1. Basics of Plate Tectonics

2. Review of Density Density= mass/volume
Substances of a low density will float on substances of a higher density

3. Internal Structure of Earth

4. Internal Structure
Core- innermost layer; composed mostly of mixtures, or alloys, of iron
Solid inner core and liquid outer core
Mantle- flows like a liquid; very hot near the melting point of rocks
Lithosphere- consists of the rust and the uppermost part of the mantle

5. Internal Structure (2)
Crust- outermost layer; extremely thin and rigid
Rest on solid upper mantle that floats on denser plastic middle mantle (like molten tar)
Constantly changing by erosion and glacial melting reduces continental mass (floats higher on mantle)
Oceanic crust- made of basalt and is denser
Continental crust- made of granite, high in aluminum silicates; is less dense; average thickness is 35km

6. Continental Drift
At the beginning of the 19th century scientists thought that the positions of the continent were fixed
Scientists today realize that the plates are moving
1. Tectonics is the process of plates moving
2. As plates move they create mountains on earth and various sea floor features

7. Alfred Wegener
1. Proposed the idea of Pangea that all continents were once a single land mass
2. Evidence
a. Fit coastlines together
b. Similar fossils on the matching coastlines
c. Magnetic reversals (he did not know this)

8. How Plates Move Plates move by the mechanism of sea floor spreading
1. Sea-floor spreading is driven by convection currents
Plate Tectonics occurs at plate boundaries

9. Convection Currents in Mantle

10. Divergent Boundaries
Plates move in opposite directions away from each other and rift (crack in earth’s crust); called sea-floor spreading
New oceanic crust is formed here
Plates move apart at the rate of 0.8”-7”/year
Example: Mid-ocean ridge

11. Convergent Boundaries
Plates Collide

12. Convergent (Oceanic-Continental)
Ocean plate sinks and is subducted creating a trench
Creates volcanic arcs/mountains on land
Earthquakes can also occur
Examples: Mt. St. Helens

13. Convergent (Oceanic-Oceanic)
One of the plates will be subducted forming an island chain or arc
Example: Aleutian Islands off the coast of Alaska

14. Convergent (Continental-Continental)
As plates collide they are welded together forming mountain ranges
Example: Himalaya Mountains