Download plate tectonics

Constructive Forces
Building the Crust
I. Plate Tectonics
A. Moving Plates
- current theory is that Earth’s surface is
composed of sections of the lithosphere
called plates
- Lithosphere is composed of the crust and
the rigid mantle
- The study of the movement of these plates
is called plate tectonics
- original concept was proposed by Alfred
Wegener as part of his theory of continental
drift
Plate Tectonics (contd.)
- Pangaea - original
supercontinent
- Concept that all of
the continents were
combined at one
time to make one,
giant landmass
- Geologic forces
caused Pangaea to
split up
Plate Tectonics (contd.)
B. Plate Thickness
- lithospheric plates
are approximately
100 km thick
- ocean basin is
mostly basalt
- continental crust is
mostly granite
Plate Tectonics (contd.)
C. Plate Movement
- asthenosphere partially melted layer
of the mantle that
the lithosphere
floats on
- convection currents
occur inside the
asthenosphere
Plate Tectonics (contd.)
- hot material moves
toward the crust
because it is less dense
- As material rises, ir
cools and becomes
more dense
- More dense material
sinks back towards the
bottom of the
asthenosphere
- Movement of mantle
material pushes the
lithospheric plates that
float on top of it
Plate Tectonics (cpnyd.)
D. Evidence of Plate
Tectonics
1. Africa and S. America
fit together like the
pieces of a jigsaw
puzzle
2. Earthquakes and
volcanoes occur along
plate boundaries
Plate Tectonics (contd.)
3. Paleomagnetism oceanic bedrock
shows patterns of
magnetic reversal that
could only be caused
by seafloor spreading
4. Seafloor spreading oceanic crust is
growing at the midocean ridges - crust
increases in age as
you move away from
the ridges
Plate Tectonics (contd.)
II. Types of Plate
Boundaries
A. Divergent - where two
or more plates are
moving away from one
another
- occurs primarily at the
mid-ocean ridges
- ex.: Mid-Atlantic Ridge
- Great Rift Valley
(Africa)
Plate Tectonics (contd.)
B. Convergent - where two
or more plates are
colliding
1. Subduction Boundary
- where continental
and oceanic plates
collide
- oceanic crust is more
dense than continental
crust, so it sinks
underneath the
continental crust and
is pushed into the
mantle
Plate Tectonics (contd.)
- as the oceanic crust
melts, pockets of magma
are created
- Magma rises through the
crust at the boundaries
and forms a volcanic
mountain on the
overriding continental
plate
- Also forms deep sea
trenches where the
oceanic crust plunges into
the mantle
- Ex.: Pacific NW - Cascade
Mtns.
Plate Tectonics (contd.)
C. Transform Boundary
- where plates slide past
one another
- Can be right-moving or
left-moving
- frequently create faults cracks in the crust where
movement occurs
- May see faults further
inland from the plate
boundary
- Ex.: San Andreas fault,
CA
Plate Tectonics (contd.)
III. Moving Plates
- plate movement is
evidenced from the
occurence of
earthquakes and
volcanoes
- different hypotheses
about the causes of
plate movement
Plate Tectonics (contd.)
A. Mantle Convection
- asthenosphere is
semi-solid, like slush
or putty
- as heat radiates from
the mantle, it causes
movement in the
asthenosphere
- Creates convection
currents
- Movement is caused
by changes in density
Plate Tectonics (contd.)
- Hotter material is
less dense - floats
upward
- rises, cools, and
sinks back down
towards the bottom
of the
asthenosphere
Plate Tectonics (contd.)
B. Ridge Push
- occurs at mid-ocean
ridges
- As new rock is formed, it
is less dense and hotter
than older rock
- New rock slides down
the side of the ridge
- Cools and hardens becomes more dense
- Creates force pushing on
lithosphere from the
force of gravity pulling on
the denser rock
Plate Tectonics (contd.)
C. Slab Pull
- at subduction boundaries,
one plate is colder and
more dense than the other
- One plate sinks into the
mantle
- The edge of the sinking
plate is still colder and
more dense than the
mantle
- As the plate sinks into the
mantle, it pulls the rest of
the plate behind it
- Considered to be stronger
than ridge push
Plate Tectonics (contd.)
IV. Continental Growth
A. Craton - ancient
continental cores
- Continents were
originally much
smaller
- large amounts of
crustal material has
been added to the
continental plates over
time
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Plate Tectonics (contd.)
B. Sources of Crustal
Material
1. Deep sea
sediments
2. River sediments
(both make
sedimentary rock)
3. Volcanic eruptions
Plate Tectonics (contd.)
C. Thin-skinned
thrusting
- pushing of thin,
horizontal sheets of
rock from
continental margins
over great distances
along level fault
surfaces
Plate Tectonics (contd.)
D. Terranes - large
block of a
lithospheric plate
that has been
moved and attached
to the edge of a
continent