Continents Change Continents Change Positions Over Positions Over TimeTimehttp://www.youtube.com/watch?http://www.youtube.com/watch?v=RuScA58BgRE&feature=relatedv=RuScA58BgRE&feature=related

Continental Drift Continental Drift Theory developed in 1912 by German Theory developed in 1912 by German

scientist Alfred Wegener.scientist Alfred Wegener. States that Earth’s continents were once States that Earth’s continents were once

joined in a single landmass and gradually joined in a single landmass and gradually moved, or drifted apart .moved, or drifted apart .

Evidence for Continental Evidence for Continental Drift- FossilsDrift- Fossils

Fossils of an ancient reptile were of an ancient reptile were discovered in eastern South America and discovered in eastern South America and western Africa.western Africa. These fossils were not found anywhere These fossils were not found anywhere

else in the world.else in the world.

Evidence for Continental Evidence for Continental Drift- ClimateDrift- Climate Fossils of tropical plants can be of tropical plants can be

found on Greenland’s shores.found on Greenland’s shores. South Africa’s rocks were deeply South Africa’s rocks were deeply

scratched by ice sheets that once scratched by ice sheets that once covered the area.covered the area.

Evidence for Continental Evidence for Continental Drift- GeologyDrift- Geology Types of rock found in Brazil matched rock Types of rock found in Brazil matched rock

found in western Africa.found in western Africa. Limestone rocks in the Appalachians were rocks in the Appalachians were

exactly like those in Scotland’s Highlands.exactly like those in Scotland’s Highlands.

Pangaea from the Greek word meaning “all lands”; from the Greek word meaning “all lands”;

Wegener’s name for the joined Wegener’s name for the joined continents (supercontinent) continents (supercontinent)

Mid-Ocean Ridge

Huge underwater mountain range. Huge underwater mountain range.

Sea-Floor SpreadingSea-Floor Spreading

Ridges form cracks along the crust.Ridges form cracks along the crust. Melted rock rises through these cracks, Melted rock rises through these cracks,

cools and forms new oceanic crust.cools and forms new oceanic crust. These areas are called spreading These areas are called spreading

centers.centers.

Age of the Sea FloorAge of the Sea Floor Evidence that the sea floor is spreading Evidence that the sea floor is spreading

apart came from the age of rocks in the apart came from the age of rocks in the crust.crust.

The youngest rock is closest to the ridge, The youngest rock is closest to the ridge, while the oldest rock is farthest away.while the oldest rock is farthest away.

Continental crust (4 billion years old) is Continental crust (4 billion years old) is much older than oceanic crust (160-180 much older than oceanic crust (160-180 million million

Ocean TrenchesOcean Trenches In these deep In these deep

trenches, dense trenches, dense oceanic crust is oceanic crust is sinking into the sinking into the asthenosphere.asthenosphere.

Old crust is being Old crust is being destroyed at the same destroyed at the same rate that new crust is rate that new crust is being formed.being formed.

Causes of Plate Causes of Plate MovementMovement

Tectonic plates rest on the Tectonic plates rest on the asthenosphere, a layer of soft, hot rock.asthenosphere, a layer of soft, hot rock.

Rock in this layer moves by Rock in this layer moves by convectionconvection..

Causes of Plate Causes of Plate MovementMovement

Rocks in the asthenosphere act in a similar Rocks in the asthenosphere act in a similar way.way. Hot, soft rock rises, cools and sinks, only to Hot, soft rock rises, cools and sinks, only to

be heated and rise again.be heated and rise again. This sinking and rising motion, if it This sinking and rising motion, if it

continues, is called a continues, is called a convection currentconvection current. . This current moves rocks in the mantle This current moves rocks in the mantle only a few centimeters a year. Over only a few centimeters a year. Over millions of years, this moves plates millions of years, this moves plates thousands of kilometers.thousands of kilometers.

Causes of Plate Causes of Plate MovementMovement

Slab pull and ridge push also move these Slab pull and ridge push also move these huge plates.huge plates. slab pull- gravity pulls the edge of a cool, slab pull- gravity pulls the edge of a cool,

dense plate into the asthenosphere and the dense plate into the asthenosphere and the entire plate is dragged along.entire plate is dragged along.

Ridge push- material from a mid-ocean ridge Ridge push- material from a mid-ocean ridge slides downhill from the ridge, pushing the slides downhill from the ridge, pushing the rest of the plate.rest of the plate.

Theory of plate tectonics Theory of plate tectonics

States the Earth’s lithosphere is made up States the Earth’s lithosphere is made up of huge plates that move over the surface of huge plates that move over the surface of the Earth. of the Earth. Plates can move apart, push together, or Plates can move apart, push together, or

scrape past each other.scrape past each other. Most major earthquakes, volcanoes, and Most major earthquakes, volcanoes, and

mountain ranges appear where tectonic mountain ranges appear where tectonic plates meet.plates meet.

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