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Plate Tectonics
I. Continental Drift
A. Alfred Wegener
1. First proposed the hypothesis,
(1915)
2. German Meteorologist &
Geophysicist
3. Published The Origin of
Continents and Oceans
B. Continental Drift Hypothesis
1. Super continent called Pangaea
began breaking apart about 200
million years ago.
2. Continents “drifted” to present
positions
3. Continents “broke” through
oceanic crust
C. Evidence for Continental Drift
1. Map Fit- some of the
continents seem to fit
together like a jigsaw
puzzle.
Ex: S. America & Africa
2. FossilsFossils of certain species of
ancient organisms were found
on separate continents
throughout the world.
Mesosaurus
The continents must have been
as ONE in order for these
species to be found on
different, widespread
landmasses.
Organisms were either too
large or small to swim or fly to
other continents without them
being linked.
Land bridges could not have
been the solution, because the
remnants are nowhere to be
found.
3. Rock Structures
Mountain chains of
comparable age that appear to
be severed by the oceans
(Appalachians)
When these landmasses are
reassembled they form a
continuous mountain belt.
N. America & Europe
S. America & Africa
S. America & Antarctica
4. Ancient Climates
Glacial deposits show that
most of the Southern
Hemisphere was covered by
ice sheets.
Rock deposits left behind by
moving glaciers were found in
all southern continents & India.
Coal fields produced by
ancient tropical plant fossils
indicate that the Northern
Hemisphere was once a
tropical environment.
Main objection to Wegener’s
hypothesis was its inability to
provide a mechanism for the
movement of the continents.
II. Plate Tectonics
A. What is the Theory?
The crust is broken into large
moving sections called plates
Lithosphere: rigid layer made
of the crust & upper mantle
Asthenosphere: weaker upper
part of the mantle
Explains the formation,
movements, collisions, &
destruction of the Earth’s crust.
Seven major plates:
Pacific (largest)
N. American
S. American
African
Eurasian
Australian
Antarctic
Plates include a continent &
mostly ocean floor.
Plates are slowly moving
continuously.
B. Plate Boundaries
1. What occurs at plate
boundaries?
a. Seismic activity
b. Volcanism
c. Mountain building
2. Types of Plate Boundaries
a. Divergent Boundaries
1. Where plates move apart
(Constructive)
Rift Valley
Midocean Ridge
2.
Seafloor Spreading
* As divergent plates spread
apart creating a fissure,
magma flows through the
fissure (rift valley.)
*The lava then cools & crystallizes
to form new seafloor mountains
(midocean ridges.)
*Younger rock is at the midocean
ridges & the older rock is at the
continental margins.
b. Convergent Boundaries
1. Where plates move
together, resulting in
the subduction of
oceanic lithosphere
into the mantle.
2. Types of Convergent Boundaries
a. Oceanic-Continental
*Forms a subduction zone
producing a deep-ocean
trench.
* Continental volcanic arcs are
mountains formed by igneous activity
associated with the subduction of
oceanic lithosphere
beneath a continent.
Ex: Andes, Cascades, Sierra Nevada
b.) Oceanic-Oceanic
* Occurs when two oceanic
plates converge, one plate
subducts below another.
* Volcanic island arc forms as
volcanoes emerge from
underwater.
Ex: Mariana Islands
Aleutian Islands
c. Continental-Continental
* Neither plate will subduct
* Causes the continents to fold &
buckle upwards forming
mountains.
Ex: Himalayas, Alps, Appalachians,Urals
c. Transform Fault Boundary
1. Where plates grind past
one another without the
production or destruction
of lithosphere
2. Most transform faults are
located within the
ocean basins, a
few cut through
continental crust
San Andreas Fault
C. Evidence
1. Paleomagnetism
*Study of ancient
magnetism preserved in
rocks
*Shows the location of the
magnetic poles at the time
of the rock’s formation.
a. Polar Wandering
* Study showing that the
magnetism of older rocks point
in different directions.
* Showed that the magnetic
poles either moved or the
continents moved.
* The Earth’s magnetic poles
should remain close to the
geographic poles
* If the magnetic poles don’t
move, the apparent movement
must have been produced by
the continents moving.
b. Magnetic Reversals
*Earth’s magnetic field reverses
its polarity periodically.
*These reversals have been
shown in the crust of the
ocean floor
*Proves seafloor spreading.
2. Earthquake Patterns
* If earthquake epicenters are
plotted on the globe, it shows
the outline of the plates.
3. Ocean Drilling
*Drilling showed that the youngest
rock is near the ridges & the
oldest is further from the ridge
*Proves seafloor spreading
4. Hot Spots
* Rising plumes of magma
* As the plate moves above a hot
spot, it will create volcanoes that
produce underwater mountains
and volcanic islands
Hawaiian & Midway Islands
* Used to track the direction a
plate is moving.
Hawaiian Islands
D. Breakup of Pangaea
1. Fragmented landmasses
collided 500 to 225 million
years ago to form Pangaea
2. Pangaea began to break
apart 200 million years ago
during the Jurassic Period
3. Break up formed two
subcontinents : Laurasia
& Gondwanaland
4. Those subcontinents
began dividing about 165
& 100 mya
E. Mechanisms & Methods
1.Convection Current Hypothesis
*Hotter magma rises in the mantle
*The magma cools and spreads
*The spreading magma moves the
plates laterally
*The cooled magma then sinks
back into the mantle & reheats
2. Slab-Pull & Slab-Push Hypotheses
*Colder oceanic plate will
sink/subduct, pulling the rest
of the plate with it.
*As new crust is formed at a rift
valley, it pushes the plate
forward.
3. Hot Plumes Hypothesis
* Plumes of hot magma rise
through the mantle, striking and
pushing the plates in different
directions