Theory of Plate Tectonics

Theory of Plate Tectonics The earth’s crust and rigid upper mantle are broken into enormous slabs called plates Tectonic plates move in different directions and at different rates over Earth’s surface

Plate Boundaries Plates interact at places called plate boundaries Plates can interact in several ways: (1) Converge (2) Diverge (3) Transform

Divergent Boundaries Places where two tectonic plates are moving apart are called divergent boundaries Most divergent boundaries are found on the seafloor, where they form ocean ridges Seafloor spreading along a divergent boundary causes ocean basins to grow wider

Ted Talks Deep Sea Creatures Seafloor Spreading Ted Talks Deep Sea Creatures

Divergent Boundaries Sometimes occur on continents When continental crust separates, the stretched crust forms a long, narrow depression called a rift valley

East Africa Rift Valley

Convergent Boundaries Places where two tectonic plates are moving towards each other are called convergent boundaries Three types: (1) Oceanic-oceanic (2) Oceanic-continental (3) Continental-continental

Oceanic-Oceanic Plate Interaction Subduction occurs when one oceanic plate slides under another oceanic plate The oceanic plate that has cooled more is the more dense plate, and therefore will slide under the other oceanic plate to melt into magma Some of the magma formed is forced back to surface to form an arc of volcanic islands Creates a deep-sea trench and volcanic islands

Marianas Trench and Marianas Islands

Oceanic-Continental Interaction Oceanic and continental plates meet The more dense oceanic crust will slide under the less dense continental crust Also creates a trench, but this time a mountain range with volcanoes EX: Peru-Chile Trench and Andes Mountains

Andes Mountains

Continental-Continental Interaction Occurs when ocean basin between two continental crusts is entirely subducted. The two continental crusts collide to form mountain ranges EX: India land mass gradually moved northeast until it collided with the Eurasian Plate.  It is currently moving at a rate of 67 mm/year, or about 2.6 inches. The collision formed the Himalayas Mountains.

Himalayas Mountains

Transform Boundaries Two plates slide past each other horizontally Crust is only deformed or fractured Characterized by long faults and shallow earthquakes

San Andreas Fault EX: San Andreas fault in California responsible for majority of earthquakes in Ca

Causes of Plate Motions What forces causes plates to move? Believed to be convection currents in the mantle Recall that heated magma is less dense, and rises to the surface where the crust is located. As the magma moves further from its heat source, it becomes more dense and sinks down. As it sinks, it warms up again, forming a circular current motion.