Plate tectonics

As plate tectonics is called the structure of the outer earth's crust, the lithosphere ( crust and uppermost mantle ), in lithospheric plates ( colloquially called tectonic plates ) which lie the deep mantle and then wander ( continental drift ).

Originally, the term plate tectonics for the corresponding model of thinking in the geosciences. The theory of large-scale tectonic processes in the outer earth's crust has now been proven many times and is one of the fundamental theories of the endogenous dynamics of the Earth.

The risks associated with plate tectonics phenomena include formation of fold mountains ( orogeny ) by the pressure of colliding continents as well as the most common forms of volcanic activity and earthquakes.

• 2.1 Continental Drift
• 2.2 From 1960: ocean floor, subduction, Geodesy

• 3.1 Constructive ( Divergent ) Plate Boundaries 3.1.1 mid-ocean ridge
• 3.1.2 Intra Continental trenches ( rift zones )

• 3.2.1 Cordilleras or Andes type
• 3.2.2 Volcanic island arcs ( Mariana type)
• 3.2.3 collision type

• 4.1 convection
• 4.2 Active lithospheric

Overview

The lithospheric plates

Fundamental to the plate tectonics is the fragmented structure of the lithosphere. It is divided into seven major lithospheric plates, which are called (especially by non- geologists ) as continental plates also be tectonic plates or:

• The North American plate and the Eurasian plate,
• The South American Plate and the African Plate,
• The Antarctic Plate and the Australian Plate
• As well as the Pacific plate, the only one of the large plates without significant amounts of continental crust.

There are also a number of small plates such as the Nazca plate, the Indian plate, the Philippine plate, the Arabian Plate, the Caribbean Plate, the Cocos, the Scotia Plate and other microplates, on their differentiation, however, partially little is known or whose existence is only suspected.

The movements of the plates

The plate boundaries are represented at the surface usually either mid-ocean ridges or deep-sea trenches. At the back of the adjacent plates drift apart ( divergent plate boundary ), which basaltic magma rises from the upper mantle and new oceanic lithosphere is formed. This process is also known as seafloor spreading Seafloor Spreading or. He is accompanied by intense, mostly untermeerischem volcanism.

Where in return decreases the oceanic crust deep into the mantle, arise deep-sea trenches. At this convergent plate boundary emerges one of the two oceanic lithospheric plates under the other panel ( subduction ). By dewatering processes in the subducting plate also creates a pronounced volcanism.

The actual continental blocks or continental blocks of predominantly granitic material - pushed as on a slow assembly line of the off spreading zones and subduction zones back - along with the surrounding ocean floor and beneath it the lithospheric mantle, respectively. Just a collision of two continental blocks can stop this movement. Because the continental crust is less dense than the oceanic crust, they appeared at the subduction zone not affiliated with the oceanic plate from, but bulges out instead to a mountain range on ( orogeny ). Here it comes to complex deformation processes. Such a continent - continent collision takes place, for example, between the Eurasian and the Indian plate and led to the formation of the Himalayas.

In addition, two plates can also easily slide past each other horizontally ( conservative plate boundary ). In this case, the plate boundary is referred to as transform fault ( fault Transform ).

As Hautptursache for plate motions convection currents in the mantle are assumed.

History of the theory of plate tectonics

Continental drift

After some researchers already held similar thoughts, it was mainly Alfred Wegener, who concluded the origin of continents and oceans of the exact fit of the coastlines of South America and Africa in his 1915 published book that these fragments might have been a once larger continent, was broken apart in the geological past. The fit is more accurate, if one considers not the shorelines, but the shelf edges, ie the submerged parts of the continents. In addition, Wegener collected many more arguments for his theory. However, he could not name any convincing reasons for the continental drift. A promising hypothesis came from Arthur Holmes (1928 ), who suggested that heat flows within the Earth could generate enough force to move the tectonic plates. However, at this time his hypothesis could not prevail.

From 1960: ocean floor, subduction, Geodesy

The paradigm shift to Mobilism continued therefore until around 1960, especially through the work of Harry Hammond Hess, Robert S. Dietz, Bruce C. Heezen, Marie Tharp, John Tuzo Wilson and Samuel Warren Carey than is generally fundamentally new insights into the geology the ocean floors attained.

• It was recognized, for example, that the mid-ocean ridges are volcanically active, and that there on long columns break large amounts of basaltic lava emerge, usually in the form of pillow lava.
• For paleomagnetic measurements of these basalts was discovered that repeated reversal of the Earth's magnetic field was generated in the course of the earth a mirror- symmetric " stripe pattern " on both sides of the Mid-Atlantic Ridge.
• In addition, it was recognized that the sedimentary rocks that cover the ocean floors, at a greater distance are also more powerful and older from the mid-ocean ridges.

The most plausible explanation for these phenomena was that the permanent outlet is basaltic magmas to the elongated mid-oceanic fracture zones part of a process through which the ocean floor forced apart in opposite directions, so that it continues to expand over time ( Seafloor Spreading ).

Since there is no indication that the radius of the earth increases continuously during their existence, as requested, for example, in Carey's expansion theory, the idea is suggested that the oceanic crust in the newly formed Earth's surface at another location of the earth's surface must vanish. This approach is supported by the fact that ( apart from tectonic exceptional items such as the Mediterranean) found in today's oceans no lithosphere, which is older than 200 million years old. Half of all oceans is not even older than 65 million years. Thus, the old idea was refuted by which the oceans ancient wells were, that would have, as the continents, already formed with the formation of the first solid crust around the glutflüssige Primal Earth. Instead, pass the ocean floors, compared with the continents, from geologically very young rocks.

The place of this disappearance of oceanic lithosphere, the deep-sea trenches were recognized in the 1970s, for example, surrounding the Pacific Ocean. Due to the relatively strong seismic and volcanic activity, this zone is also called the Pacific Ring of Fire.

• Geophysical measurements disclosed there obliquely inclined seismic reflection surfaces ( Benioff zone), where oceanic crust under continental (or other oceanic ) crust is pushed and falls. Typical of these zones are the deep earthquakes whose hypocenters may lie in depths of 320-720 km. This finding is explained by the phase transitions of minerals in the subducted plate.
• The substrate on which the lithosphere may drift sideways, around 100 km thick asthenosphere applies. It is also " low-velocity zone " (Eng. " zone slow speed " ), because the seismic P- and S- waves slow hinfortbewegen through it. This viscous zone declared to be by partially molten and thus flowing over geological time package of rocks below the relatively rigid, 70 to 120 km thick lithosphere.

The new methods of satellite geodesy and VLBI, which converged in the 1990s centimeter accuracy of which provide direct evidence of continental drift. The speed of ocean floor spreading is a few centimeters per year, but varies between the different oceans. The geodetically determined drift rates between the large plates are between 2 and 20 cm per year and are consistent with the geophysical NUVEL models are largely the same.

In addition to Wegener's theory of continental drift plate tectonics also contains elements of the flow theory of Otto Ampferer (see also history of geology, Permanenztheorie ).

Orogeny and volcanism in the light of plate tectonics

In contrast to the classical theory Geosynklinal it is now believed that most orogenic and volcanic processes are bound to the plate edges. Here arise as side effects of the moving plates for humans significant natural phenomena such as volcanic eruptions, earthquakes and tsunamis.

There are one-dimensional plate boundaries where two tectonic plates meet and triple points, where three tectonic plates meet. Not bound to plate boundaries are hotspots caused by thermal anomalies in the lower mantle.

Constructive ( Divergent ) Plate Boundaries

The drifting apart of two plates is called divergence. Here is new lithosphere.

Mid-ocean ridge

The mid-ocean ridge (MOR ) are considered ( as a so-called back and thresholds) with a total length of about 70,000 km, the largest contiguous mountain systems of the planet Earth.

The flanks of the MOR are very shallow. The comb region has long stretches depressions - the Central Graben. There, the actual formation of Earth's crust and lithosphere by large amounts of hot, mostly basaltic magmas ascend and crystallize. Only a small fraction of this reaches the seafloor as lava. The newly crystallized rock in comparison to older oceanic lithosphere is less dense. This is the reason that the MOR rise above the adjacent ocean floor. With increasing age of the basaltic rocks whose density, which is why the MOR with increasing distance from the ridge region are becoming flatter increases. Extend transversely to the central trench breaking lines in which the individual sections of the MOR are mutually offset. Therefore, the MOR have no continuous ridge line.

A strange volcanic phenomenon that is bound to the mid-ocean ridges, are the blacks and whites Smoking - hydrothermal vents, where they exit superheated, mineral- saturated water. This leads to the black smokers for the deposition of ores, which then form the so-called sedimentary exhalative - deposits.

Intra Continental trenches ( rift zones )

Also rift zones such as the East African rift, which may be regarded as the first phase of an ocean formation are associated with volcanic activity. However, it is not for constructive plate boundaries in the true sense. The plate divergence is here compensated to a large extent by the sinking and tilting of continental crustal blocks. Characteristic is the bulging of the surrounding continental crust, resulting from the heating and consequent decrease in density of the thinned lithosphere and manifests itself in the form of prominent basement massifs which form the Riftflankengebirge ( Riftschultern ) of the grave system.

Grave systems such as the East African rift caused by the activity of so-called Manteldiapire. These heat the lithosphere to thin them out and curl it like a dome on. The resulting tensions eventually cause the crust gives way and three-engine grave systems, starting from the dome-like swellings spread radially, with successive directed Riftstrahlen grow together and form an elongated grave system. Wither The remaining branches of the rift system. At the deep -reaching fractures in the crust that result from these processes, magma rises, which provides for the typical alkaline volcanism of continental rift zones.

With increasing expansion of the fracture zones of narrow, elongated marine basin, which, as the Red Sea, are highlighted already with oceanic crust and can escalate over time into vast ocean basins. Constitute

Destructive ( Convergent ) plate boundaries

The oppositely directed movement of two plates is called convergence. Either a thrust occurs at a subduction along the denser is pushed under the lower density plate ( subduction ) or a collision, wherein one or both of the plates in the edge areas are folded.

Cordilleran or Andean type

The classic Kordillerentyp the cordillera found on those subduction zones, where oceanic lithosphere is subducted directly under continental lithosphere, as on the west coast of South America.

By the descent of the oceanic plate under the continental block is located directly on the subduction a deep trough. On the continent caused by the horizontal pressure exerted by the subducted plate, a fold mountain, but without extensive Ceilings thrusts. The elevated pressures and temperatures of mountain building can lead to regional metamorphosis and reflows ( anatexis ) in the affected continental crust areas.

Within the Fold Mountains, a volcanic arc formed. This goes back to the fact that the subducted plate bound in the rock fluids - particularly water - transported into the depths. Under the prevailing pressure and temperature conditions, there is phase transformations in the rock, where water from the subducting plate is released into the overlying mantle. Wherein the melting temperature of the sheath and the rock is reduced, there is a partial melting. The first basaltic melt rises through the overlying lithosphere, and differentiates itself thereby in part gravitationally or mixed with crustal material. The resulting viscous andesitic to rhyolite magmas can reach up to the surface and go there often highly explosive volcanic eruptions produced. The Andean region as a type of Andean -type subduction are correspondingly also exemplifies the associated volcanism, represented by numerous active volcanoes such as Cerro Hudson or the Corcovado, but also by widespread lava flows and ignimbrites fossil.

In the collision of oceanic with continental crust of the ocean floor is not always completely subducted. Little remains of sea floor sediments and basaltic material ( ophiolites ) are sometimes in the subduction of its base " scraped " ( sheared ) and not sink into the upper mantle. Instead, they are wedge-shaped pushed onto the continental margin ( autopsy ) and in the cordillera and thus integrates the continental crust. Since they are the closest subduction, they experience the highest pressure and folded together with the other rocks of the continental margin and subjected to high-pressure low-temperature metamorphism.

Volcanic island arcs ( Mariana type)

On the western edge of the Pacific Ocean and the Caribbean oceanic crust is subducted beneath other oceanic crust. There, too, form deep-sea trenches and volcanic arcs. The latter are called island arcs, because only the highest parts of the volcanic arcs are above sea level. The arc shape is due to the geometric behavior of a spherical surface, such as the earth's crust at the snapping and immersion of the plate member. The convex side of the sheet has always in the direction of the subducted plate. Examples include the Mariana Islands, the Aleutian Islands, the Kuril Islands or the Japanese Islands and the Lesser and Greater Antilles.

Typical of subduction zones from the Mariana type are called backarc basin (of English. Back for behind ' and for arc, arc '). The name refers to the fact that this strain zones (seen from the subducted plate ) in the crust behind the island arc are.

Collision type

If the oceanic crust has been completely subducted between two continental blocks, it comes to a collision-type mountain building, such as in the case of the Himalayas by the collision of the Indian subcontinent with the Eurasian landmass. In such a collision, the lithosphere is thickened enormously through the formation of tectonic ceilings. After a polyphase orogeny, ie, time-offset collisions of several small continents or volcanic island arcs (called terranes ) with a larger continental block and intermittent Subduktionsphasen, have the preserved ophiolite zones, the boundary between the individual small continental blocks (see also Geosutur ). Both in the West and on the east coast of North America, there are signs that the North American continent by such multiphase mountain building during its geological history ansetzte more and more crust.

The picture can be more complicated case of oblique collision of the blocks, as in the Apennine peninsula in the Mediterranean. There is evidence that oceanic crust was subducted Mediterranean temporarily under both the African and within the Eurasian plate, while the Iberian Peninsula, the Sardo - Corsican block and the Apennine peninsula between the major continental blocks rotated counterclockwise.

Conservative plate boundaries ( transform faults)

At conservative plate boundaries or transform faults lithosphere is neither new nor formed subducted because the lithospheric plates "slide" past each other here. At and near the surface, where the rocks are brittle, such a plate border is designed as a leaf offset. With increasing depth the rock is due to the high temperatures are not brittle but high viscosity, ie, it behaves like an extremely viscous mass. Therefore, the strike-slip turns into a so-called ductile shear zone at depth.

Transform faults in continental crust can grow to a considerable length and include, like all plate boundaries to the earthquake focus. Well-known examples are the San Andreas Fault in California, or the North Anatolian fault in Turkey.

At the mid-ocean ridge (MOR ), there is not only volcanically active longitudinal trenches, but also transverse faults, which are likewise to slip faults and shear zones. This cut the edges of the MOR at irregular intervals and divide the back into single, staggered sections. However, only the areas of the disturbances, which extend between the central grooves of two adjacent MOR - sections, indeed conservative plate boundaries and thus transform faults in the proper sense. Also, the transform faults are seismically active of the MOR.

Hotspots

The hotspot volcanism represents a special feature, since it is not tied to plate boundaries. Both, for example, in Iceland as in Hawaii are from stationary sources in the lower mantle, promoting the so-called diapirs or plumes, material, melt out the basaltic lavas in the upper mantle with a specific chemical composition. While Iceland, however, exactly on the Mid Atlantic Ridge and is perhaps actively involved in the spreading of the North Atlantic Ocean, Hawaii is located in the middle of the Pacific Plate. The long chains of islands of the South Pacific explained by the fact that the oceanic lithosphere has slipped continuously over a stationary hot spot, whose volcanic vents have hit the ocean floor at regular intervals.

At least for the islands of Hawaii suggest new findings that there is not a completely stationary, but a mobile hot spot. Scientists studied the orientation of the magnetic field in the formerly molten rock, which almost freezes the prevailing at the time the magnetic field during solidification. The results are not consistent with the previous estimate, but suggest that moves the heat source under the tectonic plate. However, the proper motion of the diapir runs much slower than the motion of the Pacific plate.

Causes of plate tectonics and unsolved problems

If the reality of continental drift is among geoscientists also little doubt there is about the forces within the earth that cause the movements of the plates and drive, yet almost as much uncertainty as to times Wegener (see also mantle convection ). The two theories presented here were considered to be contrary and incompatible with each other. After today, they are increasingly seen as complementary.

Convection

The most today held view is based on slow convection currents that result from the transfer of heat between the hot earth's core and mantle. The mantle is hereby heated from below. The energy for the heating of the cladding material could for a conceptual model still arise from the Akkretionsenergie, which was released during the formation of the Earth. In part, also contribute radioactive decay processes for heating. The friction energy of the tidal effect of the moon on the earth system can probably be neglected. However, forming convection currents under laboratory conditions, for example in heated viscous fluids, very highly structured and symmetrical forms of, for example, have a honeycomb structure. This is hardly compatible with the actually observed shape of the tectonic plates and their movements.

Another theory is based on only two opposing Konvektionszentren. A dominant today cell would be less than Africa, which would explain the local prevalence of stretch breaks and the lack of a subduction zone at the edge of the African plate. The other convection cell would be on the opposite side of the globe - under the Pacific plate, having lost size. The Pacific, which, interestingly, no continental crust contains, would thus be the remnant of an ancient ocean Panthalassa Super, which had once surrounded Pangaea. Only when all the continents were reunited to form a new supercontinent in the area of ​​today's Pacific, the movement would reverse ( Wilson cycle). The new Pangaea would break apart again to include the new super ocean that would have made ​​of Atlantic, Indian and Arctic Ocean once more.

Active lithospheric

Other authors see the plates not only passively lying on the mantle. How does the thickness and the density of an oceanic lithospheric plate steadily while away from mid-ocean ridges and cool, which they are already a little sinks into the mantle and thus can be more easily inserted from the top plate. After sliding under the top plate subducted the rock is finally converted into rock higher density under the pressure and temperature conditions with increasing depth. So, for example, is from the basalt of the oceanic crust eclogite, whereby the density of the subducted plate may even exceed the density of the underlying portion of the mantle. Therefore, the decline in a collision in the shell plate is pulled low by their own weight, plate material, in extreme cases may fall to near the bottom of the mantle. The force exerted on the lithospheric force is called Plattenzug ( engl. slab pull, of pull, pull '; slab, plate '). A smaller by about a factor of 10 force is generated in addition to the the mid-ocean ridge facing side of a lithospheric plate, as there hollowed crust undergoes a downhill force, the back pressure ( engl. ridge push, from ridge, back ' and push, push '). Also on the opposite, not falling into the shell plate acts in a subduction zone, a force, a tensile stress. The speed at which an oceanic lithosphere, however, is actually moving, also depends on the size of the opposing forces.

Plate tectonics on other celestial bodies

According to the current state of research, the mechanism of plate tectonics on the earth only appears to be effective. This is still plausible for the small planets Mercury and for the large moons of the gas planet and the Earth's moon. The lithosphere of the Earth relative to the much smaller celestial bodies in relation to powerful to be mobile in the form of plates can. However, the crust of Jupiter's moon Ganymede shows signs of a standstill of plate tectonics. In the nearly Earth-sized Venus is again difficult to understand why a plate tectonics despite strong volcanism may not be under way. A significant role could play, which occurs only on earth free water. Obviously, it is here down to the crystal lattice plane as a friction reducing " lubricant ". The sediments of the ocean floor billions of tons of water are taken into the depth to the subduction zones of the earth in the pore space, which melts the overlying mantle partial. On Venus, liquid water and therefore seas are at least no longer available.

Mars, however, appears to have an intermediate position. Water or ice is present and you feel you can identify approaches to plate tectonics. The array of giant shield volcanoes and grave systems that span half the planet, remember in some way to the rifting on Earth. This again contrasts with the lack of clear Verschluckungszonen. Probably enough internal heat generation and the resulting convection in this relatively small planet but not quite, to really set the mechanism in motion, or the operation was already in the early history of the planet to a halt again.

Whether a species Plate takes place on constructed differently celestial bodies, is not known, but conceivable. As a candidate for konvektionsgetriebene long-range horizontal crustal shifts the moons Europa and Enceladus may apply. The nearly erdmondgroße Europe has a layer of ice of about 100 km thickness over a rocky lunar body that could be melted in the lower regions of partially or completely, so that the ice sheet may swims like ice on an ocean. The only 500 km at a small Enceladus is probably heated by tidal forces. Liquid water or ductile by high pressure ice could in two celestial bodies rise to profound disturbances, and press the brittle crust of ice to the side, what could follow in turn that should be swallowed elsewhere crust. The surface of these moons is certainly geologically active, or at least been active, and shows signs that took place there crustal renewal. The volcanism on Io, however, seems to be so strong that stable crustal areas are not even emerged in the types of plates.