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• Earth Science

• Chapter 9

• Plate Tectonics

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Graphics by Tasa Graphic Arts. Inc.

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upper

mantle

lithospheric plate

asthenosphere

lithosphere

crust

lower

mantle

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• I. Earth’s surface is made up of lithospheric plates.

• A. Lithospheric plates are composed of the crust

and part of the upper mantle.

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• 1. The lithospheric plates are rigid and broken into pieces.

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• 2. The plates are composed of basalt (very dense) except for

the continents which are composed of granite (less dense)

basalt

granite

Lithospheric

basalt

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• 3. Lithospheric plates float about on the asthenosphere.

continental crust

lithosphere

asthenosphere

oceanic

crust

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• B. asthenosphere- partially melted, plastic-like, fluid-like,

containing convection currents which rise upon heating and

sink upon cooling.

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• C. there are 12 major plates:

• 1. African

• 2. Antarctic

• 3. Arabian

• 4. Caribbean

• 5. Cocos

• 6. Eurasian

• 7. Indian

• 8. Nazca

• 9. North American

• 10. Pacific

• 11. Philippine

• 12. South American

• 13. Juan de Fuca

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Pacific Plate

N. Am. Plate

S. Am

Plate

Antarctic Plate

Eurasian Plate

African Plate

Indian-

Australian Plate

Caribbean

Plate

Cocos Plate Arabian

PlatePhilippine

PlateNazca Plate

Juan de Fuca Plate

Eurasian Plate

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• II. Wegnener’s Theory- 1912- theory of continental drift.

• A. in the history of the earth, one or 2 supercontinents formed

and broke up----the last supercontinent that formed was a C-

• 1. Panthalassa the

ocean surrounding

Pangea

• 2. Tethys Sea the

sea inside Pangea

•shaped continent called Pangea,

when it broke up, it formed

•the present day

•continents.

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300 million years ago

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200 million years ago

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50 million years ago

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Present

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• B. Evidence

• 1. similar shape of the coasts of continents—Africa and

South America

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• 2. similar fossils—Mesosaurus

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• 3. Rock Type and Structural Similarities—mountain

ranges that match up on different continents.

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• 4. Paleoclimatic Evidence-evidence of similar climates on

different continents, i.e. grooves on rock indicating

glacial coverage, as well as glacial till.

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• 5. Earthquakes and Volcanoes

• a. earthquakes & volcanoes occur close to the plate

boundaries

• b. plate boundaries are active areas—areas where one

plate is moving relative to another plate. produces

stress—leads to fractures, earthquakes, heat and

volcanoes.

• c. ring of fire—largest active volcano belt

surrounding the Pacific Ocean. 90% of the

earthquakes and volcanoes.

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• 6. Paleomagnetism—some igneous minerals contain iron

minerals that are magnetic. When these minerals form and

hardened they preserve a record of the magnetic poles at the

time the rock was formed.

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• Evidence shows our poles have reversed several times.

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Age in

N

S

•a. Used to compare new sea floor in the ocean on

either side of the mid-Atlantic ridge.

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• b. Spreading centers—where lithospheric plates are

moving apart, new sea floor is forming

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• III. Three types of boundaries

• A. diverging boundaries

• 1. lithospheric plates

are moving apart

• a. mid-oceanic ridge

and the African Rift

Valley

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• B. transform (sliding) boundaries

• 1. lithospheric plates are

moving past each other

• a. San Andreas fault,

California

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• C. converging boundaries: lithospheric plates are

moving toward each other. NOTE: generally, oceanic

plates (basaltic) are more dense than continental plates

(granitic).

• 1. 2 main types

• a. Collision

• b. Subduction

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• a. collision

• 1) continental plates crash together and the plates

weld together—

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• 2) features include:

• a) pushed up mountain ranges

• b) earthquakes

• c) Indian and Eurasian –Himalayans Mts.

Highest mountains in the world.

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• b. subduction

• 1) when one lithospheric plate plunges under another

plate, which overrides the other

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• 2) features include:

• a) deep sea trench

• b) chain of volcanoes (volcano island arcs)

• c) mountain ranges

• d) deep earthquakes

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• 3) two subtypes

• a) convergence of 2 oceanic plates

• 1. Mariana trench; Philippine Plate over the Pacific Plate

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• b) convergence of an oceanic plate with a continental

plate

• 1. Andes Mountains; Nazca under the South American

Plate

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• IV. Driving Mechanism

• A. Convection currents-

• 1. heat from the interior of the earth warms up the asthenosphere.

• 2. This causes expansion of the rock material making it less dense and causing the semi-fluid material to rise.

• 3. As it reached the upper level of the asthenosphere it loses its heat and cools down.

• 4. This causes it to contract and become more dense.

• 5. Now the material sinks back down to the lower level where it once again heats up.

• 6. The cycle repeats.

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• B. Slab-Push and Slab-Pull—

• 1. Cold, dense oceanic slabs are descending in the oceanic trenches. These are heavy, moving plates that pull the trailing lithosphere with it. Hence, slab-pull.

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• 2. Where spreading centers occur, the hot material produces an uplifted ridge. New material is being produced and pushes the material on either side away from the ridge. Therefore it is known as slab-push.

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Slab push and slab pull

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• C. Hot Plumes-

• 1. Thermal plumes may exist that contribute to plate motion. Deep in the mantle there appear to be areas that consistently produce extremely hot moltenmaterial. This may be due to radioactive elements that are breaking down producing heat or some other as of yet unexplained heat source. “Hot spots”exist through out the world,

• a) example: Hawaiian Islands.

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• V. Continental Growth

• A. Craton-original or ancient continent core

• 1. usually smaller

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• 2. material has been added from

• a. deep sea sediments.

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• b. volcanic rock

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• c. sediments from rivers

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• B. thin skinned thrusting-

• 1. the pushing of this horizontal sheets of rock along

nearly level fault surfaces.

• 2. Appalachian Mountains

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• C. terrane

• 1. a large block of lithospheric plate that has been moved and attached to the edge of a continent.

• 2. identified by

• a. terrane block is bounded on all sides by major faults

• b. fossils do not match surrounding area

• c. magnetic polarity does not match area surrounding it.

• 3. Example: India

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