The important geologic theory of plate tectonics (tecton - "to build") asserts that the Earth's surface is composed of about seven major blocks, or plates, and many more minor plates that move across the Earth's surface at a very slow rate. Although the plates creep along at just centimeters per year, this movement still causes stress and strain in the Earth's crustal structure. These shifting plates cause faults, mountains, trenches, mid-oceanic ridges and rifts, and even ocean basins Some movements may result in violent earthquakes and volcanic eruptions.
Understanding plate tectonics requires knowledge of the Earth's interior, which can be subdivided based on either structure or composition. The traditional subdivisions of crust, mantle and core are primarily compositional. Subdivision based on structure results in an upper layer known as the lithosphere, that is hard and brittle. It consists of the crust and a small part of the upper mantle. The lithosphere is not laterally continuous, but rather is broken up into large segments, called plates. Below the lithospheric plates lies the asthenosphere, a weak layer of plastic rock that probably contains a small amount (1-5%) of magma, or molten rock. It is believed that the lithosphere moves along on top of the asthenosphere; the plastic nature of the asthenosphere is probably what makes this possible. The exact cause of this movement is not entirely understood, but the concept of plate tectonics has proven to be the most fruitful theory in geology, and possibly one of the most in all of science.
The paradigm shift that stimulated the plate tectonic revolution began in the late 1950s to early 1960s and is linked to the theories of continental drift and seafloor spreading. During this time, there was a crisis in geology, due to the development of larger and larger amounts of unexplained and seemingly unexplainable evidence of mobility in the Earth's crust. As more and more researchers examined the available data, a new view of the Earth slowly began to materialize. However, this did not happen overnight, rather it came together in bits and pieces. Finally, in the late 1960s, a number of synthesis papers were published that combined the various new geologic concepts, such as seafloor spreading and a somewhat modified version of the theory of continental drift, into one theory--plate tectonics. Consequently, no one scientist can be credited with the theory; rather, it was a collaborative effort that continues today.
The theory of continental drift is most closely associated with Alfred Lothar Wegener, a German scientist who used geologic similarities of various continents to suggest that these continental areas had once been joined together. For example, he showed that certain glacial deposits, fossils, rocks, and other geologic structures appeared alike on opposite sides of the Atlantic Ocean. In his 1915 book The Origin of Continents and Oceans, Wegener postulated that, prior to 200 million years ago, all the continents were part of one landmass, or supercontinent, which he called Pangaea, or "all lands". He believed that in the last 200 million years, the continents broke apart and "drifted" to their current positions. Probably the main reason Wegener's continental drift theory failed to be more widely accepted was that it lacked a viable driving mechanism. However, Wegener was not without his supporters. For example, in the 1930s Arthur Holmes (1890-1965), a British geologist, suggested that upwelling mantle material might be causing land masses, or crust, to shift. However, this idea could not be tested until the development of new technologies, such as sonar, during and after World War II, allowed for more detailed studies of the seafloor. Unfortunately, Wegener died in 1930, long before plate tectonics arrived on the scene.
In the 1940s and 1950s, extensive seafloor surveys were undertaken by researchers such as Harry Hammond Hess, an American geologist. These surveys revealed a variety of seafloor features, including mid-ocean ridges that he thought could be areas where mantle material wells up to form new seafloor. Hess' hypothesis was termed seafloor spreading. This proposal was later confirmed by other researchers through evidence from earthquake, volcano, and heat-flow patterns discovered around the ridges. The distribution of ridges and deep ocean trenches provided additional details of plate boundaries.
Oceanographers and scientists have identified seven major plates: the North American Plate, Pacific Plate, South American Plate, Antarctic Plate, African Plate, Eurasian Plate, and Indian-Australian Plate. The latter probably is in the process of sub-dividing into two separate plates, and may, in fact, be two distinctly separate slabs of lithosphere. There are about a dozen minor plates as well, including the Nazca Plate, the Scotia Plate, the Caribbean Plate, the Cocos Plate, the Arabian Plate, and many more, all generally 60-120 mi (100-200 km) thick. The movement and interaction of these different plates can be determined by studying the nature of the plate boundaries, as well as the earthquakes and volcanic activities associated with them.
There are three different types of boundaries: divergent, convergent, and transform margins. The divergent boundary occurs where two plates are separating, resulting in the development of a rift and, generally, formation of new seafloor; however, they may form within a continent, such as the East African Rift. Divergence is associated with upward movement of magma, resulting in volcanism, such as at Iceland. The convergent plate boundary occurs where two plate margins collide head on. When they meet, one plate may be forced to dive, or subduct under another, forming a chain of volcanoes like the Cascade Mountains or the Marianas Islands and a deep ocean trench, like the Marianas Trench. Alternatively, both plates may be intensely deformed without significant subduction and so form a massive mountain range such as the Alps or the Himalayas. The Himalayas were formed when India collided with the Asian continent. The transform fault, the third type of boundary, develops when two plates laterally slide by one another. The San Andreas Fault exists because the North American plate and the Pacific plate are in contact. Nearly all plate boundaries are associated with some degree of earthquake activity.
The nature of Earth's lithosphere, that is, its crust and uppermost mantle, determines the character of interaction at an active plate margin. When new igneous rocks are being formed at a divergent margin, this expansion causes convergence at another margin. The nature of this convergence depends on the density of the rocks involved. Continental margins are composed of less dense rock than oceanic margins. When two oceanic plate margins converge, the denser of the two plate margins, which ever it happens to be, will inevitably sink, or subduct, below the other; when a continental margin collides with an oceanic one, the oceanic plate margin will inevitably subduct below the continental. When two continental margins collide, the colliding lithospheric plates will be intensely deformed without significant subduction, due to their low density as compared to the asthenosphere. Along transform margins, faulting is common; however, subduction and significant deformation do not occur in this tectonic setting.
Where subduction does occur—at an oceanic-oceanic or oceanic-continental, convergent margin--the descending plate can reach depths of 435 mi (700 km). At these depths, temperatures are sufficiently high for the rocks to undergo metamorphism; usually melting will also occur. The magma then convects upwards because it is lighter than the surrounding rock. As the magma makes its way up through the overlying rock, it eithers cool at depth, forming large bodies of igneous rock such as granite, or reaches the surface and erupts as a volcano. When submarine volcanoes erupt, a new island may eventually form. This is how the Aleutian Islands developed.
Although it is a topic of intense debate in the late 1990s, many geologists believe that convection of solid rock within the mantle is the driving force behind plate tectonics. This rock is perhaps heated indirectly by the Earth's core, making it lighter than the surrounding rock, and causing it to slowly convect upwards. As the rock reaches relatively shallow depths within the upper mantle, the decrease in pressure causes it to begin to melt. This provides the magma necessary for new seafloor and helps generate the mid-oceanic ridge where it extrudes. However, the convection does not stop there; rather it forms a continuous cell, which flows along the base of the plate. The mantle material then subsides, perhaps below the subducting plate at the adjacent trench, after it cools. The role this convection cell plays in plate motion and subduction remains unclear. However, the plastic nature of the asthenosphere probably is important for plate movement, as it reduces the friction involved, allowing the plate to glide along at the interface between the lithosphere and the asthenosphere.
Opponents of the mantle convection model believe that the plates generate forces that tend to lead to plate movement. For instance, as the plate moves away from the mid-oceanic trench, it cools, which increases the density. The increased density of the plate may pull the plate downward, forming a trench and creating a force that helps separate the plates at the mid-oceanic ridge, making room for new seafloor. There are several variations on this theme; all involve resolving forces created by the plates themselves.
Tectonic forces have been at work since the Earth's earliest existence, and some form of plate tectonics may have been active fairly early in Earth history. However, in 1998, some 30 years after the plate tectonic theory was first proposed, the time at which this began, its rapidity, and the early mechanics are still unclear. Geologists generally agree that plate tectonics probably began on an Earth devoid of continents and most accept that tectonic recycling of rock--melting, convection, creation of new seafloor, subduction, melting, convection etc.- --was responsible for deriving the less dense, granitic rock of the continents from the basaltic rock of the ocean crust. However, the timing and rate of continent formation is one of the most hotly contested topics in geology. Conjecture on continental origin generally falls into two categories: very early with rapid formation rates versus early with moderately rapid formation. Very early translates to 4.0 billion years ago (the Earth is about 4.6 billion years old) whereas early means about 3.0 billion years ago. The oldest known continental rocks are themselves 4.0 billion years old, making progress on this question difficult to achieve. The oceans have almost nothing to lend to the argument, since, due to subduction, the oldest oceanic crust is only about 190 million years old.
The future of plate tectonics is more certain. Modeling the effects of the slow, yet relentless, movements of lithospheric plates enables geologists to predict the expansion and contraction of oceans and the molding of new landmasses over hundreds of millions of years. For example, based on current plate activities, the Atlantic Ocean will continue to expand while the Pacific Ocean slowly shrinks. Eventually, the Mediterranean will be consumed by the convergence of the African and Eurasian Plates. The southern California coast west of the San Andreas Fault will shift northward, and Los Angeles will become a twin city with San Francisco. Asia might one day collide with North America creating a mountain range to rival the Himalayas. It is even possible, geologists speculate, that once again all the continents could reunite into one large supercontinent, a "neopangaea", creating an Earth that has little or no resemblance to our present-day world.
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