1. The Theory of Plate Tectonics

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

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

6. 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

7. Divergent Boundary

9. 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

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

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

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

16. 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

18. 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

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