Plate Tectonics Part II: Plate Boundaries

Three Types of Plate Boundaries

DIVERGENT: Move Apart  Oceanic - Oceanic • Oceanic ridges – elevated areas where new seafloor forms (seafloor spreading.) • Rift valleys are deep faulted structures

East African Rift Valley Continental – Continental Continental Rifts • rift valley on land landmass may split into two, forming a rift. East African Rift Valley

CONVERGENT: Move together 2 plates collide. Different things happen depending on the type of crust involved. Oceanic – dense – basaltic Continental – less dense – granitic

 Oceanic-Continental • subduction zone occurs when the oceanic plate is forced below the continental plate. Trenches form. • Continental volcanic arcs form when the subduction of oceanic lithosphere beneath a continent melts and rises. • Examples include the Andes, Cascades, and the Sierra Nevadas.

 Oceanic-Oceanic • Two oceanic slabs converge and one descends beneath the other. • Volcanic island arcs form as volcanoes emerge from the sea. • Examples include the Aleutian, Mariana, and Tonga islands.

 Continental-Continental • Two continents collide and the land buckles upwards • This kind of boundary can produce new mountain ranges, such as the Himalayas.

Transform: Slide past each other  At a transform fault boundary, plates grind past each other without destroying the lithosphere. • Most join two segments of a mid-ocean ridge.

San Andreas Fault

HOT SPOT Some volcanoes form from hot spots Hot mantle plumes reach the surface, forming a volcano far away from plate boundaries