1. Plate Tectonics

2. Layers of the Earth
The Earth is made of several layers that can be classified by chemical and physical properties.
The Lithosphere makes up the crust; Earth’s rigid outer shell.

3. There are two types of crust
Continental
Less dense, granitic composition
Oceanic
More dense, basaltic composition

4. The Lithosphere is broken into several large pieces called plates

5. Asthenosphere is plastic (ooey-gooey) layer under the lithosphere.
It is part of the upper mantle.
Under the asthenosphere is the mantle.
Under the mantle there is a liquid outer core surrounding a solid inner core.

6. Continental Drift Alfred Wegener (1880 – 1930)
According to Wegener’s hypothesis of continental drift, the continents had once been joined together to form a single supercontinent.
He named the supercontinent Pangaea, meaning “all land”

7. Evidence Continents look like they fit together like a puzzle
Several fossil organisms found on different land masses

8. More Evidence Matching rock and mountain belts across continents
Glacial till deposits across most continents

9. But there was a problem!
His hypothesis did not become accepted until after he died 
Why do you think Wegener’s hypothesis faced a great deal of criticism at the time?

10. Answer: He could not explain how the land was moving!

11. New Data!
During the early 1900’s, Sonar made it possible to map the ocean floors
deep-ocean trenches and mid-ocean ridges were discovered
In 1963, Harry Hess published his hypothesis for sea-floor spreading: new ocean floor forms along ridges and slowly move outward

12. Evidence for Sea-Floor Spreading
Patterns of earthquakes near trenches (where 1 plate goes under another plate)
Paleomagnetism - the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. 
- As rocks form, the iron-rich minerals align with Earth’s magnetic poles
- As the poles move, the mineral alignment also moves
- Bands/Stripes of opposite polarity form in ocean floor rock.

13. Age of ocean floor Oldest rocks are near the perimeter of ocean basins
Newest rocks are near mid-ocean ridges

14. Go Wilson!
Canadian geologist J. Tuzo Wilson (1908 – 1993) explained the “HOW?!”
Suggest the lithosphere is broken into plates
Movement of the plates is driven by convection currents in the mantle (asthenosphere)

15. Plate Tectonics
Earth’s lithospheric plates move slowly relative to each other, driven by convection currents in the mantle.