subduction zones
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Transcript subduction zones
Lecture #10Subduction 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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