1. Lecture-10 1 Lecture #10- Subduction Zones

2. Lecture-10 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath the other F The plate boundary regions where this occurs are called subduction zones

3. Lecture-10 3 Subduction Zones F There are two types of lithosphere, oceanic and continental, so there are three possibilities at a convergent boundary: –oceanic and oceanic –oceanic and continental –continental and continental F In which of these cases can subduction occur ?

4. Lecture-10 4 Subduction Zones F Subduction zones only occur at convergent boundaries between oceans and continents, and oceans and oceans F When oceanic lithosphere converges with continental lithosphere it is the oceanic material that is always subducted beneath the continental material. F When the convergent boundary is between two oceans it the older (heavier) plate which usually subducts.

5. Lecture-10 5 Subduction Zones F 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

6. Lecture-10 6 Subduction Zones F Examples of oceanic lithosphere subducting beneath oceanic lithosphere of 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

7. Lecture-10 7 General Picture of Subduction

8. Lecture-10 8 General Picture of Ocean-Ocean Convergence

9. Lecture-10 9 General Picture of Ocean-Continent Subduction

10. Lecture-10 10 Second General Example of Ocean-Continent Subduction

11. Lecture-10 11 The Termination of a Subduction Zone: Indian-Eurasian Boundary

12. Lecture-10 12 Subduction Zones F Two dominant features associated with subduction zones are: – deep earthquakes – volcanoes

13. Lecture-10 13 Subduction Zones and Deep Earthquakes F Earthquakes can only occur in brittle material (high viscosity) F It follows that earthquakes happen only in the lithosphere, which is usually 100-200 km thick F However, we observe earthquakes down to a depth of 700 km ???

14. Lecture-10 14 Subduction Zones and Deep Earthquakes F It turns out the the deep earthquakes we observe (depth > 200 km) are occurring in lithosphere that has been subducted. F Deep earthquakes do not occur in any place except for subduction zones since this is the only place where brittle material (lithosphere) exists below its normal depth.

15. Lecture-10 15 Subduction Zones and Deep Earthquakes F Deep earthquakes occur in planar (2D) arrangements called Wadati-Benioff Zones F Seismologists use the locations of deep earthquakes to map out the geometry of subducting lithosphere.

16. Lecture-10 16 Sometimes Slab Geometry is Simple

17. Lecture-10 17 Often it is Complicated (South America)

18. Lecture-10 18 Often it is Complicated (Tonga)

19. Lecture-10 19 Subduction Zones and Volcanoes F Volcanic activity is associated with all active subduction zones F We see dormant and “fossil” volcanoes at places where subduction used to occur F This type of volcanic activity is fundamentally different than volcanoes at mid-ocean ridges and hot-spots

20. Lecture-10 20 Subduction Zones and Volcanoes F As oceanic crust ages and moves away from the ridge where it was formed it accumulates sediments which are rich is water F Water also reacts with the newly formed crust and becomes chemically bound to it

21. Lecture-10 21 Subduction Zones and Volcanoes F Some sediment layers get “scraped off” the oceanic crust when it subducts at a trench; however a large amount of water is retained in the subductiong slab of oceanic material. F Thus, some water gets transported into the mantle while chemically bound to the rocks.

22. Lecture-10 22 F At about a depth of 100 km the temperature becomes hot enough that a chemical reaction takes place and the water is liberated from the material which carried it down into the mantle. F This is called a dehydration reaction. Subduction Zones and Volcanoes

23. Lecture-10 23 Subduction Zones and Volcanoes F The free water that has just been liberated immediately starts to percolate upwards and begins to partially melt the asthenosphere above it. F This partially molten material, and water, is much lighter than the surrounding material and begins rising …

24. Lecture-10 24 Subduction Zones and Volcanoes F When the partially molten material nears the surface it often becomes fully molten because of decreasing pressure – now we call it magma. F The outermost crust at the Earth’s surface is cold, brittle and strong so it is difficult for the magma to break-through F Thus magma will often pond beneath volcanoes in a magma chamber until the pressure becomes high enough for it to break though the outermost crust and erupt

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28. Lecture-10 28 Subduction Zone Summary F Subduction zones occur at convergent plate boundaries; they are “burial grounds” F Oceanic material can subduct beneath oceanic material on another plate or beneath continental material on another plate F Continental lithosphere never subducts

29. Lecture-10 29 Subduction Zone Summary F Subduction zones are the only place where deep (> 200 km) earthquakes occur F The deep earthquakes “line up” on planar structure that delineate the subducting oceanic plates F These seismicity patterns are called Wadati- Benioff zones

30. Lecture-10 30 Subduction Zone Summary F Volcanoes are also prevalent at subduction zones. F They are formed from water that dehydrates from the subducting slab (at about 100 km) and the percolates upward causing magma formation. F This volcanic material is chemically distinct from MOR volcanoes and hot-spots