasters. Active volcanoes frequently erupt, while the dormant ones are unpredictable. Thousands of years can pass until dormant volcanoes erupt, making their eruption unpredictable. Extinct volcanoes are those that have never erupted in history.

The Earth is not the only planet in the solar system with volcanoes. Many volcanoes exist on several other planets, such as Mars, Venus, etc. Venus is the one planet with the most volcanoes in our solar system. Extremely high temperatures and pressure cause rocks in the volcano to melt and become liquid. This is referred to as magma, and when magma reaches the Earth’s surface, it gets called lava. On Earth, seafloors and common mountains were born from volcanic eruptions in the past.

What Is A Volcano And How Is It Formed?

A volcano is an opening on the Earth’s crust from where molten lava, rocks, and natural gases come out. It is formed when tectonic plates shift or when the ocean plate sinks. Volcano shapes are formed when molten rock, ash, and lava are released from the Earth’s surface and solidify.

Types Of Volcanoes

Given below various types of volcanoes –

1. Shield Volcano

It has gentle sliding slopes and ejects basaltic lava. These are created by the low-viscosity lava eruption that can reach a great distance from a vent.

2. Composite Volcano (Strato)

A composite volcano can stand thousands of meters tall and feature mudflow and pyroclastic deposits.

3. Caldera Volcano

When a volcano explodes and collapses, a large depression is formed, which is called the Caldera.

4. Cinder Cone Volcano

It’s a steep conical hill formed from hardened lava, tephra, and ash deposits.

Causes Of Volcano Eruptions

Following are the most common causes of volcano eruptions:

1. Shifting Of Tectonic Plates

When tectonic plates slide below one another, water is trapped, and pressure builds up by squeezing the plates. This produces enough heat, and gases rise in the chambers, leading to an explosion from underwater to the surface.

2. Environmental Conditions

Sometimes drastic changes in natural environments can lead to volcanoes becoming active again.

3. Natural Phenomena

We all understand that the Earth’s mantle is very hot. So, the rock present in it melts due to high temperature. This thin lava travels to the crust as it can float easily. As the area’s density is compromised, the magma gets to the surface and explodes.

How Does Volcano Affect Human Life?

Active volcanoes threaten human life since they often erupt and affect the environment. It forces people to migrate far away as the amount of heat and poisonous gases it emits cannot be tolerated by humans.

Here are some interesting facts:

• The lava is extremely hot!
• The liquid inside a volcano is known as magma. The liquid outside is called it is lava.
• The largest volcano in the solar system is found on Mars.
• Mauna Loa in Hawaii is the largest volcano on Earth.
• Volcanoes are found where tectonic plates meet and move.

Your child will learn a lot about how Earth works and why volcanoes are classified as natural disasters, what are their types and how they are formed.

Now that you know enough about volcanoes, you can start writing the essay. For more information on volcanoes, be sure to read and explore more.

Tsunami Essay for Kids Essay on Earthquake in English for Children How to Write An Essay On Environmental Pollution for Kids

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What Causes a Volcano to Erupt?

Volcanic eruptions are among the most stunning phenomena in the natural world. Volcanoes erupt because of the way  heat  moves beneath  Earth ’s surface. Heat is conveyed from the planet’s interior to its surface largely by  convection —the transfer of heat by movement of a heated fluid. In this case, the fluid is magma —molten or partially molten rock —which is formed by the partial melting of Earth's mantle and crust. The magma rises, and, in the last step in this heat-releasing process, erupts at the surface through volcanoes.

Most volcanoes are associated with  plate tectonic activity. For example, volcanoes of  Japan ,  Iceland ,  Indonesia , and numerous other places occur on the margins of the massive solid rocky plates that make up Earth’s surface. When one plate slides under another, water trapped in the subducted, sinking plate is squeezed out of it by enormous pressure, which produces enough heat to melt nearby rock, forming magma. Since the magma is more buoyant than the surrounding rock, it rises, and it may collect in chambers nearer to the surface. As a chamber fills up, the pressure inside may increase. When the downward pressure produced by the weight of rock above the chamber is less than the upward pressure produced by rock below the chamber, cracks often form above. Eventually the upward pressure may push the magma through the cracks and out of vents at the surface, where it becomes  lava . In fact, strictly speaking, the term  volcano  refers to just such a vent, although it can also refer to the landform created by the accumulation of solidified lava and volcanic debris near the vent.

Far from tectonic plate boundaries, a smaller number of volcanoes occur at hotspots , where rising magma melts through the crust. The volcanoes of  Hawaii  are good examples of hotspot volcanoes.

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Introduction

A volcano is an opening in Earth ’s crust. When a volcano erupts, hot gases and melted rock from deep within Earth find their way up to the surface. This material may flow slowly out of a fissure, or crack, in the ground, or it may explode suddenly into the air. Volcanic eruptions may be very destructive. But they also create new landforms. There are more than 1,500 potentially active volcanoes in the world today.

Volcanic Eruptions

Strong volcanic eruptions throw bits of magma into the air. These bits cool into tiny pieces of rock, called volcanic dust or volcanic ash. Wind can carry volcanic dust thousands of miles away. Volcanic ash can coat the land for miles around the volcano.

Where Volcanoes Form

Some of the most violent eruptions take place where the edge of one plate is forced beneath the edge of another. This forces magma to rise to the surface. Hot gases in the magma make these volcanoes very explosive. Most volcanoes of this type are found around the edges of the Pacific Ocean . This huge circle of volcanoes is known as the Ring of Fire .

A small number of volcanoes are not located along the edges of plates. They form at “hot spots” in Earth’s crust. At a hot spot, molten rock rises from deep below the crust. The volcanoes of Hawaii are the best examples of hot-spot volcanoes.

Volcanic Landforms

Stratovolcanoes, also called composite volcanoes, are mountains shaped like cones. They have a narrow top with steep sides and a wide bottom. A crater, or bowl-shaped pit, usually lies at the top. Stratovolcanoes are made up of layers of hardened lava and ash. Thousands of eruptions left these layers over millions of years. Mount Fuji in Japan is a stratovolcano.

Shield volcanoes are dome-shaped mountains built by lava flows. They are not as steep as stratovolcanoes, though they can be quite large. Some shield volcanoes that erupt under the sea grow high enough to create islands. The volcanoes of Hawaii are shield volcanoes.

A complex volcano has more than one vent. A volcano can have more than one vent when two cones overlap one another. Or a volcano can form new vents during an explosion.

Hot Springs, Geysers, and Fumaroles

Studying Volcanoes

Uses of Volcanoes

The effects of volcanoes are not entirely harmful. Volcanic ash soil—called andisol—is good for growing crops. In addition, the volcanic glass called obsidian has been used by many of the world’s peoples for weapons, tools, and ornaments. People also use the volcanic stone called pumice for cleaning wood, metal, and other surfaces and in producing building materials.

The heat within Earth that is released in volcanoes is an enormous potential source of energy . This energy, called geothermal energy, is difficult for people to control. However, hot water and steam trapped below the surface have been used to heat homes and greenhouses and to produce electric power in several countries, including Italy , New Zealand , Japan, Iceland, and the United States.

The word volcano comes from the name of Vulcan , the ancient Roman god of fire and metalworking . The Romans believed that volcanic eruptions resulted when Vulcan made thunderbolts and weapons for the gods. Other cultures explained volcanoes as outbursts of anger from a god or goddess. Pele was the name of the volcano goddess of the native Hawaiians.

Volcanoes have a long history of destruction. In 79 ce the eruption of Mount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum.

Two of the deadliest volcanic eruptions happened on islands in what is now Indonesia . There was the 1815 Mount Tambora eruption and one in 1883. In 1883 the volcano Krakatoa exploded and collapsed, triggering a colossal sea wave known as a tsunami . Tens of thousands of people were killed by these events.

On May 8, 1902, Mount Pelée erupted on the Caribbean island of Martinique . Although very little lava flowed, an unstoppable black cloud of hot gases and ash engulfed the city of Saint-Pierre, killing almost all of its 30,000 people. The birth of a volcano was witnessed between 1943 and 1952, when a smoking hole in a Mexican farmer’s cornfield erupted into a new mountain called Paricutín that eventually stood 1,400 feet (425 meters) above the level ground around it.

Another notable event took place in 1963, when a new volcanic island called Surtsey rose up from the Atlantic Ocean near Iceland. Within a few years it built up to an area of 1 square mile (2.5 square kilometers), with a peak more than 560 feet (170 meters) above sea level.

The 1980 eruption of Mount Saint Helens, in the U.S. state of Washington , was one of the biggest in North America. The 1991 eruption of Mount Pinatubo, in the Philippines , was the largest of the 1900s. These eruptions killed fewer people than earlier volcanoes, but they still destroyed much property.

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Geography Notes

Volcanoes: compilation of essays on volcanoes | disasters | geology.

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Here is a compilation of essays on ‘Volcanoes’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Volcanoes’ especially written for school and college students.

Essay on Volcanoes

Essay Contents:

• Essay on the Effects of Volcanic Activity

Essay # 1. Meaning of Volcanoes:

Sometimes the molten rock, ash, steam and other gases find their way to the surface of the earth through some vents or openings. These ejected materials accumulate around the vent and give rise to a volcanic cone or a hill. The conical hill along with the vent is known as volcano.

The mouth of a volcano is funnel shaped and hollow. This funnel-shaped hole is called the crater and the opening through which molten rock materials come out from inside is called the pipe or vent.

Fuji Yama in Japan, Visuvius in Italy, Cotopaxi in Equador and Barren Island are good examples of volcanic mountains.

The ejection of materials through the crater is called volcanic eruption. Volcanoes are generally classified into three types. These are active, dormant and extinct.

The active volcanoes generally erupt fairly frequently and are of two types, namely, permanent and intermittent:

(a) Permanent volcanoes:

The volcanoes that eject molten materials continuously are called permanent volcanoes.

(b) Intermittent volcanoes:

These are those volcanoes in which an eruption occurs at the end of a certain period of time. Stromboli of Italy and Etna of Sicily are the examples of active volcanoes.

A volcano which erupted sometime in the past, and is now passive, but may erupt again, is called a dormant volcano. Fuji Yama of Japan and Vesuvius of Italy are the dormant volcanoes.

Extinct volcanoes are those whose activity has not been known to us during the historical time. Kilimanjaro of Africa and Chimborazo of Equador are the examples of extinct volcanoes.

Sometimes it is difficult to say whether a volcano is extinct or dormant. For example, the Vesuvius, the Krakatoa, Mt. Unzen of Japan and Mt. Pintubo of Philippines once thought to be extinct became active recently.

Essay # 2. Formation of Volcanoes :

The actual causes of the formation of volcanoes are not yet definitely known.

The probable causes of their origin are given below:

(a) Difference of Pressure on the Surface:

Pressure below the earth’s surface increases with depth. The interior of the earth is under high pressure. It is at the same time very hot. Because of high pressure and high temperature the materials of the earth’s interior are in the viscous state. Owing to the release of pressure from the top portion for some reason, the viscous materials get expanded. There is then a natural tendency for molten materials to make an upward drive through the fissures or weak zones of the earth. By this way a volcano may be formed.

(b) Chemical Reaction:

There are some radioactive minerals in the interior of the earth. They emit heat continuously and as a result of this, the other materials get expanded and create enormous pressure. Then the underlying expands and molten materials may come out through some fissures forming volcanoes.

(c) Percolation of W ater to Earth’s Interior:

Besides those two causes, a volcano may be caused when water percolating through the surface meets the molten materials. In this case water turns into steam of great volume. Steam, lava, and other materials may then come out to the surface forming a volcano.

Essay # 3. Arisen of Volcanoes:

It is a well-known fact that in all the northern and central part of Europe at the present time there are no active volcanoes. But, the geologist professes to be perfectly certain that there were, during many of the past geological periods, many volcanoes in our island. Before we can understand how he has come to this conclusion we must, according to Hutton’s method, find out how any known volcano has arisen and what traces it has left after it has become extinct.

The nearest European volcano to our islands is Vesuvius, and it is fortunate that we know a great deal about Vesuvius during the last two thousand years. The Neapolitan volcanoes extend from Vesuvius on the east to the Islands of Procida and Ischia on the west.

Before the Christian era, as far back as any tradition is known, there seems to have been no activity at Vesuvius. But a Greek colony, which settled on Ischia about 380 B.C., had to abandon the island owing to an eruption, and there were traditions of volcanic eruptions driving earlier colonists away as well.

Since the date above mentioned there has been complete cessation of vulcanicity in that island except at one time, when there was an emission of lava. On the mainland to the west of Naples there was Lake Avernus, occupying the inside of an ancient crater, where probably poisonous gases were emitted, if what Lucretius tells us is true, that birds could not fly over it without being stifled.

Thus we see that just before and during the beginning of the Christian epoch there was some evidence that the volcanic forces, which had in far-off ages built up high mountains near the Bay of Naples, were not quite extinct.

Vesuvius, which we now recognise as a modern cone within an ancient one, the remains of the latter being called Monte Somma, was, up to the first century, a single regular truncated cone. Plutarch writes of its old crater having steep cliffs and being covered with wild vines. The flanks of the mountain were of a rich soil and were covered with fertile fields stretching down towards the busy towns of Pompeii and Herculaneum.

An especially violent shock on the 5th of February A.D. 63 gave warning of a recrudescence of vulcanicity. Pompeii was badly damaged, many buildings being thrown down. But the inhabitants were rich and at once set about restoring what was damaged. After sixteen years most of the houses were in good repair, and at least two damaged temples had been rebuilt. In August A.D. 79 the great catastrophe occurred, and when the Forum was dug out a few years ago, it was seen that the west side of the colonnade, which had been almost completed when the earthquake of A.D. 63 threw it down, had been only partly rebuilt, and the area was strewn with blocks which were being made ready for the rebuilding by the stone masons.

On August 24 the elder Pliny, who was commander of the Roman fleet, went with his ships to rescue the people at the foot of Vesuvius from danger. Arriving too late he went to Stabiae where, on the next morning, he was suffo­cated by fumes..

His nephew, the younger Pliny, tells the story of what he saw in a letter to Tacitus. It is easy to follow the course of events. A column of vapour ascended from Vesuvius and then spread out on all sides. This caused intense dark­ness, which was lit up at times by flashes of lightning.

Volcanic ashes, as the fragments blown out from a volcano are called, fell on ships many miles away, earthquake shocks were frequent, and the sea went back, leaving marine animals high and dry.

Pompeii and Herculaneum:

But the ashes did not only fall on ships and the earthquakes were only, as it were, premonitory warnings of what was to follow. For at last a series of gigantic explosions blew one side of Monte Somma into the air and the enormous mass of material on falling buried up the prosperous cities of Pompeii and Herculaneum.

On the west side the descending ashes mingled with torrents of rain and so gave rise to a huge stream of mud, which poured down upon Herculaneum and sealed it up, on the south side, driven by the wind, the ashes spread out like a cloud and covered Pompeii. From this cloud there descended first broken fragments of pumice about the size of a walnut, which rained down to a depth of about 9 feet, then came fine ash wet with rain and settled for a depth of some 6 or 7 feet over the doomed town.

When the sun set the storm of ashes had ceased, but by that time Herculaneum had vanished and only the roofs of the houses of Pompeii projected above the surface. As far as we can tell, no lava flowed from the mountain during this period. But after this year other periods of activity occurred, and in 1036 we learn that a lava stream did make its way down the mountain-side.

Other times of activity were in the years 1049, 1138, 1306, 1500, and 1631, though the activity in 1500 was very slight. During the long pause before 1631 there occurred in the Phlegraean Fields to the west an event of some importance. In 1538 a new mountain was suddenly formed; it was named Monte Nuovo, and good accounts of its formation have come down to us.

Monte Nuovo:

After several shocks during September 28, we are told that very early on the 29th flames of fire were seen, and shortly after the earth burst open and ashes and pumice were blown out. Later on the inhabitants of the neighbouring town fled in terror to Naples, and it is stated that the sea had retired and left multitudes of fish to die on the shore.

The eruption continued for two days and nights, and on the third day, signs of activity having ceased, several people went up the new hill which was over 400 feet high and 8000 feet round its base. When they looked down into the crater they saw a boiling cauldron. Some days afterwards fresh explosions took place, but with decreasing activity.

No lava was extruded during the eruption, and the whole mountain was built up of large and small fragments which were blown up into the air by explosions and fell close to the orifice. No further volcanic action has occurred here since the time of the formation of the hill and it is now completely covered with grass.

For nearly a century after the birth of Monte Nuovo Vesuvius continued tranquil, and there was no violent eruption for nearly five hundred years. At the bottom of its crater cattle grazed, and its sides were covered with bushes. But in 1631 these grassy flats and woods were blown into the air and seven streams of lava poured down from the mountain and overwhelmed several villages. From the seventeenth century to the present day there has been a constant series of eruptions and no great convulsion.

In fact, the behaviour of the district is very much like that of a safety valve; when there is an opening the forces below cause more or less quiet extrusion of molten rock, but if the opening gets plugged up then the activity ceases, it may be for many years, but there comes a time when the imprisoned forces get sufficiently great to blow away the overlying tract which had sealed up any opening that had before existed.

Probably the most violent volcanic eruption of recent times occurred in the Islands of Krakatoa in August 1883. The Royal Society appointed a committee to investigate the eruption, and the report of this committee is a very remarkable production and stands for all time as an example of what such a report should be.

Krakatoa is an island lying between Java and Sumatra. Although prior to the catastrophe this island had attracted but little attention, it lies in the heart of the great focus of volcanic activity in the world at the present time. Java itself contains forty-nine great volcanic mountains, and from it a chain of volcanoes is continued to the east and to the west.

Krakatoa did not show the regular conical shape of a volcano, but was only a part of a huge crater of which other smaller islands close by were also fragments.

The volcano of which these islands are the remnant must have been formerly one of gigantic dimensions, probably some twenty-five miles in circumference at the sea-level and over 10,000 feet in height. It was built up almost entirely of lava, and rested on beds of post-Tertiary age.

At some unknown date the central mass of the volcano was blown away, and only irregular portions of its crater- ring were left. Then occurred quiet eruptions at the bottom of the crater, causing the formation of small cones which filled up the older crater in parts, and raised these above sea-level. At one edge of the old crater eruptions built up the cone of Krakatoa from lava extrusions and outbursts of ashes.

In historic times the lateral cone of Krakatoa and the other land which was part of the original volcano were seen to be covered with luxuriant vegetation, and the products of its forests attracted the inhabitants of neighbouring islands.

The first eruption recorded as having taken place at Krakatoa is in 1680, though tradition points to former ones having occurred while man lived in the neighbourhood. The eruption was of a moderate type and the island soon recovered its former state, and the vegetation which had been destroyed once more spread and for two hundred years the volcanic forces lay dormant.

In 1880 earthquakes were frequent, and in May 1883 the inhabitants of the town of Batavia, 100 miles away from Krakatoa, heard booming sounds, and for many hours their doors and windows were rattled. On the next day ashes fell on Java and Sumatra at places opposite to Krakatoa, and later a column of steam from Krakatoa showed the position of the disturbance.

The height of the cloud of dust above the island was estimated to be seven miles, and falls of dust were noticed three hundred miles away. This eruption soon decreased in intensity.

An excursion party from Batavia landed on the island and found some depth of fine ash scattered over it, and many” trees had been stripped of their leaves and branches by the fall of this deposit.

On August 11 another visit to Krakatoa showed that the forests had been completely destroyed, and that the fine ash covered the surface of the ground near the shore to a depth of 20 inches. Some fourteen foci of eruption were seen, from which steam and dust were being sent out.

This recrudescence of activity finally culminated on August 26 and 27 in a series of tremendous explosions. During these days three European ships were actually in the Straits in which Krakatoa lies, and as they escaped destruction those on them have been able to give a record of their observations.

The course of the events which happened seem to have been as follows- During the night of the 26th white-hot fragments of lava were blown out and rolled down the mountain-sides, smaller cinders were blown out to sea, and finer ashes, remaining in the air, caused intense darkness in the early morning.

From sunset till midnight there was a continuous roar which moderated in the early morning. Each explosive outburst of steam blew off the crust which formed on the lava and left the white-hot molten material to throw its light on to the overhanging pall of ash, and so lit up the scene for miles around.

But now the somewhat peculiar situation of the volcano made itself felt. It lies close to the sea-level, and so the removal of so much material by the constant explosions allowed the waters of the sea to make their way into the heated mass of lava. This must have chilled and frozen much of the molten rock.

Some flashing into steam of the water no doubt occurred, and this gave rise to waves in the sea which travelled to the shores of Java and Sumatra, and were noted on the evening of the 26th and through the night.

The general result of this action of the sea-water was the closing of the safety-valve while the forces below it were still active, and finally there resulted four tremendous explosions between 5.30 a.m. and 10.52 a.m. on the morning of the 27th.

What happened in this space of time to the island itself was that the whole of the northern and lower portion of the island was blown into the air, while a large part of the cone which existed there also vanished. Where the island had risen to heights of between 300 and 400 feet was now in some places more than 1000 feet below sea-level.

These terrible explosions caused huge waves to travel away from the centre of destruction and, rushing up on to the coasts of Java and Sumatra, stranded all vessels near the shore, devastated towns and villages, destroyed two lighthouses, and caused the deaths of over 36,000 people.

But not only were sea-waves produced, but there were also air-waves. Some were so rapid that they produced sounds even at a place 3080 miles away, others were larger and broke in windows and cracked walls a hundred miles away at Batavia, where also lamps were thrown down, gas- jets extinguished, and a gasometer leaped out of its well.

Other air-waves were traced in their course by barometers at various places scattered widely over the surface of the world, and so we know that they travelled several times round the globe.

At Greenwich the depressions of the barometer due to these air-waves were recorded on every day between noon on August 27 to early in the morning of September 1. During the whole of August 27 eruptive action seems to have continued. Three vessels remained beating about in the Straits near the island, and their crews were busy for hours shovelling off their decks the volcanic dust which descended.

The finer particles of dust, however, floated still farther away. Between 7 and 10 in the morning the sky began to darken at Batavia, soon after lamps had to be lit, and a descent of fine dust began about 10.30 and lasted for nearly an hour, when complete darkness came on, the heavy dust- rain continuing till 1, and a less heavy precipitation till 3 P.M.

Complete darkness was experienced still farther to the east at a place one hundred and fifty miles from the volcano.

What happened during the days succeeding August 27 is doubtful; there may have been several small eruptions, but we know that on October 10 there was an explosion of some magnitude, and a large sea-wave. The commotion in the sea due to the cataclysmic convulsion of August 27 travelled far and wide.

Its terrible effects on the neighbour­ing islands of Java and Sumatra have already been noticed, and, of course, as it passed away from the centre of dis­turbance it gradually lost its intensity, still, tide-gauges and eye-observations tell us that to the west it was noticed up both the coasts of India, along the south of Arabia, round the south coast of Africa, to the east coast of Central America and Terra del Fuego, while it also spread to the north and reached the coast of France and was recorded at Devonport and Portland.

To the east it spread to Australia and New Zealand, to Alaska and San Francisco.

In Australia sudden rises of 5 and 6 feet were recorded on the west coast; on the shores of Java the rise was one of about 50 feet, while in the English Channel it was only barely noticeable. But if the effect on the sea near England was so slight there was another effect so pronounced that it must have forced itself on the attention of almost everyone in our islands.

The coarser particles which were shot into the air fell close to the island, finer ones rained down, as we have noted, a hundred miles away in Java, but still finer ones, shot up into the air to enormous heights by the terrific explosions, were carried in the upper regions of the atmosphere all round the globe. One effect of these fine particles floating in the air was to give most extraordinary sunset effects on clear evenings.

The writer very vividly remembers some of them he saw in Kent. The brilliant, almost lurid glows in the western sky lasted long after the usual sunset effects would have passed away, and were far brighter than the normal sunset colours.

These splendid sunset effects were noted in Australia, in India, and throughout Europe and America.

In Surrey particularly fine sunset colourings were noted on September 8, and on several occasions between that date and early November. On November 9 the effect was most wonderful and magnificent. Afterglows were recorded throughout November, December, and January, but during February and March the coloration decreased, and after March no peculiar illumination was observed.

Essay # 4. General Course of an Volcanic Eruption:

The general course of a volcanic eruption becomes plain when we consider such cases as those of Vesuvius, Monte Nuovo, and Krakatoa.

A district may have been quiescent for centuries, but then from some cause the earth’s internal heat produces stresses at some point in the crust below that district and the crust gives. Such a crack even though it be exceedingly small in amount causes a tremor, and things on the surface are shaken. It is rare that any actual Crack appears on the actual surface of the ground, but this does occur some­times.

The molten rock which is pressed up in some cases finds its way along these cracks and may eventually solidify in them without ever reaching the surface; these rock masses are spoken of as Intrusions. At other times water makes its way down to the heated area and, flashing into steam, produces an explosion.

This, if powerful enough, may blow off a huge amount of the superincumbent crust and cause vast damage, but if an opening has once been drilled the explosions are less violent and merely cause the ejection of blocks of stone or masses of molten rock. These may either fall back into the crater or be scattered over the mountain-sides round it.

Such mountains as Monte Nuovo and many of the so-called cinder cones of Central France have been formed in this way and show no other signs of volcanic activity. But in other cases molten rock wells up the central crater and pours over or breaches its cone and flows down its sides. In this way we get lava streams, and these, owing to subsequent periods of explosive violence, may get covered by volcanic ashes.

But the story of a volcano is not quite so simple as this, for as the volcano rises higher the pressure needed to force the column of molten rock up to near its summit must grow greater, and so it often happens that the internal forces find it easier to produce an opening on the sides of the mountain and so get formed those lateral cones, which in some cases are of such frequent occurrence. These may be merely cinder cones or they may pour out molten rock.

The wind which prevails during an eruption sends the ashes this way or that, and so the deposit of volcanic detritus round a volcano is not so continuous as an aqueous sediment. Lavas and ashes may be mixed up, and the lava stream may end off with great abruptness both at its lower end and along its sides. Moreover, when the whole mass has become firm, subsequent shocks may crack it, and into those cracks molten rock may be squeezed, giving rise to what are called dykes.

In general, therefore, beds which owe their formation to igneous activity do not show anything like the same regularity that is seen in the case of aqueous sediments. Moreover, as a rule, volcanic detritus encloses no life, for though the living things, such as the inhabitants of Pompeii, may get killed on the surface of the earth when it has ashes poured down on to it, yet the mass of the ash above contains no life of the period. It may, however, contain blocks of rocks which enclose fossils of their own age, and so be useful in giving something of a clue to the age of the ash.

Fossiliferous Ash Beds:

But there is another possibility; some ash may fall into the sea, and then as it settles it will entomb some of the life of the period, and successive erup­tions may build up a great thickness of rock in which after­wards fossils in plenty may be found. Such a deposit will often be in part like a terrestrial ash bed, and in part like a marine sediment, and these volcanic deposits are the most hopeful ones in which to search for fossils.

When denudation gets to work on a volcano it is obvious that loose ash beds will be scoured away with rapidity, and so when the denuding agents get either to the solid plug of frozen lava in the vent, or to streams of lava down the sides, or to the dyke intrusions, these hard rocks will be left as prominences above the softer and more easily eroded beds.

The discussion of vulcanicity is, however, incomplete unless one considers the origin of the molten rock; here we cannot appeal to active or modern volcanoes, as in their cases we can only see surface phenomena. But, after ages of denudation, ancient volcanoes have been dissected, and their roots, as it were, disclosed.

We then find that there was a large mass of molten rock below them which no doubt was compressed at different times and squeezed upwards. These huge masses of liquid material ultimately froze, and so have arisen the enormous masses of granite so well known in parts of our islands. Such a rock is spoken of as plutonic to distinguish it from a molten rock which solidified on the earth’s surface and is called volcanic.

If one asks what caused the pressure one must remember that our earth has a hot interior, as proved by every coal-mine and deep boring, and a cooled crust, and that as the earth slowly gives out its heat it shrinks, and the crust settles on to the shrinking nucleus. This settling down must give rise to enormous stresses, and it is no wonder that cracks appear and molten material is squeezed up them.

Fissure Eruptions:

But though the Vesuvian type of eruption is the one nearest to our doors and the one most commonly found at present on the earth, if we go to Iceland we shall find another and a very different type of eruption allied to what is called the Fissure type.

In Iceland, though there are volcanoes of the Vesuvian type, the most common eruptions take place from a chain of volcanoes arranged along a large fissure. Small ash cones may be built up at points on the fissure and the position of actual eruption moves on so that the rings of the cones intersect, but at times flows of lava pour out from the fissure without any cones at all, flood over the surrounding country, fill up the valleys, and instead of an undulating plain produce a more or less level lava desert. Such was the course of events in 1783 when one of the greatest extrusions of lava in modern times occurred in Iceland.

When such an outpouring takes place, the rivers which afterwards arise cut their way into the lavas and gradually remove them from the older rocks which they overlie or rest against. Ultimately the lava plain may become a plateau scored by streams that wind over it.

This type of volcanic action differs from the Vesuvian type in that it possesses few, if any, ash beds, and in its occurring with great regularity over a vast area.

The fissure type of rock being pre-eminently a lava there seems little hope of finding fossil evidence of its age. But when we remember that extrusion is not continuous and that rivers make their way over the plateau and form gravels, and that these gravels may be covered by a sub­sequent flow, we see that it is possible for fossil evidence to be found even amongst the lavas of the fissure type. Where the centre of volcanic activity lay beneath the sea the beds due to the eruption will get covered by marine deposits and so evidence will accumulate to give the age of the eruption, if in after times the sea-floor is raised above sea-level.

Essay # 5. Volcanic Deposits in the British Islands:

Now when we examine the rocks of our islands we find repeated evidence that in various periods there existed centres of vulcanicity in many places. The Pre-Cambrian rocks of Charnwood in Leicestershire, of Shropshire, of the Malverns and else­where contain undoubted beds of volcanic ash, while when we come to the succeeding Primary rocks we find abundant proof that volcanic action took place during their formation.

North Wales is one of the districts which must again and again have included volcanoes throughout Cambrian and Ordovician times, while on the other side of. St. George’s Channel Ordovician lavas and ashes are found to the north of Dublin and in the Waterford area. Nearer the centre of Ireland such rocks are seen close to Kildare, and in the west, in Galway and Mayo, extensive coarse and fine ash deposits and lavas have been proved to be of Ordovician age.

In South Wales Ordovician lavas and ashes are seen forming Cader Idris, and they occur in Shropshire, in the Lake District, and in Ayrshire. The Ordovician period, in fact, was one, as far as our islands are concerned, of almost universal volcanic activity. It will be noticed that much of our mountainous country owes its ruggedness and its height to the Ordovician volcanic rocks, which by their hardness have been able to withstand denuding forces.

After this widespread outburst of volcanic forces came the Silurian period, and for long it was thought that there was then a complete cessation of the ejectment of ashes and the outflow of lavas. Comparatively recently, however, it has been shown that Silurian lavas and ashes are to be seen in Kerry in Ireland, to the north of Bristol, and in the Mendips, and in South-west Wales. But elsewhere, as far as we know, throughout our islands deposition of marine sediment went on quietly throughout Silurian times.

After Silurian times there came a period of uplift from Central England northwards, and the formation of a series of large sea lochs in which the Old Red Sandstone was laid down.

The tract of land stretching north from the Cheviots contained many active volcanoes. In South Devon also there was much lava poured out and ash accumulated near what must have been the northern margin of a great ocean stretching southwards.

In South Scotland many of the plugs of igneous rocks which filled up old vents in Old Red Sandstone times have proved much harder to wear down than the surrounding ash cones, and so have remained forming hills standing up somewhat abruptly amongst softer strata. This volcanic tract of Argyllshire is continued to the west in Ulster.

To the south of this line volcanoes of Old Red Sandstone age occurred in the Pentland Hills, where numerous “necks ” of this Age are found. These form the western end of a long line of active volcanoes which stretches westwards to the Ayrshire coast.

The upper Old Red Sandstone beds but rarely enclose evidence of igneous activity, but in Carboniferous times once more the subterranean forces burst into vigorous action.

Carboniferous volcanoes were most abundant in Scotland, and they persisted from the lowest beds to the basement beds of the coal-measures. They were distributed over the central valley from the south of Kintyre to the Firth of Forth.

In England Carboniferous volcanic rocks are seen in Derbyshire, in the Mendips, and near Weston-super-Mare. In the Isle of Man there are the relics of a group of Carboni­ferous vents, but in Ireland the only evidence of activity during this period is in King’s County and near Limerick.

In Scotland the predominant type of outflow of lava is of the plateau type, but the cinder-cone type of volcano is seen abundantly in the Firth of Forth region and forms the regular type seen elsewhere in the British Isles.

The plateau type of eruption built up enormous sheets of igneous rock which now form the prominent escarp­ments of many hills in South Scotland, and the vents up which the molten rock rose were plugged up by the frozen lava, and now remain as numerous prominent hills rising abruptly above the softer sandstone around them. Instances of these hills are North Berwick Law and the Bass Rock.

Eventually, but before the close of the Carboniferous period, the plateau extrusions were submerged and buried under the Carboniferous limestone series, and then a new type of eruption began, the cinder-cone type. The relics of these cones are abundant in the Carboniferous beds of Scotland.

After a very considerable time earth movements ridged up the coal-measures and formed a series of inland seas, and in the Permian period we find feeble and short-lived volcanoes in Ayrshire, in East Fife, and in South Devon. In Ayrshire there are several volcanic necks, which descend vertically through the surrounding rocks and form vertical columns of volcanic material. This material is usually a coarse ash, but in some cases molten rock has risen up the vent.

In East Fife, along the shore near St. Andrews and Elie, there are abundant small necks piercing the Carboniferous strata. That they are older than the beds they pierce is proved by the blocks which fill them being of the same mineral character as the Carboniferous beds, while they also contain the same fossils.

At Largo a coarse volcanic ash lies unconformably on the ridges of the newest Carboniferous beds, and though no Permian sedimentary beds are known in this district, these volcanic beds, being post- Carboniferous, are considered to be of Permian age.

In East Fife some sixty necks can be seen, and the larger ones form conspicuous hills such as Largo Law.

In Devonshire lavas and volcanic ashes are found near Exeter, but, as in Ayrshire, there is no thick accumula­tion of the ashes. In Devon, however, the volcanic activity was far feebler than in Scotland.

The long story of Igneous activity in our islands now comes to an end for an enormous length of time. It is to be noticed that a region of vulcanicity frequently continues to be one throughout the Primary period. Thus in South Scotland we have volcanoes in Cambrian and Ordovician times, then came a rest during Silurian times, which was followed by a renewed outburst on a gigantic scale in the Old Red Sandstone and Carboniferous periods; this activity then became feebler, and though we do find evidences of volcanic action there in Permian times, the volcanic forces were evidently dying out.

After this for untold ages throughout the whole of the Secondary period came silence. No explosions rent the ground, no faulting caused earth­quakes, there was no ejectment of lava, the Jurassic period passed and was succeeded by the Cretaceous, and bed after bed was laid down over the greater part of our island. Time after time parts of the land were submerged or were upraised, but of all the enormous thickness of rock which accumulated not one inch is of volcanic origin.

Then we enter Tertiary times, and then once more the subterranean forces got to work and produced, effects on a grander scale than had ever before been witnessed in our island region.

The Tertiary Fissure-Eruptions:

The type of eruption which was characteristic of this Tertiary period of activity was the fissure type; very occasionally there was a small amount of crater building, as is proved by the occurrence of ashes, but the mass of igneous rock extruded was through almost innumerable fissures.

The tract of ground covered by these lavas was immense, and though the outpouring took place in Tertiary times the effect of marine and surface denudation has been so pronounced that the vast original plateau is now represented by mere scattered fragments above sea-level. The chief places where it is to be seen are in Antrim, in Staffa and Iona, in Mull and in Skye, but the plateau extended north­wards to the Faroe Islands, and the Iceland volcanoes of to-day are the sole remaining places where the eruptive forces still effect the extrusion of molten rock as they did  in days gone by.

The lava is of the type called a basalt, an almost black rock and one which frequently gives rise to a columnar structure, as is well seen in Staffa. This basalt rests on chalk in the Antrim district, but in Skye is found on much older rocks.

This basalt, which builds up huge thicknesses of rock in this north-western area, must not be imagined as coming out in one continuous flow. There were pauses between extrusions, and during those pauses rivers flowed over the plateaus, cut channels in them and produced beds formed of rolled blocks of the lava and of other rocks brought down from the hills; then more molten rock flowed out and buried these channels and their beds of conglomerate or gravel.

Thus we find in amongst the lavas water-formed beds, and some of them in Antrim and in the islands off the coast of Scotland have yielded plant remains, leaves and wood and a portion of a fresh-water fish.

After the building up of vast thicknesses of rock by repeated outwellings of lava there came a later period when igneous rock of a somewhat similar nature was intruded in sheets amongst the lavas. This rock withstands the action of the weather better than the softer basalts, and so as the basalts are now as a rule more or less horizontal, the two sets of rocks produce a very definite type of scenery.

The hills which have been carved out of them are flat-topped, and the sides are ringed with a series of horizontal scarps with almost vertical edges, these being formed by the hard intrusions. As the basalts on weathering form fertile ground, the slopes below the dark cliffs of intrusive rock are commonly covered by rich green grass.

Such scenery is well seen along the west coast of Skye and any one sailing up the west coast of Scotland will again and again see instances of it.

A consideration of the height to which the basalt plateau now rises enables one to see what an enormous amount of denudation has gone on since the Tertiary time, during which the lavas were poured out.

Originally the plateau extended from the island of Mull to the mainland to the north-east, where the promontories of Morven and Ardnamurchan are now seen. But between Mull and the mainland the Sound of Mull now exists twenty miles long and two miles broad. From the deepest part of the Sound to the top of the plateau in Mull is nearly 4000 feet, and the huge mass of basalt which used to occupy and cover the present Sound has been completely washed away.

Elsewhere the same story is told, and we must imagine that in Tertiary times a huge plateau of Basalt stretched from Antrim up between the present west coast of Scotland and the Hebrides, and beyond to the Faroe Islands.

From this general consideration of the history of British vulcanicity throughout past geological ages certain general facts become obvious.

The regions of vulcanicity in our islands all lie to the west of a line from the most south-easterly point of Scotland to Exeter. In this western region volcanic activity has persisted from the very earliest times of which we have any knowledge down to Tertiary times. Not only is this general persistence to be noted, but also the fact that particular portions of the volcanic region have been the sites of recrudescence of action again and again.

In the south-west of England we find volcanoes in Silurian, in Old Red Sandstone, in Carboniferous, and in Permian times.

In the south of Scotland we find plenty of evidence of vulcanicity all through the Ordovician period, in the Old Red Sandstone, in Carboniferous, and in Permian times, while there are also plenty of dykes there originally full of molten rock which rose up the cracks in the Tertiary period.

But not only are certain well-marked areas again and again the scenes of volcanic action, others are quite as con­spicuous for the absence of that action.

The Central Highlands of Scotland, though they abut on to the Old Red Sandstone, Carboniferous, and Permian volcanic areas to the south, themselves contain no traces of vents. The southern uplands, almost surrounded by volcanic rocks of various ages, are themselves perfectly free of any opening through which either lavas or ashes were sent out.

There seem to be, therefore, certain regions which are regions of weakness through which the volcanic forces were able to make openings to the surface, and these regions remained weak even though at one time or another those forces failed to continue ejecting matter through them.

Another point to be noticed is that we have no evidence of any slackening of the forces which produce volcanic action, and a further point is that the types of vulcanicity now known as existing at the present day have been seen again and again during the history of the world.

The volcanoes such as Vesuvius can be matched in the Primary period, during which, for instance, material in our English Lake District was accumulated to a depth of some 8000 feet.

The small ash-cones of the Neapolitan area are strikingly alike to the Tertiary cones of the Puy de Dôme district in France, and to the small cones of the Carboniferous period as seen in Scotland.

Lastly, the modern eruptions of Iceland resemble the Tertiary fissure-eruptions of North-east Ireland and West Scotland, and the Carboniferous fissure-eruptions of South Scotland.

Although the Tertiary outburst proves that as a whole the volcanic forces had by that time suffered no diminution, yet when one traces the history of volcanic action it is seen to have waxed and waned.

The enormous outpouring of lavas and ejection of ashes in Ordovician times was followed by an almost complete cessation of volcanic action in the following Silurian period. The succeeding Old Red Sandstone period gives evidence of a vulcanicity somewhat less pronounced than that of Ordo­vician times, and is succeeded by smaller outpourings in the Carboniferous period, while the following Permian vol­canoes are comparatively few and unimportant.

Then for the whole of the Secondary period there was a complete absence of any vulcanicity at all. Then in the older times of the Tertiary age came a period of great violence, which died down, and from that time to this the sound of a volcanic outburst has been unheard in our islands. But the history of our islands shows that from such a long spell of quiescence we cannot draw the con­clusion that those sounds will never be heard again.

The time may come when Scotland, or Wales, or the Midland district of England will be deluged with lava, or their lands desolated by downfalls of volcanic ash, when Glasgow or Birmingham may repeat the story of Pompeii, and future ages, by excavation at those places, may learn of the civilisa­tion of the twentieth century.

Essay # 6. Distribution of Volcanoes :

Almost all the active volcanoes are in the young folded mountain region, and the fault zones of Africa. There are more than 1000 volcanoes in the world and out of these about 500 are active or dormant.

These are distributed in three belts:

(b) Mid-World Mountain Belt:

This belt is extended from the Mediterranean Alps to the Himalayan region. Visuvius, Stromboli, Barren Island, Krakatoa, etc., are some of the best known volcanoes of this belt. There are 83 active volcanoes in this belt.

(c) African Rift Valley Belt:

This belt goes from Palestine in the north to Malagasy Island, through the East African region. The Kilimanjaro in Tanzania is a well-known dormant volcano of this belt.

Over and above these, some scattered volcanoes occur in some islands of the Pacific, Atlantic and Indian Oceans.

Essay # 7. Effects of Volcanic Activity :

Like earthquake, volcanic activity also changes the features of the surface of the earth. These changes are both destructive and constructive.

Great calamities take place due to volcanic activity. Often volcanic eruptions bury many beautiful towns. For example, Harculium and Pompii, the two beautiful Italian towns were completely buried by the erupted materials of Visuvius. Volcanic eruptions may be violent if it originates under sea. It causes strong waves which are highly destructive to life and settlements of coastal regions.

On the other hand, volcanoes may have some good effects. It sometimes creates extensive basaltic plateaus. Generally, the volcanic region is rich in minerals and the volcanic soils are fertile.

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Updated: 29 March, 2024

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• A vent, hill or mountain from which molten or hot rocks with gaseous material have been ejected
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The Philippines sits on a unique tectonic setting ideal to volcanism and earthquake activity. It is situated at the boundaries of two tectonic plates – the Philippine Sea Plate and the Eurasian plate –  both of which subduct or dive beneath the archipelago along the deep trenches along its east and west seaboard.

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Eruption Summary, Characters and Themes

“ Eruption ” is a gripping action thriller co-authored by Michael Crichton and James Patterson. The novel follows the dramatic events surrounding the eruption of Hawaii’s Mauna Loa volcano, which threatens not just the island but the entire planet. 

At the heart of the story is John “Mac” MacGregor, a determined volcanologist, and General Mark Rivers, a strategic military leader . Together, they must navigate a volatile mix of natural disaster, scientific conflict, and human ego to prevent a catastrophic environmental apocalypse caused by a deadly chemical weapon hidden deep within the island’s core.

In March 2016, a disturbing discovery is made at the Hilo Botanical Gardens on Hawaii’s Big Island. Biologist Rachel Sherrill notices an unusual phenomenon: several trees have mysteriously blackened. Her ex-boyfriend, who works at a nearby military base, is called in, leading to the arrival of soldiers equipped with hazmat suits. 

They begin treating the area urgently, signaling that something far more dangerous than a natural event is unfolding.

Fast forward to 2025, and John “Mac” MacGregor, a seasoned volcanologist, is enjoying a serene day of surfing with local boys in Hilo when he receives a call that changes everything. Jenny Kimura, his lab director, informs him that seismic activity beneath Mauna Loa is intensifying, pointing to an imminent eruption. 

Despite the growing concerns, Mac reassures the public that Hilo isn’t in immediate danger. 

However, behind closed doors, he convenes with his team to explore the possibility of venting the volcano with explosives to prevent a catastrophic eruption. Initially skeptical, Mac’s doubts grow after reading a classified government report dismissing this approach.

Mac’s world is further upended when he receives an urgent message from a retired general hospitalized in Honolulu. He learns about a top-secret military facility, the “Ice Tube,” where lethal containers of nuclear waste mixed with a defoliant known as Agent Black are stored. 

If the lava from Mauna Loa reaches this cache, it could trigger an environmental disaster that would obliterate all plant life on Earth.

With time running out, Mac and his team devise a high-risk plan to strategically detonate explosives, creating a controlled eruption that would divert the lava flow away from the Ice Tube. 

Rebecca Cruz, a demolition expert, and her team are brought in to execute the perilous task. Simultaneously, Hawaii’s Civil Defense Agency, led by Henry “Tako” Takayama, enlists the help of celebrity volcanologists Oliver and Leah Cutler, hoping their fame will lend credibility to the disaster response.

As tensions rise, billionaire J.P. Brett enters the fray, funding the Cutlers and stirring up media frenzy by predicting an apocalyptic “Big One.” This reckless fearmongering, coupled with a minor Agent Black leak that claims several lives, sends the island into chaos. 

General Rivers, tasked with maintaining order, imposes martial law, which only further inflames public panic.

Amid the growing unrest, Mac and his colleagues face devastating setbacks, including the tragic deaths of two scientists sent to the Galápagos for research. As Mauna Loa finally erupts, the situation becomes increasingly dire. 

The Cutlers and Brett, attempting to capture the eruption’s fury on film, meet a fiery end in a plane crash. Meanwhile, Mac and Rebecca narrowly escape a deadly lava flow.

In a desperate final attempt to save the island and the world, Mac boards a bomber plane with a seasoned pilot, aiming to drop ordnance on the advancing lava. 

When the bombs fail to deploy, all seems lost—until a natural barrier redirects the flow, averting disaster at the last possible moment.

In the aftermath, the military swiftly removes the remaining hazardous material, and Mac, weary but relieved, decides to leave Hawaii with Rebecca for a quieter life in Houston. 

The Ice Tube, and the nightmare it represented, is buried so deep it becomes nothing more than a dark chapter in history .

John “Mac” MacGregor

John “Mac” MacGregor is the central protagonist of Eruption , a seasoned volcanologist who embodies the quintessential traits of a hero in an action thriller. His deep understanding of volcanic activity and his commitment to protecting the island of Hawai‘i make him a pivotal figure in the narrative.

Mac is portrayed as a man of science , deeply rooted in rationality and skepticism, which is evident when he initially doubts the plan to vent the volcano with explosives. Despite his reservations, Mac is open to considering unconventional solutions, reflecting his adaptability in the face of crisis.

His relationship with Rebecca Cruz, which develops as they work closely together, adds a humanizing dimension to his character. This relationship shows that even in the midst of chaos, personal connections are crucial.

Mac’s journey is marked by a transformation from a solitary scientist to a leader who must navigate the complexities of collaboration and the weight of responsibility in a life-or-death situation.

General Mark Rivers

General Mark Rivers, the Chairman of the Joint Chiefs, is a commanding presence in the story. His role is critical not only for military coordination but also for the decision-making that impacts the lives of thousands on the island.

Rivers is a figure of authority who understands the gravity of the situation, and his decision to declare martial law underscores his willingness to take drastic measures to protect the greater good. Despite his stern exterior, Rivers is shown to be pragmatic and somewhat paternal, especially in his interactions with Mac and the other scientists.

He is the one who brings together the diverse team of experts, demonstrating his ability to recognize and utilize the strengths of others. However, Rivers also embodies the tension between military power and civilian freedom, as his actions lead to public unrest.

His character illustrates the challenges of leadership during a crisis, where every decision can have profound consequences.

Rebecca Cruz

Rebecca Cruz, the leader of the Cruz Demolition team, is a strong and capable female character who plays a crucial role in the effort to prevent the eruption’s catastrophic consequences. Her expertise in demolition is essential to the plan of creating controlled explosions to divert the lava flow, positioning her as a key player in the narrative.

Rebecca is depicted as a no-nonsense professional who commands respect from her team and peers. Her relationship with Mac evolves from professional collaboration to a deeper personal connection, providing a subplot of romance that adds emotional depth to the story.

Rebecca’s character is a blend of toughness and vulnerability. She is not only a skilled expert in her field but also someone who navigates the emotional and physical dangers of the mission with resilience.

Her involvement highlights the importance of specialized knowledge and teamwork in dealing with unprecedented disasters.

J. P. Brett

J. P. Brett is the billionaire benefactor of the celebrity volcanologists, Oliver and Leah Cutler, and serves as the antagonist in the story. His character represents the reckless and self-serving attitudes often associated with wealth and power.

Brett’s actions are driven by his desire for fame and sensationalism, which leads to catastrophic consequences, such as sparking public panic and contributing to the spread of the deadly Agent Black. His decision to go up in a helicopter to film the volcanic activity, despite the obvious dangers, showcases his hubris and disregard for the safety of others.

Brett’s death in the eruption is emblematic of the destructive nature of his arrogance and greed. Through Brett, the narrative explores the dangers of prioritizing personal gain over collective safety and the potential consequences of such recklessness in the face of natural disasters.

Oliver and Leah Cutler

Oliver and Leah Cutler are the celebrity volcanologists brought into the situation by Henry “Tako” Takayama, the head of the Civil Defense Agency in Hawai‘i. The Cutlers are portrayed as fame-seeking scientists whose main concern is their public image and the sensationalism of the disaster, rather than the well-being of the people on the island.

Their involvement in the story serves as a counterpoint to Mac’s more grounded and ethical approach to science. The Cutlers’ actions, particularly Oliver’s inflammatory comments to the media, exacerbate the situation by inciting public fear and mistrust.

Their tragic end, dying in the volcanic eruption, serves as a cautionary tale about the dangers of putting ego and fame above responsible scientific conduct. The Cutlers’ characters underscore the theme of ethical responsibility in science, highlighting the potential for harm when that responsibility is neglected.

Henry “Tako” Takayama

Henry “Tako” Takayama is the head of the Civil Defense Agency in Hawai‘i and plays a significant role in the initial response to the volcanic threat. Takayama’s decision to bring in the Cutlers, motivated by his desire to strengthen his position in the crisis management efforts, reflects the political maneuvering that can occur in disaster situations.

Takayama’s character is not as deeply explored as the others, but he represents the bureaucratic challenges and the complexities of coordinating a response to a disaster involving multiple stakeholders with differing agendas. His character adds a layer of realism to the story, showcasing how even well-intentioned decisions can have unintended and sometimes disastrous consequences.

Briggs is the Army colonel who introduces Mac to the Ice Tube and the potential global catastrophe that could result from the eruption of Mauna Loa. As a representative of the military, Briggs is a crucial link between the scientific community and the armed forces.

He provides Mac with the critical information needed to understand the full scope of the threat posed by the volcanic eruption. Briggs’s character is largely functional, serving to advance the plot by bringing Mac into the fold of the military’s secret operations.

However, his role also highlights the theme of secrecy and the moral dilemmas that arise when dangerous information is kept hidden from the public. Through Briggs, the story touches on the ethical implications of military involvement in scientific crises and the tension between national security and public transparency.

Rachel Sherrill

Rachel Sherrill, the biologist who first notices the strange behavior of the trees in the Hilo Botanical Gardens, serves as the initial trigger for the story’s unfolding events. Her discovery of the blackened trees, which are later linked to the toxic Agent Black, ties the narrative back to the environmental consequences of human actions.

Rachel’s character, though not central to the main plot, symbolizes the interconnectedness of the natural world and the impact that even small observations can have on larger events. Her involvement also underscores the importance of vigilance and the role of scientists in detecting and responding to environmental threats.

Rachel’s character sets the stage for the larger crisis that unfolds, showing how seemingly isolated incidents can escalate into global catastrophes.

The Fragility of Human Control Over Nature and Technological Hubris

Eruption delves deeply into the theme of human arrogance in the face of nature’s overwhelming power, highlighting the precariousness of humanity’s attempts to control or manipulate natural forces. The novel presents the eruption of Mauna Loa as a symbol of the untamable forces of nature, juxtaposed against the human tendency to believe that technology and scientific prowess can mitigate or even dominate these forces.

The characters, particularly Mac and General Rivers, embody this struggle, as they endeavor to use explosives to redirect lava flows and protect the Ice Tube—a secret cache of a devastating chemical weapon. However, the narrative underscores the limits of human control, as the eruption’s unpredictable behavior thwarts their plans, leading to catastrophic consequences.

The eventual redirection of the lava by a natural barrier rather than human intervention serves as a potent reminder that nature operates on a scale and logic beyond human comprehension. This renders technological hubris not just futile but potentially disastrous.

The Ethical Ambiguities of Scientific and Military Collaboration in Crisis

The novel also explores the moral complexities that arise when science and the military intersect during a crisis, particularly when the stakes involve potentially apocalyptic outcomes. The collaboration between Mac’s scientific team and the military, led by General Rivers, highlights the ethical dilemmas inherent in such partnerships.

The decision to keep the existence of Agent Black—a chemical weapon with the potential to destroy all plant life on Earth—hidden from the public raises questions about the ethics of secrecy in the name of national security. Furthermore, the characters must navigate the thin line between necessary actions and moral compromises, as they resort to extreme measures like martial law and the potential use of explosives on a volcanic fault line.

The narrative suggests that in such high-stakes situations, the boundaries of ethical behavior become blurred. The urgency to prevent a global catastrophe justifies actions that would otherwise be considered reprehensible.

The Collision of Personal Ambition and Collective Survival in Crisis Management

Eruption portrays the destructive potential of individual ambition when it clashes with the need for collective action in the face of disaster. The novel introduces characters like the celebrity volcanologists, Oliver and Leah Cutler, and the billionaire J. P. Brett, whose personal ambitions exacerbate the crisis rather than mitigate it.

Their desire for fame and recognition leads them to undermine the efforts of those genuinely trying to avert disaster, such as Mac and General Rivers. This theme reflects the broader societal tension between self-interest and the common good, particularly in crisis situations where unity and cooperation are essential.

The Cutlers’ sensationalist claims about “the Big One” and Brett’s reckless pursuit of dramatic footage directly contribute to public panic and further complicate the efforts to manage the volcanic eruption. The novel ultimately suggests that in crises of this magnitude, personal ambition not only jeopardizes individual lives but can also threaten the survival of humanity as a whole.

The Intersection of Environmental Catastrophe and Human-Induced Technological Disaster

At its core, Eruption is a meditation on the interplay between natural disasters and the catastrophic potential of human-made technologies. The Mauna Loa eruption is not merely a natural event but becomes a potential global catastrophe due to the presence of the Ice Tube, a repository of Agent Black.

This intersection of environmental and technological threats amplifies the stakes, illustrating how human interventions can exacerbate the destructive power of natural phenomena. The narrative suggests that while humanity has developed technologies capable of incredible destruction, it has not necessarily developed the wisdom or foresight to manage these technologies responsibly.

The presence of Agent Black beneath the volcano symbolizes the latent dangers of humanity’s scientific advancements, which, if not carefully controlled, can lead to unintended and far-reaching consequences. The novel thus serves as a cautionary tale about the double-edged sword of technological progress, which holds the potential for both great benefit and unimaginable harm.

The Psychological and Social Impact of Disaster on Human Communities

Eruption also delves into the psychological and social ramifications of impending disaster on the affected communities. The narrative captures the escalating fear and paranoia among the residents of Hawai‘i as they grapple with the dual threats of the volcanic eruption and the potential release of Agent Black.

The declaration of martial law by General Rivers, the town hall meetings, and the media’s role in spreading panic all illustrate the complex social dynamics that emerge in crisis situations. The novel portrays how fear can lead to social fragmentation, as individuals and groups prioritize their survival over collective action.

This social breakdown is further exacerbated by the conflicting agendas of the various stakeholders involved, from the scientists to the military, to the media. The psychological toll of the disaster is evident in the behaviors of the characters, who must navigate not only the physical dangers of the eruption but also the mental strain of living under constant threat.

Eruption thus highlights the broader theme of how disasters, whether natural or man-made, can disrupt social cohesion and test the resilience of human communities.