plate motion essay

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Chapter 10 Plate Tectonics

10.4 Plates, Plate Motions, and Plate-Boundary Processes

Continental drift and sea-floor spreading became widely accepted around 1965 as more and more geologists started thinking in these terms. By the end of 1967, Earth’s surface had been mapped into a series of plates (Figure 10.16). The major plates are Eurasia, Pacific, India, Australia, North America, South America, Africa, and Antarctic. There are also numerous small plates (e.g., Juan de Fuca, Nazca, Scotia, Philippine, Caribbean), and many very small plates or sub-plates. For example the Juan de Fuca Plate is actually three separate plates (Gorda, Juan de Fuca, and Explorer) that all move in the same general direction but at slightly different rates.

Rates of motions of the major plates range from less than 1 cm/y to over 10 cm/y. The Pacific Plate is the fastest at over 10 cm/y in some areas, followed by the Australian and Nazca Plates. The North American Plate is one of the slowest, averaging around 1 cm/y in the south up to almost 4 cm/y in the north.

Plates move as rigid bodies, so it may seem surprising that the North American Plate can be moving at different rates in different places. The explanation is that plates move in a rotational manner. The North American Plate, for example, rotates counter-clockwise; the Eurasian Plate rotates clockwise.

Boundaries between the plates are of three types: divergent (i.e., moving apart) , convergent (i.e., moving together), and transform (moving side by side). Before we talk about processes at plate boundaries, it’s important to point out that there are never gaps between plates. The plates are made up of crust and the lithospheric part of the mantle (Figure 10.17), and even though they are moving all the time, and in different directions, there is never a significant amount of space between them. Plates are thought to move along the lithosphere-asthenosphere boundary, as the asthenosphere is the zone of partial melting. It is assumed that the relative lack of strength of the partial melting zone facilitates the sliding of the lithospheric plates.

At spreading centres, the lithospheric mantle may be very thin because the upward convective motion of hot mantle material generates temperatures that are too high for the existence of a significant thickness of rigid lithosphere (Figure 10.12). The fact that the plates include both crustal material and lithospheric mantle material makes it possible for a single plate to be made up of both oceanic and continental crust. For example, the North American Plate includes most of North America, plus half of the northern Atlantic Ocean. Similarly the South American Plate extends across the western part of the southern Atlantic Ocean, while the European and African plates each include part of the eastern Atlantic Ocean. The Pacific Plate is almost entirely oceanic, but it does include the part of California west of the San Andreas Fault.

Divergent Boundaries

Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface (Figure 10.18). The triangular zone of partial melting near the ridge crest is approximately 60 km thick and the proportion of magma is about 10% of the rock volume, thus producing crust that is about 6 km thick. Most divergent boundaries are located at the oceanic ridges (although some are on land), and the crustal material created at a spreading boundary is always oceanic in character; in other words, it is mafic igneous rock (e.g., basalt or gabbro, rich in ferromagnesian minerals). Spreading rates vary considerably, from 1 cm/y to 3 cm/y in the Atlantic, to between 6 cm/y and 10 cm/y in the Pacific. Some of the processes taking place in this setting include:

• Magma from the mantle pushing up to fill the voids left by divergence of the two plates
• Pillow lavas forming where magma is pushed out into seawater (Figure 10.19)
• Vertical sheeted dykes intruding into cracks resulting from the spreading
• Magma cooling more slowly in the lower part of the new crust and forming gabbro bodies

Spreading is hypothesized to start within a continental area with up-warping or doming related to an underlying mantle plume or series of mantle plumes. The buoyancy of the mantle plume material creates a dome within the crust, causing it to fracture in a radial pattern, with three arms spaced at approximately 120° (Figure 10.20). When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley (such as the present-day Great Rift Valley in eastern Africa). It is suggested that this type of valley eventually develops into a linear sea (such as the present-day Red Sea), and finally into an ocean (such as the Atlantic). It is likely that as many as 20 mantle plumes, many of which still exist, were responsible for the initiation of the rifting of Pangea along what is now the mid-Atlantic ridge (see Figure 10.14).

Convergent Boundaries

Convergent boundaries, where two plates are moving toward each other, are of three types, depending on the type of crust present on either side of the boundary — oceanic or continental. The types are ocean-ocean, ocean-continent, and continent-continent.

At an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted , under the other. Often it is the older and colder plate that is denser and subducts beneath the younger and hotter plate. There is commonly an ocean trench along the boundary. The subducted lithosphere descends into the hot mantle at a relatively shallow angle close to the subduction zone, but at steeper angles farther down (up to about 45°). As discussed in the context of subduction-related volcanism in Chapter 4, the significant volume of water within the subducting material is released as the subducting crust is heated. This water is mostly derived from alteration of pyroxene and olivine to serpentine near the spreading ridge shortly after the rock’s formation. It mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust’s melting point and leads to the formation of magma (flux melting). The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. A mature island arc develops into a chain of relatively large islands (such as Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands.

As described above in the context of Benioff zones (Figure 10.10), earthquakes take place close to the boundary between the subducting crust and the overriding crust. The largest earthquakes occur near the surface where the subducting plate is still cold and strong.

Examples of ocean-ocean convergent zones are subduction of the Pacific Plate south of Alaska (Aleutian Islands) and west of the Philippines, subduction of the India Plate south of Indonesia, and subduction of the Atlantic Plate beneath the Caribbean Plate (Figure 10.21).

At an ocean-continent convergent boundary, the oceanic plate is pushed under the continental plate in the same manner as at an ocean-ocean boundary. Sediment that has accumulated on the continental slope is thrust up into an accretionary wedge, and compression leads to thrusting within the continental plate (Figure 10.22). The mafic magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes.

Examples of ocean-continent convergent boundaries are subduction of the Nazca Plate under South America (which has created the Andes Range) and subduction of the Juan de Fuca Plate under North America (creating the mountains Garibaldi, Baker, St. Helens, Rainier, Hood, and Shasta, collectively known as the Cascade Range).

A continent-continent collision occurs when a continent or large island that has been moved along with subducting oceanic crust collides with another continent (Figure 10.23). The colliding continental material will not be subducted because it is too light (i.e., because it is composed largely of light continental rocks [SIAL]), but the root of the oceanic plate will eventually break off and sink into the mantle. There is tremendous deformation of the pre-existing continental rocks, and creation of mountains from that rock, from any sediments that had accumulated along the shores (i.e., within geosynclines) of both continental masses, and commonly also from some ocean crust and upper mantle material.

Examples of continent-continent convergent boundaries are the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran. The Rocky Mountains in B.C. and Alberta are also a result of continent-continent collisions.

Transform boundaries exist where one plate slides past another without production or destruction of crustal material. As explained above, most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean plate boundaries (Figure 10.15). Some transform faults connect continental parts of plates. An example is the San Andreas Fault, which connects the southern end of the Juan de Fuca Ridge with the northern end of the East Pacific Rise (ridge) in the Gulf of California (Figures 10.24 an 10.25). The part of California west of the San Andreas Fault and all of Baja California are on the Pacific Plate. Transform faults do not just connect divergent boundaries. For example, the Queen Charlotte Fault connects the north end of the Juan de Fuca Ridge, starting at the north end of Vancouver Island, to the Aleutian subduction zone.

Exercise 10.4 A Different Type of Transform Fault

This map shows the Juan de Fuca (JDF) and Explorer Plates off the coast of Vancouver Island. We know that the JDF Plate is moving toward the North American Plate at around 4 cm/y to 5 cm/y. We think that the Explorer Plate is also moving east, but we don’t know the rate, and there is evidence that it is slower than the JDF Plate.

The boundary between the two plates is the Nootka Fault, which is the location of frequent small-to-medium earthquakes (up to magnitude ~5), as depicted by the red stars. Explain why the Nootka Fault is a transform fault, and show the relative sense of motion along the fault with two small arrows.

As originally described by Wegener in 1915, the present continents were once all part of a supercontinent, which he termed Pangea ( all land ). More recent studies of continental matchups and the magnetic ages of ocean-floor rocks have enabled us to reconstruct the history of the break-up of Pangea.

Pangea began to rift apart along a line between Africa and Asia and between North America and South America at around 200 Ma. During the same period, the Atlantic Ocean began to open up between northern Africa and North America, and India broke away from Antarctica. Between 200 and 150 Ma, rifting started between South America and Africa and between North America and Europe, and India moved north toward Asia. By 80 Ma, Africa had separated from South America, most of Europe had separated from North America, and India had separated from Antarctica. By 50 Ma, Australia had separated from Antarctic, and shortly after that, India collided with Asia. To see the timing of these processes for yourself go to: http://barabus.tru.ca/geol1031/plates.html .

Within the past few million years, rifting has taken place in the Gulf of Aden and the Red Sea, and also within the Gulf of California. Incipient rifting has begun along the Great Rift Valley of eastern Africa, extending from Ethiopia and Djibouti on the Gulf of Aden (Red Sea) all the way south to Malawi.

Over the next 50 million years, it is likely that there will be full development of the east African rift and creation of new ocean floor. Eventually Africa will split apart. There will also be continued northerly movement of Australia and Indonesia. The western part of California (including Los Angeles and part of San Francisco) will split away from the rest of North America, and eventually sail right by the west coast of Vancouver Island, en route to Alaska. Because the oceanic crust formed by spreading on the mid-Atlantic ridge is not currently being subducted (except in the Caribbean), the Atlantic Ocean is slowly getting bigger, and the Pacific Ocean is getting smaller. If this continues without changing for another couple hundred million years, we will be back to where we started, with one supercontinent.

Pangea, which existed from about 350 to 200 Ma, was not the first supercontinent. It was preceded by Pannotia (600 to 540 Ma), by Rodinia (1,100 to 750 Ma), and by others before that.

In 1966, Tuzo Wilson proposed that there has been a continuous series of cycles of continental rifting and collision; that is, break-up of supercontinents, drifting, collision, and formation of other supercontinents. At present, North and South America, Europe, and Africa are moving with their respective portions of the Atlantic Ocean. The eastern margins of North and South America and the western margins of Europe and Africa are called passive margins because there is no subduction taking place along them.

This situation may not continue for too much longer, however. As the Atlantic Ocean floor gets weighed down around its margins by great thickness of continental sediments (i.e., geosynclines), it will be pushed farther and farther into the mantle, and eventually the oceanic lithosphere may break away from the continental lithosphere (Figure 10.26). A subduction zone will develop, and the oceanic plate will begin to descend under the continent. Once this happens, the continents will no longer continue to move apart because the spreading at the mid-Atlantic ridge will be taken up by subduction. If spreading along the mid-Atlantic ridge continues to be slower than spreading within the Pacific Ocean, the Atlantic Ocean will start to close up, and eventually (in a 100 million years or more) North and South America will collide with Europe and Africa.

There is strong evidence around the margins of the Atlantic Ocean that this process has taken place before. The roots of ancient mountain belts, which are present along the eastern margin of North America, the western margin of Europe, and the northwestern margin of Africa, show that these land masses once collided with each other to form a mountain chain, possibly as big as the Himalayas. The apparent line of collision runs between Norway and Sweden, between Scotland and England, through Ireland, through Newfoundland, and the Maritimes, through the northeastern and eastern states, and across the northern end of Florida. When rifting of Pangea started at approximately 200 Ma, the fissuring was along a different line from the line of the earlier collision. This is why some of the mountain chains formed during the earlier collision can be traced from Europe to North America and from Europe to Africa.

That the Atlantic Ocean rift may have occurred in approximately the same place during two separate events several hundred million years apart is probably no coincidence. The series of hot spots that has been identified in the Atlantic Ocean may also have existed for several hundred million years, and thus may have contributed to rifting in roughly the same place on at least two separate occasions (Figure 10.27).

Exercise 10.5 Getting to K now the Plates and Their Boundaries

This map shows the boundaries between the major plates. Without referring to the plate map in Figure 10.16, or any other resources, write in the names of as many of the plates as you can. Start with the major plates, and then work on the smaller ones. Don’t worry if you can’t name them all.

Once you’ve named most of the plates, draw arrows to show the general plate motions. Finally, using a highlighter or coloured pencil, label as many of the boundaries as you can as divergent, convergent, or transform. [map by SE]

Physical Geology by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Understanding plate motions

Scientists now have a fairly good understanding of how the plates move and how such movements relate to earthquake activity. Most movement occurs along narrow zones between plates where the results of plate-tectonic forces are most evident.

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Plate tectonics is the grand, unifying theory of Earth sciences, combining the concepts of continental drift and sea-floor spreading into one holistic theory that explains many of the major structural features of the Earth's surface. It explains why the oceanic lithosphere is never older than about 180 Ma and why only the continents have preserved the Earth's geological record for the past 4000 Ma. It provides the framework to explain the distribution of earthquakes and volcanoes and a mechanism for the slow drift of the continents across the Earth's surface. The theory has now reached such a level of scientific acceptance that the movement of plates, both relative to one another and to the hot-spot reference frame, are being used to infer movement of the hot-spot reference frame with respect to the Earth's rotational axis.

Plate tectonics is an expression of the convective regime in the underlying mantle, but the link between individual convection cells and plate boundaries is not direct because plate boundaries are not fixed and, like the plates, move relative to one another. Plate movements are driven by gravity, largely by cold, dense lithospheric slabs pulling younger lithosphere towards a destructive boundary. A less-powerful driving force is generated by the potential energy of spreading centres, elevated some 2-3 km above the general level of the abyssal plains.

As ideas concerning plate tectonics have evolved since the 1970s, it has become apparent that while the theory can be applied rigorously to the oceans, the same cannot be said of the continents. Because of the strength and rigidity of oceanic plates, deformation is focused into narrow linear zones along plate margins. By contrast, when continental lithosphere approaches a plate boundary, deformation can extend hundreds of kilometres into the continental interior because continental plates are less strong. Such deformation gives rise to the major mountain belts of the Earth, as exemplified by the Alpine Himalayan Chain.

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DIY Science

Plate boundaries: divergent, convergent, and transform.

Movement in narrow zones along plate boundaries causes most earthquakes. Most seismic activity occurs at three types of plate boundaries—divergent, convergent, and transform.

As the plates move past each other, they sometimes get caught and pressure builds up. When the plates finally give and slip due to the increased pressure, energy is released as seismic waves, causing the ground to shake. This is an earthquake.

Some of the plates have ocean water above them. Other plates include continents, and some plates include both continents and ocean. The movements of the plates help shape the geological features of our planet. The three main types of plate movements include:

Divergent (Spreading) :This is where two plates move away from each other. Molten rock from the mantle erupts along the opening, forming new crust. The earthquakes that occur along these zones, called spreading centers, are relatively small. The Great Rift Valley in Africa, the Red Sea and the Gulf of Aden all formed as a result of divergent plate motion.

Convergent (Colliding) : This occurs when plates move towards each other and collide. When a continental plate meets an oceanic plate, the thinner, denser, and more flexible oceanic plate sinks beneath the thicker, more rigid continental plate. This is called subduction. Subduction causes deep ocean trenches to form, such as the one along the west coast of South America. The rocks pulled down under the continent begin to melt. Sometimes the molten rock rises to the surface, through the continent, forming a line of volcanoes. About 80% of earthquakes occur where plates are pushed together, called convergent boundaries.

Another form of convergent boundary is a collision where two continental plates meet head-on. Since neither plate is stronger than the other, they crumple and are pushed up. This can lead to the formation of huge, high mountain ranges such as the Himalayas.

When two tectonic plates slide past each other, the place where they meet is a transform or lateral fault . The San Andreas Fault is one of the best examples of lateral plate motion.

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ENCYCLOPEDIC ENTRY

Plate boundaries.

Earth’s tectonic plates fit together in a jigsaw puzzle of plate boundaries.

Earth Science, Geology

Tectonic Plate Map

The movement of Earth's tectonic plates shape the planet's surface. This three-dimensional image shows a map of Earth's tectonic plates.

Image by Naeblys

Learning materials

• National Geographic MapMaker: Plate Boundaries

In some ways, Earth resembles a giant jigsaw puzzle. That is because its outer surface is composed of about 20 tectonic plates , enormous sections of Earth’s crust that roughly fit together and meet at places called plate boundaries. Plate boundaries are important because they are often associated with earthquakes and volcanoes . When Earth’s tectonic plates grind past one another, enormous amounts of energy can be released in the form of earthquakes . Volcanoes are also often found near plate boundaries because molten rock from deep within Earth—called magma—can travel upward at these intersections between plates . There are many different types of plate boundaries. For example, sections of Earth’s crust can come together and collide (a “convergent” plate boundary), spread apart (a “divergent” plate boundary), or slide past one another (a “transform” plate boundary). Each of these types of plate boundaries is associated with different geological features. Typically, a convergent plate boundary —such as the one between the Indian Plate and the Eurasian Plate —forms towering mountain ranges, like the Himalaya, as Earth’s crust is crumpled and pushed upward. In some cases, however, a convergent plate boundary can result in one tectonic plate diving underneath another. This process, called “subduction,” involves an older, denser tectonic plate being forced deep into the planet underneath a younger, less-dense tectonic plate . When this process occurs in the ocean, an trench "> ocean trench can form. These trenches are some of the deepest places in the ocean, and they are often the sites of strong earthquakes . When subduction occurs, a chain of volcanoes often develops near the convergent plate boundary . One such chain of volcanoes can be found on the western coast of the United States, spanning across the states of California, Oregon, and Washington. A divergent plate boundary often forms a mountain chain known as a ridge. This feature forms as magma escapes into the space between the spreading tectonic plates . One example of a ridge is the Mid-Atlantic Ridge, an undersea chain of mountains that formed as two pairs of tectonic plates spread apart: the North American Plate and the Eurasian Plate in the north, and the South American Plate and the African Plate in the south. Because ocean ridges are found underwater, often at great depths, they can be hard to study. In fact, scientists know more about the surfaces of some of the other planets in our solar system than they do about ocean ridges. A transform plate boundary occurs when two plates slide past each other, horizontally. A well-known transform plate boundary is the San Andreas Fault , which is responsible for many of California’s earthquakes . A single tectonic plate can have multiple types of plate boundaries with the other plates that surround it. For instance, the Pacific Plate , one of Earth’s largest tectonic plates , includes convergent, divergent, and transform plate boundaries.

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Related Resources

Introduction

Advance preparation and materials needed, geology learning outcomes, lab procedures, part i. four magnetic profiles, part 2. magnetic isochrons around the juan de fuca ridge.

• Measuring Distances
• Measuring Ages

Part 3. The Hawaii-Emperor Seamount Chain and Pacific Plate Motion

• The Hawaii-Emperor Chain of Volcanoes (Map & Diagram)
• Table of Data from the Hawaii-Emperor Chain

The outer layer of the earth consists of tectonic plates, each of which is moving in its own direction. How can we determine what direction a plate is moving and the speed at which it is moving? There are several way that this can be measured, including analyzing patterns of magnetic age markers recorded in rocks on the ocean floor, measuring ages and distances of volcanoes from a hot spot fixed beneath a moving plate, and making high precision GPS measurements of a plate to monitor its current speed and direction.

In this lab, you will use some of the same data that led scientists to discover how tectonic plates move and how to measure their rates and directions of motion. The data includes magnetic patterns on the ocean floor, the timeline of reversals of earth’s magnetic field, and ages of volcanic rocks. By combining these data with your ability to measure, plot, and analyze data using maps and graphs, you will be able to determine for yourself the directions and speeds at which the tectonic plates have been moving.

• Read this page completely and think through the questions posed in each section. They are specifically designed to prepare you for completing the lab worksheet itself. The worksheet contains a different set of more detailed questions that you will answer and turn in.
• Read the Plate Tectonics Basics page. It will acquaint you with many concepts in plate tectonic theory and contains a specific section on Paleomagnetism that will assist you in completing this lab.
• A ruler with a cm scale, a pencil, a calculator, and a sheet of graph paper.
• If you know how to produce scientifc graphs using a spreadsheet, you have the option of using a spreadsheet instead of a calculator and graph paper.

By performing and completing this lab, you will progress toward the following learning objectives for this course:

• Think, behave, and communicate scientifically by showing results of measurements and calculations with units correctly included in the mathematics and results and numbers rounded appropriately at the end, by making graphs of measured plate motions that are properly depicted and labeled, and by answering essay questions about how plates move based on your results.
• Show and explain how the earth is constantly active and always changing by measuring rates and directions of plate motion.
• Put geologic events in order and determine the age of geologic materials by analyzing ages of different parts of the ocean floor.
• Interpret spatial and quantitative information with maps and diagrams by analyzing plate tectonics using maps of magnetic anomalies on the ocean floor.
• Use measurements, numbers, calculations and graphs to derive meaning from the earth by graphing your measurements of plate motion.

To complete this lab, you will conduct the following types of observations, measurements or calculations, following the Calculations and Graphs standards

• Read, draw, and interpret maps of the earth.
• Examine, construct, and interpret diagrams, flow charts, and tables of information.
• Construct or examine graphs to analyze correlations between types of data.

In more detail, the measurements and calculations you will conduct for this lab include:

• see and measure patterns of magnetism on the ocean floor
• correlate the age and spacing of volcanoes formed at a hot spot over which a plate has been moving
• calculate rates of motion by dividing distances by times (ages)
• analyze rates of motion by graphing distances vs. times (ages)
• calculate the distance of crust of a given age from where it originated
• calculate the age of crust from its distance from where it originated.

There are three parts to this lab. For background, go to the Basics page on Plate Tectonics/Paleomagnetism .

Part 1 studies spreading of plates from a mid-ocean ridge. When completing the worksheet you will analyze magnetic profiles of the ocean floor. The profiles were measured by a ship that crossed the ridge in four places. By drawing lines on the map that join the magnetic anomalies together from one profile to another, you will see if the magnetic anomaly patterns make parallel stripes on each side of the mid-ocean ridge. These magnetic stripes parallel to the spreading ridge are known as isochrons.

Part 2 studies the spreading and faulting of plates from a mid-ocean ridge in more detail. When completing the worksheet you will interpret a map of magnetic patterns on the ocean floor. The map was compiled from dozens of magnetic profiles like those in Part I, measured from oceanographic research ships that traversed the area.

Part 3 studies the motion of a plate across a hot spot. When completing the worksheet you will analyze and graph the age and distance of volcanoes from the hot spot. This is another way of measuring the speed and direction of plate motion, independent of the magnetic profile method you will use in Parts I and II.

Refer to the Lab Assignments Grading Rubric for a reminder of what constitutes a well-performed lab.

Click on the thumbnail above for a larger version of the image that opens in a new window. This diagram is Figure 17.8 from Jones and Jones, Laboratory Manual for Physical Geology , McGraw Hill Higher Education, 2003; used with permission.

The diagram shows magnetic measurements in four profiles conducted in the mid-1960s across the Pacific-Antarctic Ridge. The data were gathered using an instrument called a magnetometer, which was towed behind the ship to measure the magnetism of the sea floor. The ship made four passes across the Pacific-Antarctic Ridge, at right angles to the ridge. These four magnetic profiles are labeled A, B, C, and D in Figure 17.8.

• The dashed line marks the crest, or axis, of the ridge.
• The positive parts (peaks) of the magnetic profiles are where the rocks are magnetized pointing to magnetic north in the same direction as magnetic north is today.
• The negative values (valleys) of the magnetic profiles are where the rocks are magnetized pointing to magnetic north, in the opposite direction from where magnetic north is today.
• Each peak or valley on the magnetic profiles marks a magnetic isochron. (See the term, magnetic isochron, in context in the Paleomagnetism section of the Plate Tectonics Basics page.)

The theory of sea floor spreading is part of plate tectonics theory. The theory of seafloor spreading predicts that the magnetic profiles on either side of a spreading ridge should be mirror images, symmetric and equally spaced. In addition, seafloor spreading theory predicts that each magnetic isochron should form a line parallel to the ridge (unless it has subsequently been offset along faults). You will investigate these predictions.

See if you can determine for yourself whether each magnetic profile shows a mirror image of magnetic isochrons on either side of the ridge.

Once you have identified the magnetic isochrons on either side of the ridge, you can use their spacing and ages to calculate the velocities of sea floor spreading. A rate of motion, also known as speed, is ound by dividing the distance something has moved by the length of time it took to move that far. The magnetic isochrons by definition have known ages, so the distance of an isochron from a ridge divided by the age of the isochron tells you the speed at which the plate has been spreading away from the ridge.

Look at the colored map of magnetic isochrons on the ocean floor, off the coast of the Pacific Northwest. This image comes from Kious and Tilling, This Dynamic Earth: The Story of Plate Tectonics , U.S. Geologial Survey, 2009. The thick dashed line is the crest of the Juan de Fuca Ridge. North is to the top, west is to the left, east is to the right. For a map scale, one cm (centimeter) equals approximately 66 km on a printout of the bigger copy.

• The colored areas are where the ocean floor has normal magnetism, with north in about the same direction as it is now.
• The white stripes between the colored stripes are areas where the ocean floor has reversed magnetism, with north and south switched around.

These data were gathered by a magnetometer towed behind a ship. Enough data were gathered to allow the magnetic isochrons to be mapped out as color contours.

The vertical column that shows age of oceanic crust in millions of years is a timeline of magnetic isochrons going back more than 10 million years. Matching an isochron on the ocean floor with its position on the timeline tells you the age of that part of the ocean floor. How precisely can you measure ages with the magnetic isochron timeline on the map - to the nearest million year or to the nearest 0.1 million year?

Oceanic crust gets its magnetism from the earth's magnetic field. As new oceanic crust forms from solidifying lava, magnetic minerals suspended in the liquid mixture line up like compasses. Then, once the new crust has become solid rock, the magnetism is frozen in permanently. It acts as a record of which way magnetic north was on earth at the time the oceanic crust formed.

The magnetic isochrons are not in their original configuration. Faults in the earth's crust have disrupted and offset the isochrons. The faults show up on the map as straight, solid back lines offsetting the colored magnetic strips. Because these are real-world data the magnetic isochrons do not form perfectly parallel stripes on either side of the ridge.

However, allowing for disruption along faults, can you see the magnetic isochrons forming a mirror image of stripes on either side of the ridge?

MEASURING DISTANCES

• The line of measurement comes out sideways from (at right angles to) the ridge crest.
• The line of measurement crosses the magnetic isochron stripes sideways (as close to right angles as possible).
• The line of measurement does not cross any faults.

MEASURING AGES

To fully analyze the Juan de Fuca magnetic isochron map you also need to determine the age of selected magnetic reversals. To determine isochron ages on this map, you can use the colors of the isochrons and the vertical column showing the age of oceanic crust. For example, when did the most recent dark green anomaly come to an end? On the timeline, it ended near, but not exactly at 8 Ma. Note that the beginning and end of each magnetic isochron is a magnetic reversal. Was the reversal that occurred at end the most recent dark green isochron a switch for normal to reverse magnetism, or reverse to normal to reverse? Even if the isochrons were not colored, they could be distingushed by matching the spacings of the dark and white pattern they form.

THE HAWAII-EMPEROR CHAIN OF VOLCANOES (MAP & DIAGRAM)

The Hawaii-Emperor chain of seamounts (volcanoes resting on the ocean floor) begins at the active, present-day location of the hot spot at the southeast side of the Big Island of Hawaii. (For more on hot spots, see the Terrane Accretion section of the Plate Tectonics Basics page.) From there the chain stretches west and north across the Pacific Ocean floor to the Aleutian trench near the Kamchatka Peninsula. There are about 110 individual volcanoes in the Hawaii-Emperor chain, which is about 6000 km (3800 miles) long altogether. The Hawaii-Emperor chain is divided into two segments, the WNW-trending Hawaiian chain and the N-trending Emperor chain. The two chains meet at a prominent bend, around the underwater seamounts Daikakuji and Yuryaku.

The active end (youngest end) of the Hawaii-Emperor chain is at the Big Island of Hawaii and the offshore, still underwater volcano Loihi just off the southeast coast of the Big Island. Kilauea volcano on the Big Island is active today, and other centers on the Big Island and on Maui have erupted recently. As one progresses towards the west-northwest, the volcanoes of the Hawaiian Islands get progressively older (see data table). Once active volcano building through eruptions of lava ceases, the erosional forces of tropical weathering, landslides, river erosion, and wave action overcome the island, and erodes it down to sea level (second figure). The extinct volcano evolves to a flat-topped mesa ringed by coral reefs, and then to an atoll with nothing but the circular reef showing. Finally the volcano sinks beneath the waves, and becomes an underwater seamount.

The Hawaii-Emperor chain is a classic example of a hot spot track. The standard explanation begins with a hot spot whose source of magma is rooted deep in the earth's mantle, below the tectonic plates. The hot spot magma source is thought to be fixed in the deeper mantle. Above a hot spot, a slab of oceanic crust and uppermost mantle - a tectonic plate - moves laterally above the hot spot. As the Pacific Plate moves over the Hawaiian hot spot, magma punches up through the Pacific Plate, creating an active volcano. Plate motion carries the active volcano away from the magma source, the volcano goes extinct, and a new volcano grows over the hot spot. As the extinct volcano is carried farther and farther from the hot spot source, the volcano sinks beneath the waves mostly due to aging and cooling of the ocean crust underneath the extinct volcano; this cooling causes subsidence of the ocean floor.

Hot spot tracks are very important geologic features for determining both the direction and speed of the plate upon which the seamounts rest. The direction of plate motion is given by the orientation of the chain of seamounts and volcanoes. The plate moves towards the older volcanoes. As can be seen in the first figure, the Pacific Plate moved almost due north during "Emperor time" (from 75 to 42 million years ago), and then changed direction about 42 million years ago (the age of the volcanoes at the bend in the chain), to move west-northwest during "Hawaii time" (from 42 Ma to the present). Note that the azimuths cited here assume no rotation of the Pacific Plate during the last 75 Ma.

Hot spot tracks also give the speed of plate motion, if the length of the chains of volcanoes and seamounts, and the ages of the volcanoes and seamounts are known (see data table below). Plates typically move about 1-15 cm/year, which is equivalent to 10-150 km/Ma (kilometers per million years). Human fingernails grow at the low end of the rates of plate motion, a few cm/yr. This may seem rather rather slow to us, but is very fast on the geological time scale. The earth is 4.55 billion years old; one million years is a brief moment in earth time. The speed of the North American plate (for example) is fairly typical for plates, about 6 cm/year, whereas the Marianas plate is one of the fastest (today), moving about 13 cm/year.

TABLE OF DATA FROM THE HAWAII-EMPEROR CHAIN

The data table below lists distances from the active Kilauea volcanic center (measured parallel to the Hawaii-Emperor chain) and ages of each volcano and seamount (Clague and Dalrymple, 1989). These data have been compiled from a wide variety of sources and researchers, which can introduce uncertainties.

In scientific data, uncertainty is not the same as being unsure. In scientific data, uncertainty is how much allowance must be made for imprecision and inconsistency in the measurements. By tracking and reporting sources of uncertainty, the interpretation of the data is made more valid, not less.

There are several sources of uncertainty in the data that go beyond the ± values in the ages and distances listed in the table. Different geochronologic laboratories determined the ages listed for the volcanic rocks from the Hawaii-Emperor chain, and different labs use different machines, different standards, and different analytical techniques. Furthermore, volcanoes do not have a single age; a typical Hawaiian volcano builds up over several hundred thousand years. Who is to say that the volcanic rocks dredged up from the underwater seamount Jingu (for example) are representative of all of Jingu's eruptive history? It is very difficult to sample Jingu's older rocks; they are covered by the young lavas.

Uncertainty in the distance measurements is compounded by the use of individual volcanoes as proxies for the hot spot location. Several volcanoes, spaced tens of km apart, may have been simultaneously active over the hot spot. The hot spot is not a point, it is a zone of volcanism over 100 km in diameter at the earth's surface.

How could you use the ages of the volcanic rocks and their distances from the hot spot, as listed in the table above, to determine the rate at which the volcanoes have been moving away from the hot spot?

According to the map of the Hawaii Emperor Chain of Volcanoes , which direction has the Pacific plate been moving during the formation of the Hawaiian Chain? Which direction was it moving during the formation of the Emperor Chain? What could account for the bend - the change in direction?

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4.5: Divergent Plate Boundaries

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Divergent boundaries are spreading boundaries, where new oceanic crust is created to fill in the space as the plates move apart. Most divergent boundaries are located along mid-ocean oceanic ridges (although some are on land). The mid-ocean ridge system is a giant undersea mountain range, and is the largest geological feature on Earth; at 65,000 km long and about 1000 km wide, it covers 23% of Earth’s surface (Figure \(\PageIndex{1}\)). Because the new crust formed at the plate boundary is warmer than the surrounding crust, it has a lower density so it sits higher on the mantle, creating the mountain chain. Running down the middle of the mid-ocean ridge is a rift valley 25-50 km wide and 1 km deep. Although oceanic spreading ridges appear to be curved features on Earth’s surface, in fact the ridges are composed of a series of straight-line segments, offset at intervals by faults perpendicular to the ridge, called transform faults . These transform faults make the mid-ocean ridge system look like a giant zipper on the seafloor (Figure \(\PageIndex{2}\)). As we will see in section 4.7, movements along transform faults between two adjacent ridge segments are responsible for many earthquakes.

The crustal material created at a spreading boundary is always oceanic in character; in other words, it is igneous rock (e.g., basalt or gabbro, rich in ferromagnesian minerals), forming from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface (Figure \(\PageIndex{3}\)). The triangular zone of partial melting near the ridge crest is approximately 60 km thick and the proportion of magma is about 10% of the rock volume, thus producing crust that is about 6 km thick. This magma oozes out onto the seafloor to form pillow basalts, breccias (fragmented basaltic rock), and flows, interbedded in some cases with limestone or chert. Over time, the igneous rock of the oceanic crust gets covered with layers of sediment, which eventually become sedimentary rock.

Spreading is hypothesized to start within a continental area with up-warping or doming of crust related to an underlying mantle plume or series of mantle plumes. The buoyancy of the mantle plume material creates a dome within the crust, causing it to fracture. When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley (such as the present-day Great Rift Valley in eastern Africa). It is suggested that this type of valley eventually develops into a linear sea (such as the present-day Red Sea), and finally into an ocean (such as the Atlantic). It is likely that as many as 20 mantle plumes, many of which still exist, were responsible for the initiation of the rifting of Pangaea along what is now the mid-Atlantic ridge.

There are multiple lines of evidence demonstrating that new oceanic crust is forming at these seafloor spreading centers:

1. Age of the crust :

Comparing the ages of the oceanic crust near a mid-ocean ridge shows that the crust is youngest right at the spreading center, and gets progressively older as you move away from the divergent boundary in either direction, aging approximately 1 million years for every 20-40 km from the ridge. Furthermore, the pattern of crust age is fairly symmetrical on either side of the ridge (Figure \(\PageIndex{4}\)).

The oldest oceanic crust is around 280 Ma in the eastern Mediterranean, and the oldest parts of the open ocean are around 180 Ma on either side of the north Atlantic. It may be surprising, considering that parts of the continental crust are close to 4,000 Ma old, that the oldest seafloor is less than 300 Ma. Of course, the reason for this is that all seafloor older than that has been either subducted (see section 4.6) or pushed up to become part of the continental crust. As one would expect, the oceanic crust is very young near the spreading ridges (Figure \(\PageIndex{4}\)), and there are obvious differences in the rate of sea-floor spreading along different ridges. The ridges in the Pacific and southeastern Indian Oceans have wide age bands, indicating rapid spreading (approaching 10 cm/year on each side in some areas), while those in the Atlantic and western Indian Oceans are spreading much more slowly (less than 2 cm/year on each side in some areas).

2. Sediment thickness:

With the development of seismic reflection sounding (similar to echo sounding described in section 1.4) it became possible to see through the seafloor sediments and map the bedrock topography and crustal thickness. Hence sediment thicknesses could be mapped, and it was soon discovered that although the sediments were up to several thousands of meters thick near the continents, they were relatively thin — or even non-existent — in the ocean ridge areas (Figure \(\PageIndex{5}\)). This makes sense when combined with the data on the age of the oceanic crust; the farther from the spreading center the older the crust, the longer it has had to accumulate sediment, and the thicker the sediment layer. Additionally, the bottom layers of sediment are older the farther you get from the ridge, indicating that they were deposited on the crust long ago when the crust was first formed at the ridge.

3. Heat flow:

Measurements of rates of heat flow through the ocean floor revealed that the rates are higher than average (about 8x higher) along the ridges, and lower than average in the trench areas (about 1/20th of the average). The areas of high heat flow are correlated with upward convection of hot mantle material as new crust is formed, and the areas of low heat flow are correlated with downward convection at subduction zones.

4. Magnetic reversals:

In section 4.2 we saw that rocks could retain magnetic information that they acquired when they were formed. However, Earth’s magnetic field is not stable over geological time. For reasons that are not completely understood, the magnetic field decays periodically and then becomes re-established. When it does re-establish, it may be oriented the way it was before the decay, or it may be oriented with the reversed polarity. During periods of reversed polarity, a compass would point south instead of north. Over the past 250 Ma, there have a few hundred magnetic field reversals, and their timing has been anything but regular. The shortest ones that geologists have been able to define lasted only a few thousand years, and the longest one was more than 30 million years, during the Cretaceous (Figure \(\PageIndex{6}\)). The present “normal” event has persisted for about 780,000 years.

Beginning in the 1950s, scientists started using magnetometer readings when studying ocean floor topography. The first comprehensive magnetic data set was compiled in 1958 for an area off the coast of British Columbia and Washington State. This survey revealed a mysterious pattern of alternating stripes of low and high magnetic intensity in sea-floor rocks (Figure \(\PageIndex{7}\)). Subsequent studies elsewhere in the ocean also observed these magnetic anomalies, and most importantly, the fact that the magnetic patterns are symmetrical with respect to ocean ridges. In the 1960s, in what would become known as the Vine-Matthews-Morley (VMM) hypothesis, it was proposed that the patterns associated with ridges were related to the magnetic reversals, and that oceanic crust created from cooling basalt during a normal event would have polarity aligned with the present magnetic field, and thus would produce a positive anomaly (a black stripe on the sea-floor magnetic map), whereas oceanic crust created during a reversed event would have polarity opposite to the present field and thus would produce a negative magnetic anomaly (a white stripe). The widths of the anomalies varied according to the spreading rates characteristic of the different ridges. This process is illustrated in Figure \(\PageIndex{8}\). New crust is formed (panel a) and takes on the existing normal magnetic polarity. Over time, as the plates continue to diverge, the magnetic polarity reverses, and new crust formed at the ridge now takes on the reversed polarity (white stripes in Figure \(\PageIndex{8}\)). In panel b, the poles have reverted to normal, so once again the new crust shows normal polarity before moving away from the ridge. Eventually, this creates a series of parallel, alternating bands of reversals, symmetrical around the spreading center (panel c).

Plate Tectonics Theory and Features Essay

Introduction, location of converging plates, location of diverging plates, the location where tectonic plates slide along each other.

The earth’s surface is composed of several plates that are in a constant state of motion. The plates are subjected to tectonic forces that compel them to move in different directions (Garrison, 2009). Generally, there are three major forces that prompt movement of plates on the earth’s crust.

These include transformational, divergent and convergent forces. Convergent forces push the earth’s plates towards each other leading to crashing effect. Transformational forces push the earth’s plates alongside each other while divergent forces tend to pull the plates apart (Gubbins, 1990).

Different features are formed depending on the kind of movements involved. Empirical studies conducted by geologists reveal that these movements result in geological features the earth’s surface. Resultant features include earthquakes, volcanic eruptions and mountain formation (Garrison, 2009).

When plates are pushed towards each other, they are eventually deformed. Additionally, certain features such as mountains and island arcs are generated (Gubbins, 1990). However, resultant features depend on which plates converge.

For instance, when two continental plates drift toward each other, they form a mountain as opposed to the case when continental and an oceanic plate converges. In this case, the two different plates form an island where the heavier continental plate lands on the oceanic plate (Garrison, 2009).

For instance, where there are volcanic eruptions during the convergence of the plates, magma solidifies on top of the oceanic crust to form a hard surface on the ocean. An example of a place commonly known for this incidence is American continent whereby the South American plate converged with Nazca plate to form Andes Mountain which is located in Chain (Garrison, 2009).

Moreover, it is evident that Tibet plateau in Asia was formed as a result of convergent tectonic forces. The feature resulted when the Indian plate slowly converged with the Asian plate. The oceanic crust between the two continental plates was submerged under them (Gubbins, 1990). Further evidence reveals that more convergence between the oceanic plates resulted into formation of Himalayas Mountain.

Divergent tectonic forces triggered plates to pull away from each other resulting into formation of a new continental crust (Garrison, 2009. Mostly, this occurs where two oceanic plates diverge giving rise to a continental crust being formed at the middle (Cox & Hart, 1986).

A good example of features resulting from divergence includes the Ethiopian highlands in Eastern Africa and ridges formed in the mid-Atlantic. It is evident that due to tectonic forces, the Atlantic Ocean is widening slowly at an approximated length of 2 cm in a year (Garrison, 2009).

In most cases, earth’s plates slip sideways alongside each other. This causes friction; thus there is an accumulation of intense pressure. There is a sudden release of the pressure being built up resulting in earth tremors (Cox & Hart, 1986). Though this transformational activity does not lead to the formation of spectacular features, faulting is a common occurrence.

For example, the San Andreas Fault was formed as a result of lateral slipping of earth plates. As mentioned earlier, earthquakes have been experienced in San Francisco during the 19th century (Garrison, 2009). These were believed to have been triggered by faulting in San Andreas. To date, there have been subsequent occurrences of earthquakes in the zone since the incidence.

To recap it all, it is imperative to reiterate that plate tectonic theory attempts to expound why certain conspicuous features are observed on the earth’s surface. Such features include volcanoes, faults, islands, ridges, plateaus, an earthquake.

It is also evident that the earth’s plates are usually subjected to three tectonic forces namely divergence, convergence and transformational forces. Through these forces, the plates tend to move in different directions thus resulting in different features. Scientists have for a long time, associated geological activities on the earth’s surface with movements in tectonic plates.

Cox, A. & Hart, R. (1986). Plate tectonics: how it works . California: Blackwell Publishing, Inc.

Garrison, T. (2009). Essentials of Oceanography . California: Cengage Learning.

Gubbins, D. (1990). Seismology and plate tectonics . New York: Cambridge University Press.

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