Lesson 6 discusses sea floor spreading and how it leads to continental drift. Magnetic field reversals recorded in oceanic crust rocks provided evidence that supported the theory of continental drift and sea floor spreading. As tectonic plates move apart, new magma fills the gap and forms new sea floor, recording the magnetic signature of the time. The older the sea floor rock, the further it is located from the mid-ocean ridge where new sea floor is being created. This magnetic reversal pattern in sea floor sediments provided strong evidence that sea floor is indeed continually being created and pushed apart as the theory of plate tectonics suggests.

1. Lesson 6 Sea Floor Spreading

2. Learning Outcomes You will be successful if you can: Know that sea-floor spreading causes the continents to move Describe how magnetic field reversals led to the acceptance of continental drift. Explain how the movement of tectonic plates leads to unpredictable earthquakes. .

3. Compare and contrast How is the model like "real-life" - i.e. the seafloor How is the model different from what really happens under the sea? What simplifications have we made?

4. Evidence for sea floor spreading Since the Earth was formed the north and south poles have periodically “________ ____”. When tectonic plates move apart and _____ fills the gap the iron particles in the magma orientate themselves in line with the Earth’s ________ field. This means that the rock formed on the sea floor contains a “magnetic __________” of the changing field: These magnetic patterns can be used to prove that sea floor spreading does happen, and at a rate of about 2 to 10 cm per _____. Words: impression, magma, swapped over, magnetic, year

5. Fact Seafloor drilling in 1969 found that sediments further away from oceanic ridges are thicker 1) How do you think this is linked to the age of the rock a) close to the ridge b) far away from the ridge? 2) How is this evidence for sea floor spreading?

6. How far does the Atlantic spread in a lifetime? (100 years) How far has it spread in all of human history? (10 000 years) How far has it moved in 100 million years?

7. Review Radioactive Decay Continental plate Magnetic reversal pattern Earthquakes Subduction High Density Fossils Oceanic Plate Rock types S America Africa Tectonic plates Low density Heat Core Mantle Crust