Plate Tectonics

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Plate tectonics

Plate tectonics, is the theory explaining geologic changes that result from the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of Earth's mantle). Plates move and shift their positions relative to one another. Movement of and contact between plates either directly or indirectly accounts for most of the major geologic features at Earth's surface.

The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Moreover, plate tectonic theory is so robust in its ability to explain and predict geological processes that it is equivalent in many regards to the fundamental and unifying principles of evolution in biology , and nucleosynthesis in physics and chemistry .

Continental drift versus plate tectonics

Based upon centuries of cartographic depictions that allowed a good fit between the Western coast of Africa and the Eastern coast of South America , in 1858, French geographer Antonio Snider-Pellegrini, published a work asserting that the two continents had once been part of larger single continent ruptured by the creation and intervention of the Atlantic Ocean. In the 1920s, German geophysicist Alfred Wegener's writings advanced the hypothesis of continental drift depicting the movement of continents through an underlying oceanic crust. Wegner's hypothesis met with wide skepticism but found support and development in the work and writings of South African geologist Alexander Du Toit who discovered a similarity in the fossils found on the coasts of Africa and South Americas that derived from a common source.

What Wegener's continental drift theory lacked was a propelling mechanism. Other scientists wanted to know what was moving these continents around. Unfortunately, Wegener could not provide a convincing answer. Therefore, other scientists heavily disputed his theory and it fell into disrepute.

The technological advances necessitated by the Second World War made possible the accumulation of significant evidence now underlying modern plate tectonic theory.

The theory of plate tectonics gained widespread acceptance only in the late 1960s to early 1970s.

An overview of tectonic theory

Plate tectonic theory asserts that Earth is divided into core, mantle, and crust. The crust is subdivided into oceanic and continental crust. The oceanic crust is thin (3–4.3 mi [5–7 km]), basaltic (<50% SiO2), dense, and young (<250 million years old). In contrast, the continental crust is thick (18.6–40 mi [30–65 km]), granitic (>60% SiO2), light, and old (250–3,700 million years old). The outer crust is further subdivided by the subdivision of the lithosperic plates, of which it is a part, into 13 major plates. These lithospheric plates, composed of crust and the outer layer of the mantle, contain a varying combination of oceanic and continental crust. The lithospheric plates move on top of mantle's athenosphere.

Boundaries are adjacent areas where plates meet. Divergent boundaries are areas under tension where plates are pushed apart by magma upwelling from the mantle. Collision boundaries are sites of compression either resulting in subduction (where lithospheric plates are driven down and destroyed in the molten mantle) or in crustal uplifting that results in orogeny (mountain building). At transform boundaries, exemplified by the San Andreas fault , the continents create a shearing force as they move laterally past one another.

New oceanic crust is created at divergent boundaries that are sites of sea-floor spreading. Because Earth remains roughly the same size, there must be a concurrent destruction or uplifting of crust so that the net area of crust remains the same. Accordingly, as crust is created at divergent boundaries, oceanic crust must be destroyed in areas of subduction underneath the lighter continental crust. The net area is also preserved by continental crust uplift that occurs when less dense continental crust collides with continental crust. Because both continental crusts resist subduction, the momentum of collision causes an uplift of crust, forming mountain chains. A vivid example of this type of collision is found in the ongoing collision of India with Asia that has resulted in the Himalayan mountains that continue to increase in height each year. This dynamic theory of plate tectonics also explained the formation of island arcs formed by rising material at sites where oceanic crust subducts under oceanic crust, the formation of mountain chains where oceanic crust subducts under continental crust (e.g., Andes mountains), and volcanic arcs in the Pacific. The evidence for deep, hot, convective currents combined with plate movement (and concurrent continental drift) also explained the mid-plate "hot spot" formation of volcanic island chains (e.g., Hawaiian islands) and the formation of rift valleys (e.g., Rift Valley of Africa). Mid-plate earthquakes, such as the powerful New Madrid earthquake in the United States in 1811, are explained by interplate pressures that bend plates much like a piece of sheet metal pressed from opposite sides.

Proofs of tectonic theory

As with continental drift theory two of the proofs of plate tectonics are based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas (e.g., between West Africa and the eastern coast of South America).

Ocean topography also provided evidence of plate tectonic theory. Nineteenth century surveys of the oceans indicated that rather than being flat featureless plains, as was previously thought, some ocean areas are mountainous while others plummet to great depths. Contemporary geologic thinking could not easily explain these topographic variations, or "oceanscapes." Surveys in the 1950s and 1960s provided an even more detailed picture of the ocean bottom. Long, continuous mountain chains appeared, as well as numerous ocean deeps shaped like troughs. Geoscientists later identified the mountainous features as the mid-oceanic ridges (MORs) where new plates form, and the deep ocean trenches as subduction zones where plates descend into the subsurface.

Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies

that measure the reflection of seismic waves off features in Earth's interior . Different materials transmit and reflect seismic shock waves in different ways, and of particular importance to theory of plate tectonics is the fact that liquid does not transmit a particular form of seismic wave known as an S wave. Because the mantle transmits S-waves, it was long thought to be a cooling solid mass . Geologists later discovered that radioactive decay provided a heat source with Earth's interior that made the athenosphere plasticine (semi-solid). Although solid-like with regard to transmission of seismic S-waves, the athenosphere contains very low velocity (inches per year) currents of mafic (magma-like) molten materials.

Another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites (slivers of oceanic floor with fossils) found in upper levels of mountain chains. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted by plate tectonic theory.

As methods of dating improved, one of the most conclusive lines of evidence in support of plate tectonics derived from the dating of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary (e.g., Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centered on the divergent boundary. More importantly, dating studies show that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and precision in rock dating also showed that rock specimen taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an area subsequently separated by movement of lithospheric plates.

Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.

The line of evidence, however, that firmly convinced modern geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries. Tremendously persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation of the rocks to known changes in Earth's magnetic field as predicted by electromagnetic theory. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents.

Paleomagnetic studies are based upon the fact that some hot igneous rocks (formed from volcanic magma) contain varying amounts of ferromagnetic minerals (e.g., Fe3O4) that magnetically orient to the prevailing magnetic field of Earth at the time they cool. Geophysical and electromagnetic theory provides clear and convincing evidence of multiple polar reversals or polar flips throughout the course of Earth's history. Where rock formations are uniform—i.e., not grossly disrupted by other geological processes—the magnetic orientation of magnetite-bearing rocks can also be used to determine the approximate latitude the rocks were at when they cooled and took on their particular magnetic orientation. Rocks with a different orientation to the current orientation of the Earth's magnetic field also produce disturbances or unexpected readings (anomalies) when scientists attempt to measure the magnetic field over a particular area.

This overwhelming support for plate tectonics came in the 1960s in the wake of the demonstration of the existence of symmetrical, equidistant magnetic anomalies centered on the Mid-Atlantic Ridge. During magnetic surveys of the deep ocean basins, geologists found areas where numerous magnetic reversals occur in the ocean crust. These look like stripes, oriented roughly parallel to one another and to the MORs. When surveys were run on the other side of the MORs, they showed that the magnetic reversal patterns were remarkably similar on both sides of the MORs. After much debate, scientists concluded that new ocean crust must form at the MORs, recording the current magnetic orientation. This new ocean crust pushes older crust out of the way, away from the MOR. When a magnetic reversal occurs, new ocean crust faithfully records it as a reversed magnetic "stripe" on both sides of the MOR. Older magnetic reversals were likewise recorded; these stripes are now located farther from the MOR.

Geologists were comfortable in accepting these magnetic anomalies located on the sea floor as evidence of sea floor spreading because they were able to correlate these anomalies with equidistant radially distributed magnetic anomalies associated with outflows of lava from land-based volcanoes.

Additional evidence continued to support a growing acceptance of tectonic theory. In addition to increased energy demands requiring enhanced exploration, during the 1950s there was an extensive effort, partly for military reasons related to what was to become an increasing reliance on submarines as a nuclear deterrent force, to map the ocean floor. These studies revealed the prominent undersea ridges with undersea rift valleys that ultimately were understood to be divergent plate boundaries. An ever-growing network of seismic reporting stations, also spurred by the Cold War need to monitor atomic testing, provided substantial data that these areas of divergence were tectonically active sites highly prone to earthquakes. Maps of the global distribution of earthquakes readily identified stressed plate boundaries. Earthquake experts recognized an interesting pattern of earthquake distribution. Most major earthquakes occur in belts rather than being randomly distributed around Earth. Most volcanoes exhibit a similar pattern. This pattern later served as evidence for the location of plate margins, that is, the zones of contact between different crustal plates. Earthquakes result from friction caused by one plate moving against another.

Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels.

In his important 1960 publication, "History of Ocean Basins," geologist and U.S. Navy Admiral Harry Hess (1906–1969) provided the missing explanatory mechanism for plate tectonic theory by suggesting that the thermal convection currents in the athenosphere provided the driving force behind plate movements. Subsequent to Hess's book, geologists Drummond Matthews (1931–1997) and Fred Vine (1939–1988) at Cambridge University used magnetometer readings previously collected to correlate the paired bands of varying magnetism and anomalies located on either side of divergent boundaries. Vine and Matthews realized that magnetic data reveling strips of polar reversals symmetrically displaced about a divergent boundary confirmed Hess's assertions regarding seafloor spreading.

In the 1960s ocean research ships began drilling into the sediments and the solid rock below the sediment, called bedrock , in the deeper parts of the ocean. Perhaps the most striking discovery was the great age difference between the oldest continental bedrock and the oldest oceanic bedrock. Continental bedrock is over a billion years old in many areas of the continents, with a maximum age of 3.6 billion years. Nowhere is the ocean crust older than 180 million years.

Marine geologists discovered another curious relationship as well. The age of the oceanic bedrock and the sediments directly above it increase as you move from the deep ocean basins to the continental margins. That is, the ocean floor is oldest next to the continents and youngest near the center of ocean basins. In addition, ocean crust on opposing sides of MORs show the same pattern of increasing age away from the MORs.

The great age of continental rocks results from their inability to be subducted. Once formed, continental crust becomes a permanent part of Earth's surface. We also know that the increase in age of ocean crust away from ocean basins results from creation of new sea floor at the MORs, with destruction of older sea floor at ocean trenches, which are often located near continental margins.

Plate movement an today be measured by sophisticated GPS and laser-based measuring systems. A much slower but certainly more spectacular proof of plate movement is exemplified by the still-ongoing formation of the Hawaiian Islands. The Pacific plate is moving north over a stationary lava source in the mantle, known as a hot spot . Lava rises upwards from this hot spot to the surface and forms a volcano . After a few million years, that volcano becomes extinct as it moves north, away from the hot spot, and a new volcano begins to form to the south. A new volcano is forming today on the ocean floor south of the island of Hawaii.

Rates of plate movement

Plates move at rates of about an inch (a few centimeters) per year. Scientists first estimated the rate of plate movement based on radiometric dating of ocean crust. By determining the age of a crustal sample, and knowing its distance from the MOR at which it formed, they estimate the rate of new ocean floor production and plate movement. Today, satellites capable of measurement of plate motion provide a more direct method. Results from these two methods agree fairly closely. The fastest plates move more than 4 in (10 cm) per year. The rate of motion of the North American plate averages 1.2 in (3 cm) per year.

Scale and number of plates

Estimates of the number of plates differ, but most geologists recognize at least fifteen and some as many as twenty. These plates have many different shapes and sizes. Some, such as the Juan de Fuca plate off the west coast of Washington State, have surface areas of a few thousand square miles. The largest, the Pacific plate, underlies most of the Pacific Ocean and covers an area of hundreds of thousands of square miles. In the distant geologic past, Earth's lithosphere perhaps consisted of many more of these smaller plates, rather than the comparatively few, larger plates now present.

Plate interactions

Tectonic plates can interact in one of three ways. They can move toward one another, or converge; move away from one another, or diverge; or slide past one another, a movement known as transform motion. All plate margins along which plate movement is occurring have one thing in common—earthquakes. In fact, most earthquakes happen along plate margins. The other types of activity that occur when two plates interact are dependent on the nature of the plate interaction and of the margins. Plate margins (or boundaries) come in three varieties: oceanic-oceanic, continental-continental, and continental-oceanic.

Oceanic-oceanic plates

Recall that plates in continental areas are thicker and less dense than in oceanic areas. When two oceanic plates converge (an oceanic-oceanic convergent margin) one of the plates subducts into a trench. The subducted plate sinks downward into the mantle where it begins to melt. Molten rock from the melting plate rises toward the surface and forms a chain of volcanic islands, or a volcanic island arc, behind the ocean trench. Subduction of the Pacific plate below the North American plate along the coast of Alaska formed the Aleutian Trench and the Aleutian Islands, a volcanic island arc. At oceanic-oceanic divergent margins, sea floor spreading occurs and the ocean slowly grows wider. Today, Europe and North America move about 3 in (7.6 cm) farther apart every year as the Atlantic Ocean grows wider.

Continental-continental plates

Due to their lower density and greater thickness, continental-continental convergent plate margins act quite differently than oceanic-oceanic margins. Continental crust is too light to be carried downward into a trench. At continental-continental convergent margins neither plate subducts. The two plates converge, buckle, fold , and fault to form complex mountains ranges of great height. Continental-continental convergence produced the Himalayas when the Indian-Australian plate collided with the Eurasian plate.

Continental-continental divergence causes a continent to separate into two or more smaller continents when it is ripped apart along a series of fractures. The forces of divergence literally tear a continent apart as the two or more blocks of continental crust begin slowly moving apart and magma pushes into the rift formed between them. Eventually, if the process of continental rifting continues (it may fail, leaving the continent fractured but whole), a new sea is born between the two continents. In this way rifting between the Arabian and African plates formed the Red Sea.

Continental-oceanic plates

When continental and oceanic plates converge, the scenario is a predictable one. Due to its greater density, the oceanic plate easily subducts below the edge of the continental plate. Again subduction of the oceanic plate leads to volcano formation, but in this setting, the chain of volcanoes forms on the continental crust. This volcanic mountain chain, known as a volcanic arc, is usually several hundred miles inland from the plate margin. The Andes Mountains of South America and the Cascade Mountains of North America are examples of volcanic arcs formed by subduction along a continental-oceanic convergent margin. Continental-oceanic convergence may form a prominent trench, but not always. No continental-oceanic divergent margins exist today. As you can imagine, they are unlikely to form and would quickly become oceanic-oceanic divergent margins as sea floor spreading occurred.

Transform margins

In addition to convergence and divergence, transform motion may occur along plate margins. Transform margins, in many ways, are less spectacular than convergent and divergent ones, and the type of plates involved is really of no significance. Along transform margins, about all that occurs are faults and earthquakes. Plate movement produces the earthquakes, as the two rock slabs slide past one another. The best known example of a transform plate margin is the San Andreas fault in California, where the Pacific and North American plates are in contact.

Continent formation

If sea floor spreading only produces basaltic (oceanic) rock, where did the continents come from? Knowledge of the processes involved is somewhat limited, but formation of the early continents resulted from subduction at oceanic-oceanic convergent margins. When plates subduct, a process known as partial melting occurs. Partial melting of mafic rock results in the production of magma that is more felsic in composition; that is, it has a composition intermediate between basalt and granite. In addition, weathering of mafic rock at the earth's surface also produces sediments with a more felsic composition. When these sediments subduct, they yield magma of felsic composition via partial melting.

Repeated episodes of subduction and partial melting, followed by volcanic eruption, produced lavas of increasingly felsic composition. Finally, this cycle formed volcanic island arcs that were too buoyant to be subducted and became a permanent part of Earth's surface. When sea floor spreading pushes one of these buoyant volcanic island arcs toward a subduction zone, rather than subducting, it welds, or accretes, onto the side of the volcanic island arc forming on the other side of the trench. Over time, these microcontinents, through accretion, formed larger continental masses.

Continents "float" on the plastic material making up the mantle like a block of wood floats on water . As erosion occurs, sediments are carried from mountains and higher elevations out to sea, where they accumulate on the continental shelf , forming wedges of sediment. Such accretionary wedges can extend far out to sea, depending on the size and shape of the continental shelf. As erosion moves sediments from the interior of the continent to the edges, the continent gets thinner but its surface area becomes larger. If conditions remain stable, accretionary wedges can go on accumulating for a very long time, reaching hundreds of miles out into the ocean. Sometimes, the wedge becomes so thick it rises above sea level to become dry land.

Continents have either passive or active margins. Passive margins are found where the continent's edge is on the same plate as the adjacent ocean, and it is along passive margins that accretionary wedges form. Active margins are found where the continent and the bordering oceanic crust are on separate plates. In these situations, a subduction zone is usually present. In general, the continents bordering the Atlantic Ocean have passive margins, while those surrounding the Pacific Ocean, which has a very active MOR, have active margins.

Driving mechanism

Most geologists believe convective cells in the earth's interior are the driving force for plate motion. If you have ever seen a rapidly boiling pot of water, then you know about convection cells. In the center of the pot, bubbles rise to the surface and push water to the sides. Along the sides, the water cools and descends back down to the bottom of the pot to be heated again.

In a similar way, convection cells in the mantle bring molten rock to the surface along MORs where it forms new ocean crust. Below the crust, pressure is exerted on the bottom of the plates by the convection cell , helping to push the plates along, and causing divergence. At the trenches, the cells may also exert a downward force on the descending plates, helping to pull them down into the mantle.

Importance of plate tectonics

Plate tectonics revolutionized the way geologists view Earth. This new paradigm brings together nearly all the divisions of geologic study. Like the theory of evolution in biology, plate tectonics is the unifying concept of geology . Plate tectonics' initial appeal and rapid acceptance resulted from its ability to provide answers to many nagging questions about a variety of seemingly unrelated phenomena. Plate tectonics also revitalized the field of geology by providing a new perspective from which to interpret many old ideas. Finally, plate tectonics explains nearly all of Earth's major surface features and activities. These include faults and earthquakes, volcanoes and volcanism, mountains and mountain building, and even the origin of the continents and ocean basins.

Resources

books

Hancock, P.L., and B.J. Skinner, eds. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

Tarbuck, Edward. D., Frederick K. Lutgens, and Tasa Dennis. Earth: An Introduction to Physical Geology. 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.

Winchester, Simon. The Map That Changed the World: WilliamSmith and the Birth of Modern Geology. New York: Harper Collins, 2001.

periodicals

Buffett, Bruce A. "Earth's Core and the Geodynamo." Science (June 16, 2000): 2007–2012.

Hellfrich, George, and Bernard Wood. "The Earth's Mantle." Nature (August 2, 2001): 501–507.

other

United States Department of the Interior, U.S. Geological Survey. "This Dynamic Earth: The Story of Plate Tectonics." February 21, 2002 [cited March 11, 2003]. <http://pubs.usgs.gov/publications/text/dynamic.html>.

K. Lee Lerner

Clay Harris

KEY TERMS

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Accretion: —The addition of sediment or rock to a plate's margin at a subduction zone. Material is scraped off the subducting plate and adheres to the edge of the overriding plate.
Basalt: —A dense, dark colored igneous rock, with a composition rich in iron and magnesium (a mafic composition).
Convection cells: —The circular movement of a fluid in response to alternating heating and cooling. Convection cells in the earth's interior involve molten rock that rises upwards below midoceanic ridges.
Convergence: —The movement of two plate margins toward one another; usually associated with plate subduction or the collision of two continents.
Crust: —The outermost layer of the earth, situated over the mantle and divided into continental and oceanic crust.
Divergence: —The separation of two plate margins as they move in opposing directions; usually associated with either sea floor spreading or continental rifting.
Granite: —A light-colored igneous rock that is less dense than basalt due to an abundance of lighter elements, such as silicon and oxygen (a felsic composition).
Hot spots: —Areas in the mantle, associated with rising plumes of molten rock, which produce frequent, localized volcanic eruptions at Earth's surface.
Magnetic reversals: —Periods during which the earth's magnetic poles flip-flop; that is, the orientation of Earth's magnetic field reverses. During these periods of reversed magnetism, compass needles point toward the south pole.
Mantle: —The thick, dense layer of rock that underlies Earth's crust.
Microcontinents: —Volcanic islands of intermediate to felsic composition that were too buoyant to subduct, and therefore formed the first continental crust.
Mid-oceanic ridges: —Continuous submarine mountain ranges, composed of basalt, where new sea floor is created.
Ocean trench: —A deep depression in the sea floor, created by an oceanic plate being forced downward into the subsurface by another, overriding plate.
Plates: —Large regions of the earth's surface, composed of the crust and uppermost mantle, which move about, forming many of Earth's major geologic surface features.
Sea-floor spreading: —The part of plate tectonics that describes the movement of the edges of two of the plates forming Earth's crust away from each other under the ocean. Sea-floor spreading results in the formation of new submarine surfaces.
Subduction: —In plate tectonics, the movement of one plate down into the mantle where the rock melts and becomes magma source material for new rock.
Transform motion: —Horizontal plate movement in which one plate margin slides past another.