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 Climates5. 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 bull-dozers. – 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”