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Plate Tectonics
History of the Theory
• First proposed by Alfred Wegener in 1912
• Not widely accepted until the 1960s
• Proposes that Earth’s outer layer is composed of plates
• These layers move
Pangea
• Describes the arrangement of the continents millions of years ago as one huge supercontinent
• Helps explain how the same animals ended up so far apart from each other
Structure of the Earth
• core- innermost solid layer• mantle- second layer containing the plastic
asthenosphere and a solid rock layer comprising part of the lithosphere
• crust- thinnest outermost layer, includes part of the lithosphere
• The rigid layer of the Earth is known as the lithosphere which is broken into plates
• These plates move across the asthenosphere
Force Behind Plate Tectonics
• The actual force behind plates tectonics is not known for certain. There are 2 possible theories:– Convection in the Earth’s
mantle pushes the plates– Gravity pulls the older
and heavier ocean floor with more force than the newer, lighter sections
Four Types of Boundaries
• There are 4 types of plate boundaries– Divergent– Convergent– Collision– Transform
Spreading Sea Floor
• The Atlantic Ocean is growing
• This occurs due to a divergent boundary in the middle of the Atlantic Ocean
• This boundary is the site of the Mid-Atlantic Ridge
Spreading Sea Floor
• As the plates move apart, magma from underneath the crust wells up between the space
• The seawater cools the exposed magma, forming rock, resulting in new sea floor
Continental Slide
• We know the Earth is not getting any bigger even though new crust is being created at the divergent boundaries
• There are also convergent boundaries where the crust of one plate slides under another
Subduction
• This process of one plate sliding under another is known as subduction
• As the crust subducts, it melts creating magma
• This magma can make its way to the surface and generate volcanoes
• Most of the world’s volcanoes occur at convergent boundaries
Continental Crush
• When an ocean plate meets a continental plate, subduction occurs
• However when two continental plates meet, they crush together in a collision boundary
Continental Crush
• This collision results in a folding and crumpling of the land
• A collision boundary results in mountain ranges such as the Himalayas
Sliding
• When a boundary neither consumes nor creates crust, it is known as a transform boundary
• Here the two plates are simply sliding past one another
Sliding
• As the two plates slide past each other, tension builds
• An earthquake results when this tension is released
• One of the most famous is the San Andreas Fault in California
• The western plate is moving northwest while the eastern plate is moving southeast
This information was found at http://www.pbs.org/wgbh/aso/tryit/tectonics/