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/