Subduction ( lat. Sub "under" and ducere "lead") is a fundamental process of plate tectonics . The term describes the immersion of oceanic lithosphere at the edge of a tectonic plate into the part of the earth's mantle below , while this plate edge is simultaneously passed over by another, adjacent lithospheric plate. When the plate descends, its crustal rocks undergo a metamorphosis . The density of the submerged part increases so that it can sink deep into the earth's mantle.
terms and definitions
For subduction to take place in this sense, two plates must move towards each other. Their contact area is called a converging plate boundary or, because crust material is "destroyed" there, a destructive plate boundary . The descending plate is called the lower plate , the overriding upper plate . The part of the lower plate that is submerged in the earth's mantle is called a slab (English for "(stone) plate"). The entire area of the lithosphere that is directly influenced by subduction is called the subduction zone . There special tectonic and magmatic phenomena occur.
Subduction, as it takes place today, requires a solid (but plastically deformable) and relatively "cold" mantle . This is why it has probably only appeared since the Mesoarchean and not since the first earth's crust was formed in the Hadadic .
For a submerged oceanic lithosphere to sink into the deep mantle, a transformation of the basic oceanic crust into eclogite is probably necessary (see causes and mechanism of subduction and drainage and metamorphosis of the submerged plate ). A geothermal gradient in which the crust material in the mantle can transform into eclogite and thus subduction and thus "real" plate tectonics is only possible, apparently continuously and everywhere on earth only for about 3 billion years ago (middle Mesoarchic). Before that, the temperature in the upper mantle was too high, so that the subducted crust was already too much drained at a relatively shallow depth. When the depth at which the pressure was sufficiently high was reached, there was no longer any water available for the advective ion transport necessary for eclogitization , so that eclogite could no longer arise.
Causes and Mechanism
Oceanic lithosphere only lingers for a relatively short time on the surface of the earth's surface, viewed in terms of geological time, because it has less buoyancy than the continental lithosphere and continues to lose buoyancy with increasing age. At present there is therefore no oceanic crust that is older than 180 million years ( Jurassic ), because older material has already been subducted again. An exception is the eastern Mediterranean, which is underlain by remnants of the oceanic crust of the Neotethys , which are very likely to have a Permian to Triassic age (230 to 280 million years).
The subduction takes place in the subduction zones, where the edge of a lithospheric plate bends downwards at a more or less steep angle. In many places on earth, such "hanging" plate ends (slabs) have been detected using seismological methods.
Submerging increases the temperature and pressure in the slab, which triggers rock transformations, especially in the crustal rock (see below ), whereby its density increases even further instead of decreasing due to the warming. The oceanic lithosphere thus retains a higher density than the material of the sublithospheric mantle from which it once emerged and does not return directly to it. Rather, the slab gravitationally - due to its gravity - pulls the part of the plate that is still on the surface of the earth's body even at greater depth. This driving force of the further subduction is called slab pull ("plate pull "). The "plate pull" is considered to be a possible driving factor for plate drift and thus for the entire plate tectonics. In what depth and in what way the sinking of a slab ends and what happens to the slab afterwards is not yet fully understood. In any case, seismic anomalies, which are interpreted as signatures of sinking slabs, were found near the core-mantle boundary .
If material disappears from the earth's surface somewhere, new material must appear somewhere else, because the surface area of the globe is constant. Therefore, in addition to the material sinks of the subduction zones, there are also material sources, above all a similarly extensive, earth-spanning network of spreading zones (see also → Central Ocean Ridge ), in which continually penetrating asthenospheric material forms new oceanic lithosphere. In addition, so-called mantle plumes rising from the core-mantle boundary also convey hot mantle material to the underside of the lithosphere and form hotspots there , which trigger a special form of volcanism independent of plate boundaries . Subduction, plate drift, ocean floor spreading and mantle plumes are expressions of the earth's mantle convection .
Beginning, course and end
Two mechanisms are considered for the creation of a subduction zone:
- Spontaneous subduction . The lithosphere basically consists of two layers. The upper layer is the earth's crust and the lower is the lithospheric mantle. The crust of the oceanic lithosphere has a somewhat lower and the lithospheric mantle a somewhat higher density than the underlying asthenosphere . If it is still young and relatively warm, the oceanic lithosphere has enough buoyancy in total to "swim" on the denser asthenosphere and thus remain on the surface of the earth. Because it cools with increasing age and increasing distance from the expansion zone and therefore becomes denser and the lithospheric mantle also grows through the accumulation (accretion) of asthenospheric material, its buoyancy disappears over the course of millions of years, so that the old, too heavy part of such a plate eventually begins to descend spontaneously into the asthenosphere. This happens either on a passive continental margin or on an already existing fracture zone .
- Induced subduction . Another plate is actively pushing itself over the relatively young oceanic lithosphere, driven by tensions that can originate from areas of the lithosphere that are sometimes very distant, for example from an extensive rift system .
Once started, the subduction is increasingly driven by the gravitational pull (slab pull) of the already submerged part of the plate (slab). If the formation of new lithosphere at the oceanic spreading zone of the sub-plate takes place more slowly than subduction, this leads to a narrowing of the corresponding ocean basin (in a plate-tectonic context, “ocean basin” is always understood to mean an area between continental margins or convergent plate margins underlaid by oceanic lithosphere, which is often not the case geographical understanding of an ocean ). As long as this difference persists, the spreading zone with its mid-ocean ridge approaches the subduction zone more and more and is ultimately also subducted itself. In English this is known as spreading ridge subduction . In such a case, the subduction is slowed down and the edge of the top plate is deformed more than usual. Gaps in the slab along the subducted part of the spreading axis ( slab windows ) can meanwhile increase the magmatism on the top plate. Because no more material is added to the oceanic lithosphere of an ocean basin after the expansion zone has been completely subduced, the speed of the narrowing increases.
If the spreading axis runs largely parallel to the edge of the upper plate and the plate on the other side of the back does not have an excessive movement component directed transversely to the edge of the upper plate, the impact of a mid-ocean ridge on a subduction zone can lead to the end or at least to a longer-lasting interruption of the subduction . The reason for this is that the extremely young oceanic lithosphere just beyond the ridge has a very low density and is therefore difficult to subduct, especially because it has no slab that could exert a gravitational pull. The same thing happened in the course of the Cenozoic, at least in sections, on the western edge of the North American Plate.
Ocean basins in the sense of plate tectonics are in fact always bounded by lithospheric areas that have more differentiated - that is, “non-oceanic” - and relatively thick, towering crusts. It is either granitic continental crust or igneous island arcs with less silicon . To simplify matters, all of these areas can be viewed as larger or smaller continental blocks. When an ocean basin is narrowed by subduction, the basin edges come closer and closer. Finally, when the ocean basin closes completely, the continental block of the basin rim of the subplate gets into the subduction zone and opposes the plate movement with increasing resistance, because a continental block with its high buoyancy cannot be deeply subducted. This leads to a collision of the continental blocks, including mountain formation and the demolition of the slab. The subduction zone has become a collision zone .
If continental crust is also subducted in the end phase of a subduction or the early phase of a collision, this tends to rise again due to its significantly lower density. Such a process is commonly known as exhumation . The sinking of crustal complexes at a depth of 100–200 km and their subsequent exhumation occurs regularly in mountain formations. Today, crust sections are known that have risen again from a depth of over 350 km.
The collision of two continental blocks severely slows down the relative movement of the plates involved and finally brings it to zero. This has an impact on the movement pattern of the neighboring plates, which are now exposed to a new geometric constraint. Continent-continent or continent-island arc collisions therefore always trigger a more or less extensive reorganization of the plate movements. As a rule, the larger the collision partners, the greater their extent.
Construction of a subduction zone
There are two types of subduction: In ocean-continent subduction , oceanic lithosphere is pushed under a continental block because of its higher density ; one speaks here of an active continental margin . In the case of ocean-ocean subduction, on the other hand, the oceanic lithosphere dips below the oceanic lithosphere of another plate.
In the submerged area of the oceanic crust, deep sea channels such as B. the deepest submarine channel on earth with up to 11,034 m , the Mariana Trench . In addition, a volcanic mountain arises on the continental block above the subduction zone, such as B. the Andes . Occasionally, the edge of the top plate can also be raised, as in the example of the central Andes. If only oceanic lithosphere is involved in the subduction, an island arc is created over the subduction zone .
Immersion angle and subduction speed of the sub-plate influence the tectonic processes in the hinterland of the island arc or the continental volcanic mountain range, the so-called backarc (literally: "back of the arch"). If the subduction speed is low and the immersion angle is steep (> 50 °), the lithosphere in the backarc often expands with the formation of a backarc basin , which can lead to the formation of a small ocean basin with a mid-ocean ridge (backarc spreading). Backarc spreading is particularly common recently in the ocean-ocean subduction zones of the western Pacific ( Mariana-type convergence ). If the subduction speed is high and the angle of immersion is flat (<30 °), the back arc area is compressed and a fold and thrust belt is created there . This is most recently the case in the ocean-continent subduction zones on the eastern edge of the Pacific ( Andean-type convergence ).
Subduction zones are at risk of earthquakes due to the opposing plate movements . When diving, the two plates get caught and build up considerable tensions in the rock, the sudden release of which on the surface of the earth can lead to earthquakes and submarine tremors (also known as seaquakes) with tsunamis . Such an earthquake in a subduction zone occurred on December 26, 2004 in the Sunda Trench (see also 2004 Indian Ocean earthquake ). The severe Tōhoku quake of March 11, 2011 , which was accompanied by a devastating tsunami, was also caused by subduction. The zone in which these earthquakes occur is called the Wadati-Benioff zone .
Drainage and metamorphosis of the descending plate
Oceanic lithosphere contains large amounts of water. This is either unbound - z. B. in the crevices of faults or in the pore space of the marine sediments that have accumulated on it - or bound in minerals. The water and other highly volatile compounds (such as CO 2 ) are released during the subduction process through the increase in pressure and temperature in several phases in the form of so-called fluids ( devolatilization ): Always "leave" when the pressure is increased minerals regain their field of stability and release volatile element compounds (e.g. water). This devolation is a sub-process of the gradual metamorphosis of the subducted rocks of the oceanic crust. Depending on the prevailing temperature conditions, MORB basalt , dolerite and gabbro , as well as the rocks spilite and amphibolite formed in the course of the ocean floor metamorphosis , run through various so-called metamorphic paths . In relatively “warm” subduction zones, a direct transformation into eclogite (a high-pressure rock consisting of the clinopyroxene mineral omphacite and garnet , as well as jadeite ) takes place at a depth of about 50 kilometers . At relatively "cold" subduction zones first one takes place blue shale facies metamorphism and the Eklogitisierung only takes place at depths greater than 100 kilometers. At subduction zones with strong heat development due to the shear forces that occur, first green slate facial metamorphosis occurs in the upper part of the lithosphere of the subducted plate and, with increasing depth, then amphibolite, followed by granulite - and finally eclogite facial metamorphosis at depths of less than 100 kilometers. The olivine of the peridotitic mantle lithosphere of the subducted plate is converted into spinel at depths between 350 and 670 kilometers , and from depths of more than 670 kilometers the conversion into perovskite and magnesiowustite takes place . With all these rock and mineral transformations there is also an increase in density. Only through the metamorphoses and the corresponding increase in density is a really deep sinking of the overturned oceanic lithosphere into the asthenosphere and later into the lower mantle possible.
Above all, the fluids released at greater depths during the eclogitization of crustal rocks, which originate from the decay of hornblende and lawsonite or clinozoisite as well as glaucophane and chlorite , are apparently also the cause of the volcanism in subduction zones.
As a direct result of subduction
The fluids released during the metamorphosis of the submerged plate - at the temperature and pressure that prevail there, water is not liquid , but supercritical - the melting point of the surrounding rock is lowered and anatexis (partial melting) of the between the upper plate and slab occurs protruding part of the asthenosphere, the so-called mantle wedge . If the required temperature and pressure values are reached, the deep areas of the accretion wedge and, in very rare cases, even the slab can partially melt . The resulting magma rises, but often remains stuck within the crust of the upper plate and solidifies there to form large plutons .
That part of the magma that completely penetrates the crust forms characteristic chains of volcanoes . When oceanic lithosphere descends below other oceanic lithosphere, island arcs form on the upper plate , such as B. the Aleutian and Kuril Islands . If, on the other hand, the oceanic lithosphere dips below the continental lithosphere, continental volcanic chains are formed, as in the Andes or in the Cascade Mountains . Because the oceanic lithosphere is dehydrated in phases as the pressure rises, if the angle of immersion is flat enough, several volcanic lines follow one another, which run parallel to each other and to the subduction front.
The andesitic melts typical of subduction zones give rise to stratovolcanoes which, due to the viscosity of their magmas, are prone to explosive eruptions. Well-known examples of particularly explosive eruptions in the recent past are those of Krakatau in 1883, Mount St. Helens in 1980 and Pinatubo in 1991.
So-called petit spots can also appear on the subplate during subduction . In 2006, these approximately 50-meter-high volcanoes were observed for the first time on a descending plate in the Japan Rift at a depth of 5000 m. The bending of the descending plate presumably creates cracks and crevices through which magma can then rise from the asthenosphere to the ocean floor.
The volcanic mountains and arches of the islands in the numerous subduction zones at the edges of the Pacific Plate form what is known as the Pacific Ring of Fire .
As an indirect consequence of subduction
Various models are being discussed today which consider subduction to be the ultimate cause of intraplate volcanism (see also hotspot ). The subduction creates chemical and thermal heterogeneities in the earth's mantle, water is brought into the earth's mantle, which lowers the solidus temperature of the rocks and can cause them to melt.
Primary deposits typical for subduction zones are porphyry copper deposits or so-called iron-oxide-copper-gold deposits (short: IOCG deposits). There are also secondary, sedimentary deposits, such as B. the salars of the Andean region; These are salt flats in which, over millions of years, lithium leached from weathering volcanic material has accumulated in minable concentrations.
- SFB 574 - Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters
- From the sea floor to the volcanic peaks Video about the work of the Kiel Collaborative Research Center (SFB) 574 Volatiles and Fluids in Subduction Zones from GEOMAR I Helmholtz Center for Ocean Research Kiel.
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