Plate Tectonics
Chapter 8
BHS Earth Science
Hollywood’s Version
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Continental Drift
Wegener’s continental drift hypothesis
stated that the continents had once been
joined to form a single supercontinent.
• Wegener proposed that the supercontinent,
Pangaea, began to break apart 200 million
years ago and form the present landmasses.
Continental Drift
Evidence:
1. Jig-Saw Fit
•
Continents fit together like Puzzle pieces
2. Fossil Evidence
•
fossils of same age and species were found in
connecting bands on different continents
3. Rock Types and Structure
•
Similar bands of rocks types and mountain belts
connect
4. Ancient Climates
5. PANGAEA!
Connecting Mountain
Ranges
Top 100 Greatest
Discovery
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Rejecting the Hypothesis
• Most scientists rejected Wegener’s
ground breaking hypothesis because he
could not provide an explanating of how
the plates moved
• A NEW THEORY EMERGES!
Seafloor Spreading
• In Wegener’s theory, continents
“plowed” through the sea like bulldozers.
– Not the case. Continents are actually
connected to plates, which move
• Continents actually move with
lithospheric plates that are pushed by
Mid-Ocean ridges
Harry Hess
• Discovered “Mid-Ocean Ridges
– Spreading centers for tectonic plates
• Noticed magnetic stripes on the ocean
floor which proved that new crust was
being created at these ocean ridges
• Provided the mechanism for how the
plates moved.
Top 100 Greatest
Discovery
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Topographic Map of the
World
Plate Tectonics
• The theory explaining how the movements of
earth’s “TECTONIC PLATES” or “moving
plates” create the geologic events like
earthquakes, volcanoes and tsunamis.
• Lithospheric Plates- giant puzzle pieces
connecting on the surface of earth.
– These sit on top of mantle so they can move
around
Plate Boundaries
• Three types:
– Divergent Boundary
– Convergent Boundary
– Transform Boundary
Divergent Boundary
• Plate moving apart
• Examples:
– Mid Atlantic Ocean Ridge
– East Pacific Rise
• Plates are being pushed apart… as they are
pushed apart, magma rises from the mantle
and fills the void.
• Mechanism for plate movement
Ocean Ridges
• Oceanic ridges are continuous elevated zones on
the floor of all major ocean basins. The rifts at the
crest of ridges represent divergent plate boundaries.
Rift Valleys
• Tears in the earth
• Also caused by Divergent Boundaries
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Convergent Boundaries
• Two plates crashing together
• Collliding
• Three types:
– Continental to Continental
– Ocean to Ocean
– Ocean to Continental
Continent to Continent
collision
• When two continents collide, you will
get mountain ranges
– Example: Himalayans (India crashing into
Asia)
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India and Asia
Ocean to Ocean
Convergance
• Two oceanic slab converge together. One is
pulled under the other
• Often forms volcanoes on seafloor
– Volcanic Island Arcs
• Aleutian Islands
– Alaska
Ocean to Ocean
Convergent
• Aleutian Island, Alaska, US
Ocean to Continental
Convergance
• ALSO KNOWN AS SUBDUCTION ZONE
• Oceanic plate is forced down into the mantle
beneath a second continental plate
• Creates volcanoes and deep ocean trenches
– Examples Andes and Marianna Trench
– Marianna Trench is deepest place in the WORLD!
35,000 feet deep!!!!!
Subduction Zone
Subduction Zne
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Subduction Zone
process
• Denser ocean slab pulled underneith
continent
• As it is pulled under, the oceanic crust
begins to melt.
• When crust begins to melt, magma rises
• Magma rises to surface and creates
volcanoes
Transform Boundaries
• Plates grind past each other without
destroying or creating new lithosphere
• Like cars passing each other on a highway
• Creates tremendous earthquakes
• Example: San Andreas Fault
Transform Boundaries
How do Plates Move?
• The crust lies on top of the mantle.
• It is believed that convection currents cause
movement in the asthenosphere. Heat rises from
interior.
• Areas where the asthenosphere is raising causes
plates to move apart (diverge) and areas where the
asthenosphere is sinking causes plates to move
together (converge).
Why do plates move?
• Clear answer is yet unknown
• There are theories for “Ridge Push”,
“Trench Suction”, and just plain old
“Gravity”