Theory of Plate Tectonics

1. Theory of Plate Tectonics • The lithosphere is broken into separate sections called plates • The theory of plate tectonics states that the pieces of Earth’s lithosphere are in constant slow motion driven by convection currents in the mantle. • No plate can move without affecting the other plates. As plates move, they change the Earth’s surface (creating volcanoes,, mountain ranges, etc.)

3. Plate boundaries • Plate boundaries are where plates meet • Faults may form along these boundaries. Faults are breaks in the Earth’s crust where rocks have slipped past each other. • There are 3 kinds of plate boundaries • Transform • Divergent • Convergent

4. Transform Boundaries • In a transform boundary, the crust is neither created or destroyed • At transform boundaries, two plates slip past each other in opposite directions.

5. Divergent Boundaries • The place where two plates move apart • This can cause a mid-ocean ridge (ocean) or a rift valley (land)

6. Divergent Boundaries • On land, when continents split apart a rift valley is formed. • The valley floor sinks lower and lower until it s below sea level. • Red Sea was formed from a rift valley.

7. Convergent Boundaries • A convergent boundary is the place where 2 plates come together resulting in a collision that can bring together. • Oceanic crust and oceanic crust • Oceanic and Continental • Continental and continental • Where these two plates collide the density of the plates determines which will come out on top • (Oceanic crust is more dense than Continental Crust)

8. When two plates collide • Oceanic → ←Continental • Because ocean is more dense than continent. The ocean plates sinks and plunges beneath continent. This is subduction (the sinking below) • Oceanic → ← Oceanic • 2 Plates meet at a trench the more dense plate drives the other under. This is subduction (the sinking below) • Continental → ← Continental • Head on collision creates mountain ranges

9. Oceanic → ←Oceanic • 2 oceanic plates collide. The older plate sinks because it is colder and denser than the younger plate. • Oceanic-Oceanic subduction can also create Deep-Ocean trenches and Island Arcs

10. Deep-Ocean trenches • Deep ocean trenches are deep canyons that form when a plate sinks. Most are found in the Pacific Ocean. Best example is Mariana’s Trench.

11. Island Arcs • These are a chain of volcanic islands that form the top plate parallel to a deep-ocean trench. Examples are the Philippine Islands, Aleutian Islands of Alaska and the islands of Japan Aleutian Island Arc Trench (darker blue)

12. Oceanic → ←Continental • Deep Ocean Trenches also occur when land and ocean plates collide and one sinks. The pacific Plate and Juan de Fuca Plate are sinking under the North American Plate and the This often causes underwater earthquakes

13. View across a bridge near Parkfield, from the North American Plate to the Pacific Plate. The bent rail is due to the motion of the San Andreas fault under the bridge since it was built.

14. Oceanic → ←Continental • Coastal Mountains in Oregon and Washington are examples of when an oceanic crust sinks under a continental crust. The continental crust will buckle to form a mountain range. These mountain ranges usually parallel a deep-ocean trench.