Dynamic Crust

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Unit III Dynamic Crust
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The outer portion of the Earth is called the
crust.
The crust is a layer of rock that is between
10-50 km. thick.
This crust is broken up into 12-14 plates that
cover the entire surface.
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The mantle makes up the greatest volume of the
Earth
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Earths Interior
The crust and solid upper mantle are called the
lithosphere.
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VIDEO-Science of Earthquakes
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The Plates that cover the Earth
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You live on the North American Plate
New York is in the center of the plate
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How do the plates of the crust interact?
The plates of the Earth interact in three ways
1. Convergent plate boundary
2. Divergent plate boundary
3. Transform plate boundaries
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Convergent and Divergent Video
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How are the plates of the Earth forced around?
CONVECTION
Convection currents in the ASTHENOSPHERE drives
the plates
This happens because the HOT magma rises to the
surface due to density differences. Hot material
is less dense and will RISE.
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A simple convection cell
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Convection Cells
Heat rises here
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I. Convergent plate boundaries
At this type of boundary, two lithospheric PLATES
MOVE TOWARDS EACH OTHER.
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It is known that most of the continental land
masses are composed of the igneous rock GRANITE.
Most of the oceanic crust is composed of the
igneous rock BASALT.
BASALT is a mafic rock, therefore it is more
dense than the continental rock.
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Convergent boundaries come in three forms
Continental
Oceanic
Continental
Continental
Oceanic
Oceanic
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Continental crust converges with oceanic crust
The more dense oceanic crust will dive beneath
the less dense continental crust.
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This happens because the denser crust will always
move below something with a lower density.
Continental
Oceanic
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This entire area is also known as a subduction
zone.
Because one plate dives beneath another it will
be exposed to extreme heat and pressure. The
rock will begin to melt. The HOT liquid magma
will rise to the surface.
On land, VOLCANOES form.
In the ocean just off shore, deep TRENCHES form.
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Volcanoes on land
Deep trenches in the ocean
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Remember, the volcanoes form because the less
dense magma will rise to the surface and
eventually explode.
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Subduction

• http//www.wwnorton.com/earth/egeo/flash/2_9.swf

• http//www.wwnorton.com/earth/egeo/flash/2_9.swf

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Continental crust collides with continental crust
The land will fold up and tall mountains will
form. This happened when India crashed into
Asia. The Himalayan Mountains were formed.
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Himalayan Mountain Chain Formed
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Himalayan video
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Oceanic crust collides with oceanic crust
One plate will have a higher density and will
dive beneath the other. Liquid magma will rise
to the surface and a Volcanic Island Arc will
form. You will also get a deep ocean trench.
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Magma melts and rises to the surface and islands
form
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II. Divergent Boundary
Here, two lithospheric plates are moving apart
from one another and new ocean crust (BASALT)
fills in.
Names for these spreading center are Ridges,
Rifts and Rises
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These types of boundaries are usually found in
the ocean.
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A famous divergent boundary is called the
MID-ATLANTIC RIDGE which is located right down
the middle of the Atlantic Ocean between South
America and Africa.
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Similarities between divergent boundaries and
volcanoes
Both have rising magma
Differences between divergent boundaries and
volcanoes
Divergent boundaries are not explosive.
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New Ocean Crust

• http//www.wwnorton.com/earth/egeo/flash/2_7.swf

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Mid Atlantic Ridge
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Sea Floor Spreading Video

• http//www.wwnorton.com/earth/egeo/flash/2_5.swf

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We have substantial proof of this spreading by
looking at the magnetic qualities of the rocks
found near the ridge.
They show magnetic alignment that coincides with
the magnetic field of the Earth.
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Magnetic Reversal
http//www.wwnorton.com/earth/egeo/flash/2_3.swf
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The rock at the ridge is fresh magma from beneath
the Earth. When it hits the cold seawater it
hardens. It is extrusive and contains a lot of
pyroxene and olivine
The youngest rock is seen here.
Where would you find the oldest rock?
Out towards the continents.
The ocean floor is spreading at about the rate of
2 cm/yr.
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Youngest Ocean Rock
Oldest Ocean Rock
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Usually as you go deeper in the ocean the
temperature decreases. If you are near a
divergent boundary the magma is so hot, is the
water around the rift hot?
Yes, remember the rock coming out of the ground
is around 2000 degrees Celsius. You will find a
unique underwater habitat there.
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III. Transform Boundary
At a transform boundary the two plates slide past
one another. This is not a smooth process. The
plates try to move and get stuck. Then when
there is enough pressure, the two plates break
free and the ground shakes. This is called an
earthquake.
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Transform Boundary
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Transform Boundary

• http//www.wwnorton.com/earth/egeo/flash/2_8.swf

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An example of a transform boundary can be seen
in California. It is called the San Andreas
Fault.
The fault runs from northwest to southeast
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Plate Boundary Review

• http//www.wwnorton.com/earth/egeo/flash/2_6.swf

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Plate Tectonics Video
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All of these interactions we have talked about
are called Plate Tectonics. Three proofs are
1. The continents appear to fit together like a
puzzle.
2. Fossils have been found in southern South
America and southern Africa that match.
3. Rock layers match in those same locations.
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1. The continents fit together like a puzzle.
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2. Fossil Evidence
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3. Sedimentary Rock Layers
South America
Africa
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Continental Drift Animation

• http//www.wwnorton.com/earth/egeo/flash/2_1.swf

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The large super-continent that was present before
the breakup was called PANGEA.
Over the past few hundred million years the
continents have been moving further apart.
Will they ever come back together?
Scientists believe so. As the Atlantic grows,
the Pacific shrinks. Eventually all of the
continents will be joined again.