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Floating…on the Plates
of the World
By: Rachel
Aldrich
The Interior Structure of the Earth
The Earth is made up of four main layers:
► Crust
► Mantle
► Outer core
► Inner core
The Crust
Continental crust forms Earth’s
continents, and is generally 30-50
km thick. Some continental crust
is as old as four billion years.
Oceanic crust
is found under
the Earth’s
oceans, and is
only 5-10 km
thick. All
oceanic crust is
younger than
200 million
years old.
The Mantle
The mantle is located under Earth’s crust. The upper
layer of the mantle, which moves like a rigid block that
carries the crust with it, is called the lithosphere.
The
asthenosphere
is below the
lithosphere. It acts
like a stiff liquid on
which the less
dense plates of
the lithosphere
“float.”
Earth’s plates are
composed of the
lithosphere!
Evidence for Movement
Fossil Evidence
Fossils of the
freshwater reptile
Mesosaurus were found
in South American and
Africa
Remains of
Glossopteris, a tropical
fern, have been found
in Australia, India,
Antarctica, South
America and Africa
How could these organisms be
found in so many different
places that are separated by
oceans today?
Puzzle Fit
Earth’s continents
fit together like the
pieces of a puzzle.
They must have
been joined
together at
some time in
the past!
Climate Clues
Fossils of tropical
plants were found
on the island of
Spitsbergen in the
Arctic Ocean.
All of these places must have had
tropical climates in the past and
were located close to the equator.
Glacial evidence has
been discovered in
Africa, South America,
India and Australia.
Rock Evidence
Parts of the
Appalachian
Mountains of
the eastern
U.S. are similar
to mountains
found in
Greenland and
western
Europe.
Rock
structures
found in
eastern South
America are
similar to
those found in
western
Africa.
These areas must
have been connected
in the past!
More Evidence for Movement
Magnetic striping on
GPS satellites
the ocean floors
monitor the
shows that the
movement of
seafloor spreads apart
Earth’s
at mid-ocean ridges.
lithospheric
Magnetite rocks obtain
plates.
the polarity of Earth’s
magnetic field as new
ocean crust forms.
The stripes are a
record of past
magnetic pole
reversals, or
paleomagnetism.
Scientists drilling into the ocean floor found
that the age of rock becomes increasingly older
further from the center of the ridge.
Plate Boundaries
There are three types of plate boundaries:
1.
2.
3.
Divergent
Convergent
Transform
Divergent Plate Boundaries
These are places
where Earth’s
lithospheric plates
are moving away
from each other.
Mid-ocean
ridges are
found in all
of the
ocean
basins.
As the plates
spread apart a
crack, or rift
forms. Magma
rises up to fill the
crack, creating
mid-ocean ridges.
An example is
the MidAtlantic Ridge,
a 10,000-mile
long stretch of
underwater
volcanoes that
runs through
the Atlantic
Ocean.
Convergent Plate Boundaries
Recall that there are
two types of crust:
•Continental
These are places where
Earth’s plates a moving
toward each other.
•Ocean
Therefore, there are
three types of
convergent boundaries:
•ocean/ocean
•ocean/continental
•continental/continental
When two oceanic
plates converge, one
plate stays at the
surface, and the other
dives down beneath it
at some angle in a
process called
subduction.
Ocean trenches and island
arc volcanoes are formed at
ocean/ocean convergent
boundaries.
When an ocean plates
collides with a
continental plate, the
denser ocean plate
slides under the lessdense continental
plate.
Island arc
volcanoes and
ocean trenches
form at the
edge of the
continental
plate.
As an oceanic plate travels from a mid-ocean ridge to a
subduction zone, hundreds of meters of sediment is
scraped off the ocean floor and added to the edge of the
continental plate. The growth of a continent along its
edge in this way is called continental accretion.
When two continental plates collide, no subduction takes place
because the lithosphere is much less dense than the mantle. Instead,
the plates collide and crumple up, forming mountain ranges.
The zone where two continents have
met and become welded into a
single continent is called a suture
zone. An example of this can be
found where the Indian Plate
collided with the Eurasian Plate,
creating the Himalayas.
Transform Plate Boundaries
These are places where
plates slide past each
other.
The San Andreas Fault is a
transform plate boundary that
forms between the North
American Plate and the Pacific
Plate.
How Do the Plates Move?
Wegener and other
scientists had
discovered evidence
to support the idea
of moving plates
and the existence of
Pangaea. However,
they could not
explain HOW or
WHY the plates
moved.
Flow of Matter and Energy within the Earth
The temperature of
the Earth increases
with depth. Sources
of Earth’s heat
include:
•The original heat of
Earth’s formation
•Impacts by
meteorites early in
Earth’s history
•Decay of radioactive
elements
Temperature affects the density of materials. As rocks in Earth’s
mantle are heated enough, their density increases. The less dense
rock rises to the surface slowly over time. When plates of the
lithosphere diverge, lava oozes out of the cracks, creating new
seafloor. As the lithospheric plates continue to diverge over time, the
new seafloor moves further away from the ridge and cools. As it cools,
it becomes more dense and sinks back into the mantle, where it is
heated and rises again.
This pattern of heating and
rising, cooling and sinking in
Earth’s mantle is known as
thermal convection. Many
scientists today believe that
these convection cells in the
mantle are the driving force
for the movement of
lithospheric plates.