Plate Tectonics
Structure of the Earth
Structure of the Earth
The Earth can be
considered as being
made up of a series of
concentric spheres, each
made up of materials
that differ in terms of
composition and
mechanical properties.
Plate Boundaries
Driving Mechanisms of Plate Tectonics
Crust and Lithosphere
Crust: the outermost layer of the earth, a hard outer shell.
Crust beneath the oceans and the continents is different:
Oceanic crust: relatively thin, varying from 5 to 8 km (but
thinner at Oceanic ridges).
Has the average composition of basaltic
rock that is rich in silica and magnesium
(Sima).
Continental Crust:
thicker and more
variable in thickness
than oceanic crust:
Thickness ranges from 20 km to about 75 km (beneath
mountain ranges).
Has the average composition of granitic
rock that is rich in Silica and
Aluminum (Sial).
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Lithosphere:
The topmost layer of
the upper mantle.
The lithosphere has the composition of the upper mantle
(Iron and Magnesium Silicates) but is rigid like the crust.
Beneath the continents
the lithosphere extends
to up to 300 km beneath
mountain ranges.
Lower temperatures and pressures allow the Lithosphere
to be rigid.
The crust and lithosphere “float” on the underlying
mantle.
Very thin (a couple of km) at the Oceanic Ridge; extends
to 80 km depth beneath old oceanic crust that is well away
from the Ridge.
Where the crust and lithosphere are thick (e.g., beneath
continental mountains) they extend deeper into the upper
mantle.
The crust and lithosphere are broken up into 25
Lithospheric Plates
The Mantle includes the Lithosphere.
Unlike the crust, the mantle is dominated by Iron and
Magnesium Silicate minerals.
Upper Mantle: near its melting point so that it behaves
like a plastic (Silly Putty is a reasonable analogy); the
upper mantle material flows under stress.
Upper mantle material flows
by convection; transfers
heat from within the Earth
towards the surface.
Lower Mantle: solid
material, rather than
plastic.
USGS
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The Core: the metallic portion of the Earth; Iron mixed
with small amounts of Nickel.
Plate Boundaries
Outer Core: probably liquid (based on studies of shock
wave passage through the Earth).
Inner Core: solid, made up of cooled liquid core material.
USGS
Plate Boundaries
Oceanic Ridge
The types of boundary between plates are distinguished by
the type of relative plate motion along the boundary:
Oceanic Ridge – Divergence
Oceanic Trench – Convergence
Transform Margins – Horizontal slip
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More-or-less continuous volcanic mountain chain
throughout the world's oceans.
The ridge is a Divergent Plate Margin and divergence
takes place by Sea Floor Spreading.
65,000 km long.
New crust is added from upwelling magma (molten
rock) from the upper mantle.
Average width approx. 1,000 km.
Rise up to 3 km above
the surrounding sea
floor.
Older crust is
pushed laterally
away from the
ridge axis – so that
the sea floor
spreads away from
the ridge axis.
Average depth approx.
2.3 km below sea level.
A kilometre deep valley
runs along much of the
length of the ridge.
From http://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/sea_flr_spread.html
Oceanic crust
becomes older
with distance
from the oceanic
ridge.
Spreading rates (distance per year that two points on
either side of a ridge move apart) vary:
N. Atlantic Ridge
3cm/yr
S. Atlantic
5cm/yr
N. Pacific
12.5cm/yr
E. Pacific
17.5 cm/yr
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Oceanic Trenches
Deep, narrow troughs the border many ocean basins.
http://www.gisdevelopment.net/technology/images/image002.gif
Thousands of kilometers long, 50 to 100 km wide and
several kilometers deep (below sea level).
Longest trench: Peru-Chile trench at 5,900 km.
Deepest trench: Mariana trench (western Pacific);
over 11 km deep.
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Trenches are termed Convergent Plate Margins
because they are locations where plates converge on,
or push against, each other.
Where oceanic crust is subducted back into the upper
mantle.
Melting crust rises and penetrates overlying crust to
form volcanoes.
Material (sediment and basaltic rock) is scraped off
the subducting crust and accreted to the over-riding
crust – termed the subduction complex.
Crust descends at angles from 35 to 90 degrees.
Crust melts as it descends, beginning at 100 to 200 km
depth and has melted completely by 700 km depth.
The zone over which melting takes place is termed the
Benioff Zone.
Island Arcs parallel many oceanic trenches: arcshaped chains of volcanic islands (e.g., Japan) due to
the rising magma from melting subducted crust.
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Convergent Plate Margins
Oceanic Crust-Oceanic Crust
The oldest, densest crust normally descends beneath
the younger crust.
Volcanic islands develop at the surface of the overriding crust (forming Island Arcs).
Oceanic Crust – Continental Crust
Basaltic oceanic crust descends beneath lighter
continental crust.
Coastal mountain chains develop due to compressive
forces and volcanics (e.g., the Andes of South
America).
Magma material rises from descending slab and builds
volcanoes in
the rising
mountains.
Continental Crust-Continental Crust
Neither plate subducts (both too light).
Compressive forces driving plates fold and thrust the
continental margins forming an extensive mountain
belt (e.g., the Himalayan Mountains).
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Transform Plate Margins
Transform faults displace the oceanic ridge.
Plate margins along which the plates slip by each
other. Termed: Transform Faults
On either side of a transform fault plate motions are in
opposite directions.
Spacing of transform faults is proportional to the rate
of spreading.
The faster the rate of
spreading the greater the
distance between
transform faults.
The San Andreas Fault is a transform fault.
The land east of the fault is on
the North American Plate; the
land west of the fault is on the
Pacific Plate.
The eastern side of the fault
moves southeast and the western
side moves to the northwest.
Total movement along the fault
has been 564 km over the past 30
million years (1.9 cm per year).
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Rupturing of the ground surface along the San Andreas Fault.
Wallace creek crossing the San Andreas Fault
To summarize……
http://www.seed.slb.com/en/scictr/watch/living_planet/mountains.htm
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But what drives plate tectonics?
Two main hypotheses:
1. Convection Cells within the upper mantle (first
postulated by Arthur Holmes a year before Wegener
died).
and
2. Ridge push and slab pull.
http://www.gisdevelopment.net/technology/images/image002.gif
Mantle Convection
Giant convection cells within the upper mantle
drag the plates along laterally.
Where convection rises sea floor spreading takes place.
Where the convection cells descend they drag crust
down, causing subduction.
Here’s a link to an animation showing how convection might drive plate
tectonics.
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Ridge push and slab pull
Where new, young crust forms its weight pushes down
slope to drive the plates laterally.
Once the crust has cooled, having been pushed away
form the ridge, it sinks into the upper mantle and
helps to pull adjacent crust along.
This pushing and pulling provides the forces that drive
plate tectonics.
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