1. Day 26  You will use the theory of plate tectonics and the changes in the earth’s surface in order to explain and discuss the evidence for sea- floor spreading.  What does the theory of plate tectonics state? ObjectiveWarm-Up

2. Pangaea Puzzle (10 minutes)  Write a short paragraph about why the continents are no longer in these positions.  Include the following: Key terms Evidence for CDT Why it was not believed

3. Exploring the Earth:  Where have you seen evidence of the movement of Earth’s plates?  Describe the relationship between continental drift, sea- floor spreading and plate tectonics.

4. Sea Floor Spreading

5. Theory of Sea Floor Spreading  States that the ocean floor spreads apart at the Mid Ocean Ridge.  As the ocean floor spreads, magma rises from the asthenosphere to form new sea floor(made of basalt).  Old ocean floor is destroyed at areas called subduction zones.  This theory was found by Harry Hess in the 1960’s

6. Sea Floor Spreading Mid-Ocean Ridge : underwater mountain chains that run through the Earth’s Basins Magma rises to the surface cools and new crust forms (Basalt) Older Crust is pushed farther away from the ridge

7. Key Vocabulary  Mid-Ocean Ridge:  Huge, underwater, volcanic mountain chain. Encircle the entire earth. Area where new ocean floor is created.  Subduction Zone:  Area where an oceanic plate descends into the upper mantle. Ocean floor is destroyed. Associated with ocean trenches.  Trench:  Deepest areas on the ocean floor. Mariana's Trench~11033 m below sea level (pacific ocean)

8. Something to think about… Pacific  It we know the Atlantic ocean is getting bigger because of divergence (moving apart) at the Mid-Ocean Ridge we can assume the Atlantic ocean is getting (bigger or smaller)? Since the Atlantic ocean is getting bigger, and the Pacific ocean is surrounded by continents which create subduction zones (point where ocean crust is destroyed) we can infer that the Pacific ocean is getting (smaller or bigger) because it is surrounded by subduction zones.

9. Sea Floor Spreading Evidence  Oldest sea floor is only 180 million years old.  Age of the earth is estimated to be 4.5 billion years old.  Sea-floor is the youngest at Mid-ocean Ridge and oldest near continents  Magnetic poles switch.  Shows that new ocean floor was being created at the mid ocean ridge.  Geologist can measure the reversal in the rock with a giant magnet! Age of the sea-floorMagnetic Reversal

10. Sea-Floor Spreading

11. Magnetic Reversal You will create a model to illustrate sea-floor spreading.

12. Closure:  What was the evidence to support sea-floor spreading? (2 pieces)  What other theory was this then able to prove?  The earth is about 4.6 billion years old. Based on observations of your sea-floor spreading model, why do you think that the oldest ocean floor is only about 200 million years old?

13. Quiz-Word Bank WegenerPangaea LithosphereMantle AsthenosphereHess Outer coreContinental drift theory Inner coreSea Floor Spreading Crustsubduction zone Oceanic crust Mid-ocean ridge Continental crustyounger Basalt