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Lecture #08Subduction Zones
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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
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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 ?
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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.

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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
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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
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General Picture of Subduction
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General Picture of Ocean-Ocean
Convergence
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General Picture of Ocean-Continent Subduction
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Second General Example of
Ocean-Continent Subduction
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The Termination of a Subduction
Zone: Indian-Eurasian Boundary
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Subduction Zones
 Two
dominant features associated with
subduction zones are:
deep earthquakes
– volcanoes
–
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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 ???
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Subduction Zones and
Deep 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.
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Subduction Zones and
Deep 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.
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Sometimes Slab Geometry is Simple
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Often it is Complicated (South America)
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Often it is Complicated (Tonga)
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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
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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
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Subduction Zones and Volcanoes
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.
 Some
 Thus,
some water gets transported into the
mantle while chemically bound to the rocks.
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Subduction Zones and Volcanoes
 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.
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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 …
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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
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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
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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 WadatiBenioff zones
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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
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