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Plate tectonics

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Plate tectonics (from the Greek for "builder", tekton) is a theory of geology developed to explain the phenomenon of continental drift. In the theory of plate tectonics the outermost part of the Earth's interior is made up of two layers, the outer lithosphere and the inner asthenosphere. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift.

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Key principles The division of the Earth's interior into lithospheric and asthenospheric components is based on their mechanical differences. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle and crust.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which "float" on the fluid-like asthenosphere. Due to convective currents in the asthenosphere, the tectonic plates undergo motion in different directions. The point where one plate meets another is known as a plate boundary and these areas are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. Plate boundaries are the home of the majority of the world's active volcanoes with the Pacific Plate's Ring of Fire being most active and famous. These zones are discussed in further detail below.

Tectonic plates are broadly divisible into two groups: continental and oceanic plates. The distinction is based on the density of their constituent materials; oceanic plates are denser than continental plates due to their greater mafic mineral content. As a result, the oceanic plates generally lie below sea level, while the continental plates project above sea level (see isostasy for explanation of this principle, which is essentially a large-scale version of Archimedes' Bath).

Types of plate boundary The different types of plate boundary are:

  • Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or an orogenic belt[?] (if the two simply collide and compress).

  • Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types.

Transform boundaries

The left- or right-lateral motion of one plate against another along transform or strike slip faults can cause highly visible surface effects. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the slipping-point of rocks on either side of the transform-faults the accumulated potential energy is released as strain, or motion along the fault. The massive amounts of energy that are released are the cause of earthquakes, a common phenomenon along transform boundaries.

A good example of this type of plate boundary is the San Andreas Fault complex, which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving North with respect to North America.

Divergent boundaries

At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The driving force that moves the plates apart is not fully understood. Two theories are the ridge-push[?] and slab-pull[?] hypotheses. In the former, upwelling convective currents in the mantle bring hot material close to the Earth's surface. As it reaches shallow levels it starts to melt and is expelled at the divergent boundary, thus forcing the plates apart. The second hypothesis suggests that, if one end of a plate is being subducted at a convergent boundary, the downgoing slab of material will exert a stress on the other end, thus pulling it away.

The genesis of divergent boundaries is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is though to be sufficient to break apart the lithosphere. The hot spot believed to have created the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimetres per century.

Divergent boundaries are typified in oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and, in continentental lithosphere by rift valleys such as the the famous East-African Rift. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different massive transform faults occur. These are the Fracture Zones[?], many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features (http://pubs.usgs.gov/publications/text/baseball) perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (due to thermal contraction and subsidence).

It is at mid-ocearn ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals[?] on opposite sides of ridge centres. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made. The magnetic banding directly corresponds with the Earth's polar reversals[?]. This was confirmed by measuring the ages of the rocks within each band. In reality the banding furnishes a map in time and space of both spreading rate and polar reversals.

Convergent boundaries

The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the oceanic side, and a mountain range on the continental side.

An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca plate[?] is being suducted beneath the continental South American Plate[?]. Where two continental plates collide, the effect is for the plates to crumple and compress, creating extensive mountain ranges, such as is occurring at the Indian and Eurasian plate-boundary with the Himalaya.

Impact on Earth Sciences The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) can be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Within a matter of only several years geophysics and geology in specific were revolutionized.

The parallel is striking: just as Pre Copernican astronomy was highly descriptive but still able to make predictions, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "how?". Before acceptance of plate tectonics, geology in particular was trapped in a 'pre-Copernican' box.

However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly accepted within a few short years in the 1960s and 1970s. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy.

One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites?", or "How did the convex and concave margins of the Alpine chain form?", any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics.

With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift sea floor into thin atmospheres. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.

Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift valley, like the one now on the other side of Africa, had split into the Atlantic--and was still at work.

We have inherited some of the old terminology, but the underlying concept is as radical and simple as "The Earth moves" was in astronomy.

See: Alfred Wegener, List of Tectonic Plate Interactions, obduction, subduction

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