1. Seafloor Spreading

2. Seafloor Spreading Video

3. Wegener could not explain how the continents moved but... Harry Hess explained HOW continents moved and he called his idea Seafloor Spreading

4. During World War II: Scientists began to experiment with new tools and equipment to learn about the oceans SONAR (SOund Navigation And Ranging) was one new technology used to map the sea floor

5. Sound waves reflect off of any hard surfaces that they hit, so… 1. Sound waves were sent to the bottom of the ocean floor from a “transmitting” device held off a boat 2. Sound waves reflected back to another “receiving” device on the boat 3. The longer it took the sound waves to return to the boat, the deeper the water.

6. Sonar technology also is what helps fisherman catch fish and our military to look for enemies

7. So what have we discovered about the ocean floor? http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_seafloorspreading.html

8. The seafloor contains mountains, volcanoes and valleys Including the Mid- Atlantic Ridge The World’s Largest Mountain Range And it is UNDERWATER!

9. Process of Seafloor Spreading: 1. Magma is forced upward at the mid-ocean ridge (divergent boundary) 2. Magma cools and sinks as it flows away forming a high ridge 3. The magma then cools to solid crust forming new ocean floor This occurs ~2.5 cm/yr in Atlantic Ocean

10. This is happening all over the Ridge

11. Areas of Seafloor Spreading

12. Features of Sea Floor Spreading Trenches form away from the divergent boundary near subduction zone. –Subduction – more dense crust (oceanic) gets pushed under less dense crust (continental) back into the mantle.

13. Proof of Sea Floor Spreading 1. Rock samples –Youngest rock near the ridge (where magma cooled and hardened most recently) –Oldest rock near shore (where magma cooled and hardened ~180 million years ago

14. 2. Magnetic Clues Some minerals are magnetic Magnetic minerals are attracted to Earth’s magnetic field.

15. When magma containing these minerals cools, it freezes the magnetic mineral grains so they point in the direction of the Earth’s magnetic field.

16. Over time, Earth’s magnetic field changes (reverses direction) -As a result, the direction the minerals point changes with it. Earth’s magnetic field caused by movement in outer core

17. These changes result in Paleomagnetism - magnetic striping along the ocean floor. Each beige and red stripe indicates a change in Earth’s magnetic field.