1. Theory of Plate Tectonics

2. 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

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

5. 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

6. Ted Talks Deep Sea Creatures
Seafloor Spreading
Ted Talks Deep Sea Creatures

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

8. East Africa Rift Valley

9. 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

10. 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

11. Marianas Trench and Marianas Islands

12. 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

13. Andes Mountains

14. 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.

15. Himalayas Mountains

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

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

18. 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.