Plate Boundaries 17 3 Theory of Plate Tectonics

Plate Boundaries 17. 3

Theory of Plate Tectonics • Continental and oceanic crust move as enormous slabs known as tectonic plates • Huge pieces of crust and rigid upper mantle that fit together at their edges to cover Earth’s surface. • 8 major plates and several smaller ones • Move in different directions and at different rates relative to one another

Divergent Boundary • Tectonic plates move away from each other • Found along seafloor in rift valleys • Rising magma forms a ridge trough the rift’s faults • New crust is formed • Rift valley – narrow depression formed by separating continental crust.

Convergent Boundary • Tectonic plates are moving toward each other • Subduction - when 2 plates collide the denser plate descends below the less-dense plate. • Continental crust is less dense than oceanic crust. • 3 different types: • Oceanic-oceanic • Oceanic-continental • Continental-continental

Oceanic-oceanic • A subduction zone is formed when a denser oceanic plate descends below another oceanic plate. • Process creates an ocean trench • Examples: • Mariana Trench and Mariana Islands

Oceanic-continental • A subduction zone is created when an oceanic plate descends below a continental plate. • This causes a trench and a volcanic arc along the edge of the continental plate. • Example: Peru-Chile Trench and the Andes Mountains

Continental-continental • Formed a long time after an oceanic-continental convergence. • The edges of 2 continents collide and the edges crumple, fold, and rise from the ground to form mountain ranges • Example: Himalayas

Transform Boundary • A region where 2 plates slide horizontally past one another • Many earthquakes take place at these boundaries • Crust is somewhat deformed or fractured • Example: San Andreas Fault