SUBDUCTION ZONE

SUBDUCTION • When two tectonic plates converge often one will get buried or subducted beneath the

other.

• The plate boundary regions where this occurs are called subduction zones.

• There are two types of lithosphere, oceanic and continental, so there are three possibilities at a convergent boundary:

1. oceanic and oceanic

2. oceanic and continental

3. continental and continental

• In which of these cases can subductionoccur ?

• Subduction zones only occur at convergent boundaries between oceans and continents, and oceans and oceans

SUBDUCTION • When oceanic lithosphere converges with continental lithosphere it is the oceanic material that

is always subducted beneath the continental material.

• When the convergent boundary is between two oceans it the older (heavier) plate which usually subducts.

• Examples of an oceanic lithosphere subducting beneath a continental lithosphere:

• South America subduction zone: Nazca plate (oceanic) subducting beneath South American plate (continental)

• Aleutian subduction zone: Pacific plate (oceanic) subducting beneath North American plate (continental) – in Alaska

• Examples of oceanic lithosphere subducting beneath oceanic lithosphereof another plate:

• Marianas subduction zone: Pacific plate subducting beneath Phillipine Sea plate in western Pacific

• Tonga subduction zone: Pacific plate subducting beneath Australian plate in western Pacific.

SUBDUCTION General Picture of Subduction

SUBDUCTIONGeneral Picture of Ocean-Ocean Convergence

General Picture of Ocean-Continent Subduction

MORE ON SUBDUCTION ZONES

• Slab of lithosphere descends back into the

mantle at a deep ocean trench.

• Earthquakes trace the descent of the slab into

the mantle (Benioff Zone).

• Earthquakes can be detected to a depth of

600 km .

Example from Tonga In the western Pacific.

SUBDUCTION ZONES FEATURES

• DEEP OCEAN TRENCH

• VOLCANIC ARC

• WADATI-BENIOFF ZONE

• ACCRETIONARY WEDGE (PRISM)

• FOREARC BASIN

• BACKARC EXTENSION OR COMPRESSION

SUBDUCTION ZONES FEATURESDEEP OCEAN TRENCH:

• An oceanic trench is a narrow, deep trough parallel to the edge of a continent or an island arc

– Deepest parts of the oceans

– Benioff zone earthquake foci begin at trenches and dip landward under continents or island arcs

– Volcanoes found above upper part of Benioff zone arranged in long belts parallel to trenches

– Marked by very low heat flow and large negative gravity anomalies

OUTER TRENCH HIGH (OR OUTER BULGE)

due to flexure of the subducting plate 100-200 km from trench several hundred meters high

SUBDUCTION ZONES FEATURES

VOLCANIC ISLAND ARC

Where oceanic crust is subducted beneath oceanic

crust, melting of the slab produces volcanic island arcs

SUBDUCTION ZONES FEATURES

THE ACCRETIONARY PRISM:

Sediment eroded from the orogenic belt accumulates in the trench and is intensely deformed as the plates converge. Like the wedge of earth ahead of a bulldozer, the sediment thickens until it is capable of resisting further deformation.

SUBDUCTION ZONES FEATURES• FOREARC BASINS: the low-lying region between the volcanic arc and the accretionary

wedge into which sediments, mostly from the arc, are deposited.

SUBDUCTION ZONES FEATURES• BACKARC BASINS - back-arc basin, submarine basin that forms behind an island arc.

Such basins are typically found along the western margin of the Pacific Ocean near the convergence of two tectonic plates. A back-arc basin is formed by the process of back-arc spreading, which begins when one tectonic plate subducts under (underthrusts) another. Subduction creates a trench between the two plates and melts the mantle in the overlying plate, which causes magma to rise toward the surface. Rising magma increases the pressure at the top of the overlying plate that creates rifts in the crust above and causes the volcanoes on the island arc to erupt.

SUBDUCTION ZONES FEATURESREMNANT ARCS: The Mariana Islands and Lesser Antiles of the Caribbean are active volcanic island arcs located above a subducting slab. Both have backarc basins. Beyond the backarc basins are extinct volcanic ridges - the remnant of former volcanic island arcs. The site of subduction has apparently stepped back to it current position

METAMORPHIC ZONESOne of the best indicators of former subduction is the presence of paired metamorphic belts, a belt of typical Greenschist and Amphibolite metamorphism flanked by a belt of Blueschist metamorphism.

METAMORPHIC ZONESGreenschist-Amphibolite Metamorphism:The rising magma from the descending plate heats the crust, resulting in greenschist and amphibolite metamorphism in the igneous arc. At very high temperatures, rocks become very dehydrated; even muscovite mica breaks down to potassium feldspar and amphibole to pyroxene. This sort of metamorphism, called granulite metamorphism, occurs deep in the crust just about everywhere simply due to the normal geothermal gradient. At 25 degrees per kilometer, the temperature at the base of the crust, 40 kilometers deep, is 1000 degrees C. Of course, unusually intense heating can cause it to occur at shallower levels.

Blueschist Metamorphism At high pressures but low temperatures, rocks are metamorphosed to blueschist grade. The reason temperatures are abnormally low is that the descending slab is still cool and helps keep adjac.ent rocks cool as well.

Eclogite Metamorphism At about 100 kilometers depth, pyroxene, olivine and plagioclase recrystallize to a denser form to produce sodium-bearing pyroxene and garnet. The result is one of the most beautiful of rocks, eclogite, a mass of light green pyroxene enclosing pink garnets.

WHAT ROLE DO SUBDUCTION ZONES PLAY IN THE RECYCLING OF EARTH'S CRUST? At subduction zones, the cold oceanic crust gets subducted beneath another oceanic crust, or beneath a continental crust because it is more dense. The subducted crust originated from a constructive plate margin and over millions of years has formed and has moved along to a convergent plate margin to be subducted. This plate then subducts, and melts. Some of this material will rise up to the surface on the non-subducted plate to form a volcano. Over millions of years, the volcano will get eroded and weathered and the eroded material will be transported by rivers etc, into the sea and will be deposited on the sea bed. And this will be subducted again in the future... And that is how it gets recycled.

It is important as it gives us a variety of habitats and ecosystems for animals of all kind to live in. The new oceanic crust and formation of ocean basins can give us seas, to transport people and material in boats, and can provide us, our seafood etc. The volcanoes formed, fertilise soils when the ash falls on the land etc.

THANK YOU