Lecture-10 1

Lecture #10- Subduction Zones

Lecture-10 2

Subduction Zones

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

Lecture-10 3

Subduction Zones

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

In which of these cases can subduction occur ?

Lecture-10 4

Subduction Zones

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

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.

Lecture-10 5

Subduction Zones

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

Lecture-10 6

Subduction Zones

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

Lecture-10 7

General Picture of Subduction

Lecture-10 8

General Picture of Ocean-Ocean Convergence

Lecture-10 9

General Picture of Ocean-Continent Subduction

Lecture-10 10

Second General Example of Ocean-Continent Subduction

Lecture-10 11

The Termination of a Subduction Zone: Indian-Eurasian Boundary

Lecture-10 12

Subduction Zones

Two dominant features associated with subduction zones are:

– deep earthquakes

– volcanoes

Lecture-10 13

Subduction Zones and Deep Earthquakes

Earthquakes can only occur in brittle material (high viscosity)

It follows that earthquakes happen only in the lithosphere, which is usually 100-200 km thick

However, we observe earthquakes down to a depth of 700 km ???

Lecture-10 14

Subduction Zones andDeep Earthquakes

It turns out the the deep earthquakes we observe (depth > 200 km) are occurring in lithosphere that has been subducted.

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.

Lecture-10 15

Subduction Zones andDeep Earthquakes

Deep earthquakes occur in planar (2D) arrangements called Wadati-Benioff Zones

Seismologists use the locations of deep earthquakes to map out the geometry of subducting lithosphere.

Lecture-10 16

Sometimes Slab Geometry is Simple

Lecture-10 17

Often it is Complicated (South America)

Lecture-10 18

Often it is Complicated (Tonga)

Lecture-10 19

Subduction Zones and Volcanoes

Volcanic activity is associated with all active subduction zones

We see dormant and “fossil” volcanoes at places where subduction used to occur

This type of volcanic activity is fundamentally different than volcanoes at mid-ocean ridges and hot-spots

Lecture-10 20

Subduction Zones and Volcanoes

As oceanic crust ages and moves away from the ridge where it was formed it accumulates sediments which are rich is water

Water also reacts with the newly formed crust and becomes chemically bound to it

Lecture-10 21

Subduction Zones and Volcanoes

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.

Thus, some water gets transported into the mantle while chemically bound to the rocks.

Lecture-10 22

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.

This is called a dehydration reaction.

Subduction Zones and Volcanoes

Lecture-10 23

Subduction Zones and Volcanoes

The free water that has just been liberated immediately starts to percolate upwards and begins to partially melt the asthenosphere above it.

This partially molten material, and water, is much lighter than the surrounding material and begins rising …

Lecture-10 24

Subduction Zones and Volcanoes

When the partially molten material nears the surface it often becomes fully molten because of decreasing pressure – now we call it magma.

The outermost crust at the Earth’s surface is cold, brittle and strong so it is difficult for the magma to break-through

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

Lecture-10 25

Lecture-10 26

Lecture-10 27

Lecture-10 28

Subduction Zone Summary

Subduction zones occur at convergent plate boundaries; they are “burial grounds”

Oceanic material can subduct beneath oceanic material on another plate or beneath continental material on another plate

Continental lithosphere never subducts

Lecture-10 29

Subduction Zone Summary

Subduction zones are the only place where deep (> 200 km) earthquakes occur

The deep earthquakes “line up” on planar structure that delineate the subducting oceanic plates

These seismicity patterns are called Wadati-Benioff zones

Lecture-10 30

Subduction Zone Summary

Volcanoes are also prevalent at subduction zones.

They are formed from water that dehydrates from the subducting slab (at about 100 km) and the percolates upward causing magma formation.

This volcanic material is chemically distinct from MOR volcanoes and hot-spots