Plate tectonics

plate tectonics is at present most important theory of geology and geophysics for the spacious operational sequence in the earth's crust. Plate tectonics describes the movements of the Lithosphären - plates - the so-called Kontinentalverschiebung - and the associated consequences. To thesecount the emergence of Faltengebirgen (Orogenese) and deep sea gutters with the pressure of the plates, as well as the phenomena of the earthquakes and volcanism.

Table of contents

general

Plattentektonik: Tektonische Platten mit Kontinenten (Hintergrund)
plate tectonics: Tectonic plates with continents(Background)

fundamental for plate tectonics is the fragmented structure of the lithosphere, which is arranged into 7 large plates:

Besides givesit still some smaller plates like for example the Caribbean plate, Cocosplatte, Nazcaplatte, Somaliplatte, Arab plate and Philippine plate), over whose connection however different data are present.

The plates are by midocean backs or by deep sea gutters(- ditches) separated. At the backs new oceanic crust from basaltischem magma, which climbs from the upper Earth's mantle, which one ocean floor , develops - spreading or Seafloor Spreading calls.

In the deep sea gutters the crust drops again into the coat and„one swallows “(subduziert). The actual continental blocks from predominantly granitischem material are pushed - together with the surrounding ocean floors - as on a slow assembly-line away from the spreading zones and/or to the Subduktionszonen. Only one collision with onethis movement can stop other continent. Since the continental crust is specifically easier however than those of the ocean floors, it cannot dive with this together into the Subduktionszone.

history of the theory of plate tectonics

firstHypotheses (17. - 19. Century)

the most obvious and therefore earliest recognized reference to continental drift is the similarity in the process of the east coast of South America and the west coast of Africa.

The oldest assumption of a horizontal shift as a cause comes from the flämischen Kartografen Abraham Ortelius, in the expenditure of its Atlas Theatrum Orbis Terrarum of 1596. As driving power of these procedures Ortelius accepted earthquakes and tides.

Antonio Snider Pellegrinis representation of the closed and opened Atlantiks.

The first reference Sir Francis Bacon becomes frequent im Jahre 1620 zugeschrieben, doch soll Bacon sich nur auf die Ähnlichkeit der Westküsten beider Kontinente, also die atlantische Küste von Afrika und die pazifischeKüste von Südamerika bezogen haben (nach Keary und Vine, Global Tectonics, 1990, Blackwell Scientific Publications,Oxford). On the other hand also theology professor Theodor Christoph Lilienthal in king mountain is to have considered the possibility in view of the similarity of the opposite coasts of South America and Africa 1756 that they lay once close together. It brought the breakup apart with a Biblical disaster inConnection.

Alexander of Humboldt described 1801 and 1845 the similarity of the opposite coasts of South America and Africa and speculated that the Atlantic was washed by a catastrophic river. In the year 1858 the American Antonio Snider Pellegrini a step continued to go, as hethe first map published, on which the old person and the new world without separating ocean were to be seen. It mutmasste that it had been the Biblical Sintflut, which separated the continents. Around the turn of the century drifting apart became of theAmerican and the African continent brought with the emergence of the moon from the Pacific in connection.

The Austrian geologist Eduard sweet (1831 - 1914) represented the face of the earth in his book series „“first the land bridge theory, around the salient similaritiesto explain between certain fossil animal and plant socializations on different today's continents. Later it postulated however the existence of two formerly together hanging large land masses. For the more southern of both sweet coined/shaped before already from other imported names the Gondwana- Country. This continent covered all today's continents of the southern hemisphere, including India, still in the Mesozoikum. At the beginning of the Känozoikums however large parts of this continent dropped and oceans became.

Alfred Wegener and its opponents

thosehere colored represented paläobiogeographischen circulation areas of Cynognathus, Mesosaurus, Glossopteris and Lystrosaurus permit the reconstruction of Gondwana and are thereby vouchers for the existence to plate tectonics
in its 1915 published book „the emergence of the continents and oceans “(leftto fourth) concluded Alfreds Wegener (1880-1930 ) from the exact fit of the coastal lines revised edition of 1929 from South America and Africa that these fragments of a formerly larger continent could have been, which was broken in the earth-history past apart. ThoseFit is still more exact, if one regards not the coastal lines, but the edges of shelf, thus the parts of a continent which are under water. Besides Wegener collected further arguments:
  • Fold belts and shear zones made of South America leave themselves in Africa with very similar rock successionsand deformation samples compare.
  • Diamond stores in South America and west Africa exhibit geological similarities.
  • On all south continents climate is - witnesses to permo carbons of the ice age.
  • Determined fossil and rezente Floren and Faunen on both sides the Atlantiks agrees:
    • Fossils of coolingloving land plants with zungenförmigenSheets (Glossopteris - Flora) were common on all south continents.
    • Fossil remnants of Mesosaurus, a reptile living in the fresh water, could be proven both in Africa and in South America.
    • The sea-cow Manati comes today both into west Africa andin central and South America forwards.
Die zu Pangäa vereinten Festlandsmassen im Unterperm
The mainland masses in the Unterperm on the basis such indications

reconstructed Wegener a supercontinent, which it called Pangäa - all country -, united to Pangäa, that not only the south continents, but all well-known continental masses covered. According to its theory, the specifically lighter continental crust or SiAl - beside silicon aluminum is the prevailing element of the granite - on the closer, basal tables underground, the SiMa, consisting of predominantly granitischem rock, should - which aluminum represented in the basalt by magnesium- „swim “, about in such a way, as an iceberg in the sea floats.

As possible Kraft, which let the continents break and float apart, Wegener suggested different astronomical forces: for example the deceleration of the earth rotation by the tidal friction of the moon, or precession forces. „The pole escape “, i.e., centrifugal force produced by the earth rotation, should move the continental masses slowly in the direction of the equator. But even Wegener was clear that in the long run these forces were not sufficient, in order to explain the drift of the continents.Straight one therefore Wegeners theory rejected during its lifetimes of most geoscience-learn.

A less scientific motive of Wegeners opponents however probably existed in the Eifersüchteleien between at that time strictly from each other the separated subsections of the geosciences. There itself Wegener originallywith astronomy, meteorology and climatology , was considered he had employed „genuine “geologist to many as an unqualified „specialist from beyond “.

starting from 1960: Approximately one used ocean floors,

Subduktion , earth measurement of the paradigm changes fundamentally new realizations over geology around 1960, asthe ocean floors attained.

  • One recognized for example that the midocean backs are volcanically active, and that there on long Bruchspalten large quantities of basaltischer lava withdraw, usually in the form of cushion lava.
  • When paläomagnetischen measurements these basalts one discovered thatthe repeated pole reversal of the earth's magnetic field in the course of geologic history an exactly mirror-symmetric „Streifenmuster “on both sides of the Mid-Atlantic Ridge had produced.
  • In addition one recognized that the sedimentary rocks, which cover the deep sea soils at larger distance of the midocean backs alsoever more powerful and older become.

The most plausible explanation for these phenomena was that basal tables the magmas, which constantly withdraw and solidify at the midocean Bruchzonen press the ocean floor apart into opposite directions, so that it itself in the course of the timecontinues to expand always (Sea Floor Spreading).

Now it gives to today no clear signs that the radius of the earth in the course of its existence would have significantly become larger, as it was demanded in the old expansion hypothesis. This suggested the thoughts,that the again formed oceanic crust in other place must be destroyed again.

  • It suggests that one discovered to today no ocean floor, which would be older than 200 million years. Half of all oceans is not older even than 65Millions years. Hereby the old, fixedistic conception, was disproved after which the oceans were age-old collapse basins, which would have formed, like the continents, already soon after figuration of the first firm crust around the glow-liquid Urerde.

As place of the destructionoceanic crust into the 1970er years the deep sea gutters were recognized, which particularly surround the Pacific ocean. Because of their strong seismic and volcanic activity this zone is called also „Pacific Feuerring “.

  • Geophysical measurements revealed there diagonally bent seismic Reflection surfaces (Benioff zone), at which heavy oceanic crust is apparent pushed under continental (or other oceanic) crust and drops. For these zones the deep earthquakes , whose Hypozentren can lie in depths from 320 to 720 km, are typical.This findings are explained with the strong friction between the dropping plates and the surrounding rock, which finally leads to breaking and melt opens of the subduzierten plate.
  • As substrate, on which the crust can shift laterally, those is considered to approximately 100km powerful „Low Velocity zone “(asthenosphere), in which the seismic „p and S-waves “spread only slowly. This avowed one rock packages below rigid, 70-120 km powerful lithosphere, fluid with the existence of partly melted .

Thosenew methods of the satellite geodesy and the VLBI, which approached into the 1990ern of the cm accuracy, supply now a direct proof of continental drift. The speed of ocean floor spreading amounts to on the average some centimeters per year, varies however between the particularsOceans. The geodetically determined drift rates between the large plates lie between 2 and 20 cm and agree with the geophysical NUVEL - models to a large extent.

Can recapitulatory be said: Alfred Wegener (1880-1930) postulated already 1915 that the relatively easy, granitischenRocks of the continental crust such as wedges on the closer, more viscous basalts of the oceanic crust affect and both together with the upper Earth's mantle than plates of the lithosphere the asthenosphere “swim”. But only around 1970 the findings of the individual geosciences converged, andthe plate tectonics model could replace the older theories for Gebirgsbildung and for the structure of the earth's surface. These theories were the Geosynklinal developing on the contraction hypothesis - theory, the expansion theory and the Pulsationshypothese.

Beside Wegeners theory of continental drift contains plate tectonics also of elementsthe lowest Rome theory of Otto Ampferer (see also: History of geology, permanence theory).

Gebirgsbildung and volcanism in the light of plate tectonics

contrary to the classical Geosynklinal theory, one assumes today the most mountainforming and volcanicProcesses are bound to the disk edges.

midocean backs

the midocean backs are regarded today (as backs and thresholds so mentioned) as the largest coherent mountain systems of the planet earth. There however expansive forces prevail forwards, so that thoseRocks not to be folded. Instead break tectonics with the formation of tectonic ditches and refuges prevails forwards. Of the midocean backs Transform disturbances, those are characteristic the volcanically active central ditches in irregular distances about right-angled cut and the individual sections against each othershift. Also the Transformstörungen is seismically active, because the tectonic tensions unload themselves here, which are developed diagonally by disk movements to the central backs. A well-known example is the earthquake-endangered San Andreas distortion in California, those the small fermenting since plate of the Pacific one Plate separates.

Its own volcanic phenomenon, which is bound to the midocean backs, are the Black Smoker, hydrothermale chimneys, at which overheated, mineral-satisfied water withdraws.

Intrakontinentale ditches

also the tectonic ditches, like the East Africa niche the ditchwhen the first phase of the ocean formation can be understood, are connected with volcanic activity. Characteristic is here the upbulging of the surrounding continental crust, which leads to lifting out of expanded basic mountain - massifs, or Schilden. The straight unusual average altitude of the African one Sign lets assume many scientists that under the African continent stationary heat sources are: several Manteldiapire in such a way specified, curve the lithosphere up and heat the earth's crust up. It comes to tears, to volcanism and to the withdrawal of magma.

With more increasinglyNarrow, long-drawn-out sea basins form expansion of the Bruchzonen, how the Red Sea, which can expand with the time to genuine oceans.

Kordilleren or type of and

the classical Kordillerentyp of the chain mountains is over the Subduktionszonen, inthose one subduziert oceanic crust directly under continental crust, as at the west coast America. Here prevail particularly compressing conditions, which lift out, in tectonic covers over push and fold the rock packages each other. In larger depths it can in these zones throughthe increased pressures and temperatures also to regional metamorphosis and melting (Anatexis) come.

Into the Subduktionszonen at the east coast of Asia, and at the deep sea gutters of the west Pacific, like the Maria EN and the Tonga ditch, it comes to the formation of curved Island elbows. The curvature is to be due to the geometrical behavior of a Kugeloberfläche like the earth's crust when bending and submerging a disk part. Examples are the Alëuten, which leads Kurilen or the Japanese islands. These island chains are to a large extent volcanic origin. Betweenthe island elbows and the pre-aging continent can still so-called „bake arc Basins “develop.

With the collision of an oceanic plate with a continental plate it can likewise come beside the mountain unfolding to volcanic education. The subduzierten plates become also here inthe heat of the upper coat melted and the andesitischen heated up to granitischen magmas penetrate through the rocks of the overlaying Faltengebirge to to the surface. There they feed the partially highly explosive volcanic eruptions.

During the collision of more oceanic with more continentalCrust is not always completely subduziert the ocean floor. Small remainders of bottom of the sea sediments and basaltischem material (Ophiolithe) “are scraped off” to Weilen with the Subduktion by their document and to sink not in the upper coat. Instead they become, together withthe rocks of the Kontinentalrandes, distorted, folded and into the respective mountain belts integrates. Usually these rock packages are pushed wedge-shaped onto the Kontinentalrand (Akkretionskeil) and become part of the continental crust. Particularly at the west coast of North America are signs,that the continents also by the collision with “micro continents” and with island elbows (over term:Terranes) ever more crust set. If in a Faltengebirge, as for instance the Himalaja or the alps, Ophiolithe are found, then this points thus to the formerExistence of whole oceans, which disappeared between two continental plates.

type of collision

if the oceanic crust between two continental blocks subduziert is complete, comes it to the type of collision of the Gebirgsbildung, like for example with the collision of the IndianSubkontinents with the eurasischen land mass in the Himalaya.

The picture can become many more complicated however when diagonal meeting one another of the blocks and presence by „micro continents like the Apenninhalbinsel in the Mediterranean. It seems that oceanic Mediterranean crust temporarily both under the African one andalso under the Eurasi plate one subduziert, while the Iberi peninsula, which Sardo korsische block and the Apenninhalbinsel between the large continental blocks the counterclockwise were rotated.

Load, but emergency leases exists also areas like the Southeast Asiatic island world or the Karibik,in those two oceanic plates in sense moving in opposite directions under another oceanic plate to be subduziert. Coincidentally the volcanic eruptions do not belong in these areas to the most enormous at all.

Hot Spots

it is not completely clear, like the so-called Hot Spot - volcanism into this picture fits. Both on Iceland, and on Hawaii, from stationary magma chambers to the upper coat, the so-called Diapiren or plume, basal tables lavas are promoted. During Iceland on the Mid-Atlantic Ridge lies however exactly andin the spreading of the North Atlantic, is Hawaii is perhaps actively involved in the middle in the Pacific plate. The long island chains of the south Pacific explain themselves thereby that the oceanic lithosphere slid continuously over a stationary Hot Spot, its volcanic chimneys inregular distances the ocean floor pierce have.

At least for the islands of Hawaii of wise new realizations on the fact that it concerns there not around a stationary, but a mobile Hot Spot. The scientists examined the adjustment of the magnetic fieldin the formerly melted rock, which with solidifying that at the time prevail magnetic field quasi freezes. The results cover themselves not with the past acceptance, but put the assumption close that the heat source under the tectonic plate moves.

Work on []

Also hardly still one doubts causes

Das Prinzip der Plattentektonik (nicht maßstäblich)
of plate tectonics and unresolved problems the principle of plate tectonics

(not full-scale) if the reality of continental drift under Geowissenschaftern, then those exists to release the movements of the plates and over the forces in the interior of the earth,advance, still nearly as much ambiguity as at times Wegeners.

Today to most represented opinion proceeds from slow convection stream, which form by the heat exchange between the hot Earth's mantle and the cooler earth's crust. The energy for the heatingthe magma supply probably radioactive decay processes. The friction energy of the tidal effect of the moon on the earth body can probably be neglected. Unfortunately convection stream on laboratory conditions , for example in tough liquids heated up, train structured very high and symmetrical forms, as for instanceBee honeycomb sample. This can hardly be agreed with the actually observed shape of the geotectonic plates and their movements.

Other authors underline rather the effect of the gravitation on the movement of the plates. While the oceanic crust of the midocean backs departsand, increases also its thickness and density cools down constantly. Finally the point is reached, at which it abtaucht under the own weight „automatically “again into the coat. Basal tables the magmas at the midocean backs became thus the ocean floordo not press actively apart, but the columns open passively, by the lateral course to the Subduktionszonen. Melting the magmas at the midocean backs is based after these conceptions rather on lateral pressure relief as on increased heat flow from thatDeep one.

Another theory proceeds itself from only two in relation to lying convection centers. One today dominant factor cell would be under Africa, which would explain the there prevailing of stretch breaks and the absence of a Subduktionszone at the edge of the African plate. The other oneConvection cell would be on the opposite side of the globe - under the Pacific plate, which constantly loses at size. The Pacific, which contained interestingly enough no continental crust, would be thus the remnants of a urzeitlichen superocean Panthalassa, once the Pangäa opposite been appropriate for thatis. Only if in the area all continents had united today's Pacific again to a new supercontinent, the movement would turn around (Wilson cycle). The new Pangäa would break again apart, around the new superocean, itself outAtlantic, Indian one and arctic ocean would have formed to close a further mark.

plate tectonics on other heavenly bodies

after the past conditions of the research seems to be effective the mechanism of plate tectonics only on earth. That is forthe small planet Merkur and for the large moons of the gas planets and the moon still plausibly. The lithosphere this relative to the earth much smaller heavenly body is simply too thick in the relationship, in order to be able to be mobile in the form of plates.However Ganymeds crust beginnings of a plate tectonics come to succumbing shows. However it is difficult with the nearly earthlarge Venus to understand, why a plate tectonics might not have come despite strong volcanism on. A substantial role could thereby only upthe earth play occurring free water. Obviously it serves here to down there on the crystal lattice level as friction-reducing „lubricant “. One knows that pulled to the Subduktionszonen of the earth not only from cut sediments of existing Akkretionskeil into the depthbecomes, but with them annually billion tons water. On the Venus is simply missing it.

Mars against it seems to stress an intermediate position. Water and/or. Ice is present and one meant to be able to recognize beginnings of a plate tectonics. The lined upgigantic sign volcanos and ditch systems, which re-clamp the half planet, remind in certain way of the Rifting on earth. Again the absence of clear swallowing zones faces that. Probably internal heat development and from this following convection on this was enoughrelative to small planets out, in order to set the mechanism really on, or the procedure did not come completely already in the early history of the planet again the stop.

Whether a kind plate tectonics on differently developed heavenly bodies takes place, is not well-known, butconceivable. As candidates for convection-driven spacious horizontal crust shifts the moons Europe and Enceladus can be considered. Scarcely moon-large Europe partly or completely exhibits an ice tank of approximately 100 km thickness over a rocky moon body, within the lower rangesto have melted could, so that the ice tank possibly swims like luggage ice on an ocean. Only about 500 km small Enceladus is probably heated by tidal forces. Liquid water or soft ice could ascend with both heavenly bodies at linear weakness zones, thatit presses stahlharte ice of the crust to the side which would let follow again that in another place crust would have to be swallowed. The surface of these moons was at least active anyhow geologically actively or, which requires mechanisms of the crust renewal. Volcanism on IO against it seemsto be so strong that stable crust ranges in the kind of the plates did not develop only at all.

disk shifts in geologic history

We know, how quickly and where the large plates move at present and permit different indicationsus to reconstruct their ways in the past. Because of their inertia they need dozens of millions of years, in order to come for the stop and still longer, in order to turn around their movement.

shifts in the past

  • One assumesthe land mass of the earth zirka 320 million years ago of essentially two continents covered, i.e. Gondwana and Laurasia.
  • Approximately 250 million years ago both to the giant continent Pangäa had grown together, which was surrounded by the giant ocean Panthalassa, and in itselffrom the east the Tethysmeer like an enormous bay inside extended.
  • Approximately 135 million years ago the continental mass broke apart. The Tethysmeer continued to open to the west and separated a south continent, which is called again Gondwana. The north continent disintegratedby the opening of the North Atlantic into the two parts North America and Eurasien.
  • To forwards approx. 100 million years the decay process of the continents continued to continue. Above all the large south continent has itself in South America, Africa, India, Antarctic and Australia split up. The Tethysmeer still separates the north continents from the south continents.

shifts in the future

all geological observations point out that the plates are further dynamic. It does not leave itself exactpredict, how the land masses will be distributed in 200 million years on the earth's surface. Forecasts can be provided on the basis an extrapolation of the current movements:

  • In 20 million years East Africa will split itself off along East Africa niches of the Grabenbruchs the remaining Africa andform a new ocean. Spain separates from France and turns thereby easily in the clockwise direction. Australia and New Zealand push themselves fast to the north, so that north Australia is now because of the equator. The black sea is completely cut off of the Mediterranean andthe gulf of Akaba opened up to Turkey.
  • In 40 million years Africa of far direction the north to move and the Mediterranean region completely transform, Sicilies is shifted northward and lies near the coast before Rome. Spain turnsfurther in the clockwise direction away from France. Central Europe could break along the Rhine apart. Australia moves far direction Southeast Asia. The Atlantic becomes broader, because America continues to itself remove from Europe and Africa.
  • In 50 million years a part solves itself California along the San Andreas distortion of the American mainland and moves after northeast. North America with Greenland moves first to the west, turns then in the clockwise direction and drifts to the south. Thus Greenland arrives into the moderate zone south 60. Degree of latitude and becomesreally green.
  • In 80 million years Africa will have so far penetrated northward the fact that into consequence of the thrust gradually to the place of Mediterranean will have stepped a new mountain chain and thus becomes finally the last tracesthe antique ones disappeared its. Australia collided to New Zealand in the meantime with Japan, the Tropics reached and the Antarctic steers on Australia too.
  • In 90 million years are separate north and South America. North America shifts south to the sideSouth America.
  • In 150 million years entire Greenland concerned south Peru at approximately 30 degrees southern latitude.
  • In 200 Millionen Jahren hat sich die Antarktis Mexiko so stark angenähert, dass beide am Äquatorliegen und die Antarktis wie zuletztin the early Mesozoikum sumptuously bewachsen will be. East Africa collided with India, Madagascar meets on Southeast Asia. Newfoundland is already with 10 degrees northern latitude and continues to move towards the equator, the Florida on its way to the southalready behind itself left. South America in the clockwise direction 90 degrees. In the last 200 million years Scandinavia and the British islands moved slowly in southeast direction.

general

India becomes stillsome time under the Himalaya push and perhaps completely under Tibet disappear. But in the north the collections in that will continue to grow to Mongolia and an expanded mountain chain forward up to the Baikalsee will finally put themselves.

Large changes are before Australia tooexpect, which drifts rapidly northward and will probably push themselves under the Sunda islands. These grow then to a new mountain chain up, which rises on the Australian plate.

Along the sea-plate East Africa niches of the ditch, the southern part of the large one African Grabenbruchs, the emergence of a new ocean is to be observed, which will connect itself with the Red Sea. As consequence a new continental plate will split itself off Africa and will eastward continue to drift. From the Grabenbruch becomes thena new midocean back become.

The Atlantic will open further along the Mid-Atlantic Ridge. In response the Pacific will become smaller gradually and will in the distant future completely disappear. The Pacific is a leftover of Panthalassa - that ocean, thatonce surrounded the supercontinent Pangäa. The midocean back of the north Pacific was subduziert under North America, before South America is approaching this process briefly. It is to be assumed this will continue.

literature

  • Wolfgang freshness, Martin Meschede: Plate tectonics,Prime US publishing house, 2005.ISBN 3-89678-525-7.
  • Oceans and continents: their origin, their history and structure, spectrum the science publishing house company, Heidelberg, 1985. ISBN 3-922508-24-3.
  • Hans Pichler: Volcanism: Force of nature, climatic factor and cosmic form strength, spectrum the science publishing house company, Heidelberg, 1985. ISBN 3-922508-32-4.
  • Miller, H. (1992): Outline thatPlate tectonics. - Enke, Stuttgart, 149 S.
  • A. Wegener (1912): The emergence of the continents, into geological ones round-look - magazine for general geology, volume III number 4, publishing house of William angel man, 9. July 1912.
  • Rainer child, Xiaohui Yuan: Colliding continents.Physics in our time 34 (5), S. to 213 - 217 (2003), ISSN 0031-9252

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