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Plate Tectonics

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Plate Tectonics• Earth’s crust is cracked and

fragmented– Fragments called plates

• The Earth’s mantle is very hot and fluid, so the plates move atop the mantle

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Earth’s Crust• Two kinds of crust exist

–Granitic (felsic) and basaltic (mafic)

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Earth’s Crust• Think of the Earth as

completely covered with a layer of thin, dense basalt rock with chunks of thicker, less dense granite here and there–The granite chunks are the

continents riding on the basalt like wood floats on water

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Plate Boundaries--where tectonic plates meet and interact

Where can you find this image?

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Plate Boundaries

• Three types–Convergent

–Divergent

–Transform

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Divergent Boundaries

• Where plates are moving apart–Cracks in the crust are filled by

upwelling molten rock

Give one example of a divergent boundary:

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Divergent Boundaries• Divergent boundaries on continents

are called rifts–Example: the African Rift Valley

• Splitting Africa in two

• Divergent boundaries under oceans form mid-ocean ridges

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Divergent Boundaries• Several plates are pulling apart

beneath the Atlantic ocean forming the Mid-Atlantic Ridge–Diverging ocean plates cause sea

floor spreading• Where new sea floor is constantly being

added

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EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT

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In Afar, Ethiopia, a 40-mile magmatic rift that opened up 1 year ago

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Spreading Center

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Mid-ocean ridges (rifts) normally form where tectonic plates are(1) converging (3) stationary(2) diverging (4) sliding past each other

The motion of the convection currents in the mantle beneath the Atlantic Ocean appears to be mainly making this ocean basin(1) deeper (3) wider(2) shallower (4) narrower

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Convergent Boundaries• Where plates are coming together

Here for example

What is the name of this boundary?

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• Where mountains form

• Where earthquakes occur (and sometimes volcanoes)–Several plates are colliding with the

Pacific Plate creating the Pacific Ring of Fire• An area that experiences lots of

earthquake and volcanic activity

Convergent Boundaries

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Convergent Boundary Types

• Ocean-Ocean

• Continental-Continental

• Ocean-Continental

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Convergent Boundaries

CONVERGENT BOUNDARIES Oceanic crust - Oceanic Crust

older, denser crust subducts forming ocean trench Oceanic Crust – Continental Crust

denser oceanic crust subducts forming ocean trench Continental Crust – Continental Crust

resists subduction - folding, faulting, and mountain building

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Ocean-Ocean Convergence

• Forms ocean trenches– Deepest places on Earth

Name one ocean-ocean boundary

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Oceanic Crust – Oceanic Crust Older, denser oceanic crust is SUBDUCTED

VOLCANIC ISLAND ARC FORMED – ALEUTIAN ISLANDS

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Ocean-Continental Convergence

Site of subduction zones• Denser ocean plate slides beneath the less

dense continental plate–Example: the Pacific Northwestern U.S.

• Creates many of the Earth’s volcanic mountain chains as the ocean plate dives down, melts and is forced back to the surface

Name one ocean-continental boundary

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Oceanic-Continental Convergent Boundary - SUBDUCTION

DENSER OCEANIC CRUST GOES DOWN INTO MANTLE

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Continental-Continental Convergence

• How non-volcanic mountains are formed– Example: the Appalachian chain and the

Himalayas

– Rising of land due to plate collisions is called uplift

Name one continental-continental boundary

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Continental Crust – Continental Crust Convergent Boundary

NO SUBDUCTION - COLLISION ZONE

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Collision of India and Asia

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Transform Fault Boundaries

TRANSFORM PLATE BOUNDARIES

At a transform fault boundary, plates grind past each other without creating or destroying the lithosphere.

SHALLOW EARTHQUAKES

Transform faults

• Most join two segments of a mid-ocean ridge.

• At the time of formation, they roughly parallel the direction of plate movement.

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Transform Fault Boundary AT MID-OCEAN RIDGES

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So, what causes plates to move?

• Convection currents: drag and move the lithospheric plates above the asthenosphere

(three sources of heat produce the convection currents):

(1)Leftover heat from earth’s formation

(2)Decay of Radioactive elements

(3)Plate friction

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Convection Cells

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Convection Currents in the mantle drag and pull the lithospheric plates

above them

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Mid-Atlantic Ridge

• As the plates pull apart new ocean floor is added

What 2 continents are moving away from each other as a result of the mid-Atlantic Ridge?

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Convection Currents• It is theorized

convection currents within the Mantle cause plates to move around

• There are other theories about what causes plates to move!

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Evidence for Plate Tectonics

Ocean Drilling• The data on the ages of seafloor sediment

confirmed what the seafloor spreading hypothesis predicted.

• The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the continental margins.

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Which graph best represents the geologic age of the surface bedrock on the ocean bottom?

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More Evidence for Plate Tectonics

Earthquake Patterns• Scientists found a close link between

deep-focus earthquakes and ocean trenches.

• The absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory.

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Convergent plate (subduction) boundaries have deeper earthquakes

Divergent plate boundaries and collision zones have shallow earthquakes

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SUMMARY 3 PLATE BOUNDARIES

DIVERGING CONVERGING (SUBDUCTION ZONE)

Oceanic Crust – Continental Crust

Oceanic Crust - Oceanic Crust CONVERGING (COLLISION ZONE)

Continental Crust – Continental Crust TRANSFORM

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Boundaries: Diverging

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Boundaries: Converging SUBDUCTION

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CONVERGING –- COLLISION ZONE

No subduction – continental crust less dense - buoyant

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Boundaries: Transform

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Due to plate tectonics the world is constantly changing