The Theory of Plate Tectonics

Plate Tectonics Became a theory in the 1960’s Describes plate movements AND explains why they move

Plate Tectonics Two types of crust: Oceanic and Continental Plates are usually made of both Plates: Chunks of lithosphere “floating” around on the asthenosphere

Plate Boundaries Three Types: Divergent Boundaries: two plates moving away from each other Plates move apart, molten rock from asthenosphere fills gap Most found on ocean floor (mid-ocean ridges) Rift valley: valley in the center of the mid-ocean ridge

Divergent Boundary

Convergent Boundaries Two plates colliding (Three types of collisions) Ocean – Continental: ocean crust is denser, so it is subducted: forced under the continental crust Ocean trench forms Subducting crust melts, some rises through the continental crust to form volcanic mountains

Convergent Boundaries Continental-Continental: neither is subducted, but both are crumpled and uplifted, forming mountain ranges

Convergent Boundaries Oceanic-Oceanic: one plate gets subducted, again forming an ocean trench Melting crust rises to form a volcanic island arc

Transform Fault Boundaries Sliding and grinding past each other Does not move smoothly – spurts of movement between periods of no motion

Causes of Plate Motion Convection Hot asthenosphere rises, cools, sinks  convection currents Lithospheric plates ride on top of these conveyer belts Evidence: More heat near divergent plate boundaries than elsewhere along ocean floor

Causes of Plate Motion Ridge Push New rock at mid-ocean ridges is hot, less dense, rises higher As it cools, it sinks away from the ridge

Causes of Plate Motion Slab Pull When a plate subducts and sinks, it pulls the rest of the plate with it