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Earth’s Structure
The Four Layers The Earth is
composed of four
different layers.
The crust is the layer
that you live on,
and it is the most
widely studied and
understood.
The mantle is much
hotter and has the
ability to flow.
The Four Layers
The outer core and
inner core are even
hotter with
pressures so great
you would be
squeezed into a
ball smaller than a
marble if you were
able to go to the
center of the Earth!
The Crust
The Earth's crust is
like the skin of an
apple and is
broken into many
pieces called
plates. It is very
thin in comparison
to the other three
layers.
The Crust
The crust is only about 3-5 miles (8 km) thick
under the oceans (oceanic crust) and
about 25 miles (32 km) thick under the
continents (continental crust).
Continental Crust
Oceanic Crust
The Crust
The plates "float"
on the soft,
semi-rigid
asthenosphere.
Lithospheric Plates
The Mantle
The crust and the upper layer of
the mantle together make up a
zone of rigid, brittle rock called the
Lithosphere.
The Crust
The crust is composed of two rocks.
- The continental crust is mostly granite.
- The oceanic crust is basalt.
Basalt is much denser than the granite.
Because of this the less dense continents ride
on the denser oceanic plates.
The Mantle
The mantle is the largest layer of the Earth.
The middle mantle is composed of very hot
dense rock that flows like asphalt under a
heavy weight.
The Mantle
The movement of the middle mantle
(asthenosphere) is the reason that the
crustal plates of the Earth move.
The Mantle
The crust and the upper layer of
the mantle together make up a
zone of rigid, brittle rock called the
Lithosphere.
The Mantle
The middle mantle "flows" because of
convection currents which are caused by
the very hot material at the deepest part of
the mantle rising, then cooling and sinking
again --repeating this cycle over and over.
The Outer Core
The core of the
Earth is like a ball
of very hot
metals. The outer
core is so hot
that the metals in
it are all in the
liquid state..
The Outer Core
The outer core is
composed of
the melted
metals of
nickel and iron.
The Inner Core
The inner core
of the Earth has
temperatures
and pressures
so great that
the metals are
squeezed
together.
The Inner Core
So much so that
they are not
able to move
about like a
liquid, but are
forced to
vibrate in place
like a solid.
Wegner
In the early 1900s, a German
scientists by the name of
Alfred Wegner became
interested in the idea of
Continental Drift while reading
about different fossil locations
around the earth.
Wegner
Wegner hypothesized that, about 200 million
years ago, there must have been one super
continent at one time that he named
Pangea.
Pangea
Scientists use the similarity of rock types and
fossil types that date to the same age to
support their theory that the continents were
connected to form a super continent.
Pangea
Continental shelfs also
support the idea of
Pangea.
Pangea
The break up
of Pangea
Pangea
Our final piece of
evidence is the
magnetic record
of the ocean floor.
This shows the
pattern of reversal
and we find a near
perfect mirror image
on opposing sides
of the ridge.
Magnetic
Reversal
Sea Floor Spreading
Magnetizing of Sea Floor
Plate Tectonics
Why do the plates move?
• No single idea explains everything but
we can identify several forces that
contribute to the movement of the
plates.
Plate Tectonics
Slab pull is the sinking of the cooled
dense oceanic plates which pulls
on the rest of the plate dragging it
down.
Plate Tectonics
Ridge rises is the material deposited
on the top of the ridge which slides
downs from the rise pushing the
plate downward.
Plate Tectonics
Last, convection
currents results in the
movement within the
mantle which
functions as the
driving force behind
the motion of the
plates.
Tectonic Plates
Today, we know there are a dozen large
lithospheric plates (smaller plates not shown).
Some plates have continents; some don’t. All are
in motion.
African Plate
Indo Australian Plate
Plate Tectonics
Tectonic plates are moving at an average of 1-10
centimeters per year (≈ rate of fingernail growth).
Modified from USGS Graphics
Plate Tectonics
We appear to be headed for another
super continent as North America,
South America, Asia and Australia converge
in the ever shrinking Pacific Ocean.
Plate Tectonics
Notice that earthquakes & volcanoes (red)
correlate with plate boundaries, and the deepest
quakes (blue) are in subduction zones.
Plate Boundaries (Faults)
There are three basic types of plate
boundaries:
1. Divergent
2. Convergent
3. Transform
Divergent Plates
A divergent
boundary forms
when plates are
pulled apart. These
occur at spreading
ocean ridges and in
continental rifts.
Examples:
 Atlantic mid-ocean ridge
 Basin and Range, USA
 African Rift Valley
 Northern Red Sea
Divergent Plates
Mid-Atlantic
Ridge
Spreading Center
New ocean floor is being
created by rising magma
Divergent Plates
Divergent Boundaries usually create a Rift
Valley.
A rift valley is a narrow valley formed as the
plates separate.
Convergent Plate Boundaries
When plates
move toward
each other and
one plate directly
collides with
another plate.
Convergent Plate Boundaries
There are 3 types of
convergent boundaries.
1. Ocean /Ocean
(Marianas)
2. Continent/Continent
(Himalayas)
3. Ocean /Continent
(Cascades)
Ocean / Ocean Convergence
Oceanic crust collides with another oceanic
crust and one plate subducts.
– This has been known to produce a chain
of islands from the resulting molten rock
rising to the surface to create volcanic
islands.
Oceanic/Continental Convergence
Oceanic crust collides with continental crust:
since oceanic crust is denser so it is forced
under the continental crust (subducted).
– Subduction Zone: region along a plate
boundary where one plate moves under
another
Oceanic/Continental Convergence
Deep Ocean Trench: usually forms along a
subduction zone as oceanic crust moves
down and melts, becoming part of the
mantle material.
Some magma rises through the continental
crust and produces volcanic mountains
Continental / Continental Convergence
Continental crust collides with
continental crust: neither plate is
subducted.
Instead, colliding edges are crumpled
and uplifted This produces large
mountain ranges.
Transform Plate Boundaries
In a transform plate boundary, Lithosphere plates
are neither produced nor destroyed, two plates
just slide past one another.
This is also called a strike-slip fault.
Strike-slip fault between two spreading ridges allows the
two plates to move apart.
Transform Plate Boundaries
Strike-slip fault between two spreading ridges
allows the two plates to move apart.
The San Andreas Fault
in California is an
example of this type of
fault.