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  • 1. Sea Floor Spreading
  • 2. Vindication for Wegener! • Do you remember why scientists did not believe Alfred Wegener? • WWII – the beginning of a significant period in geology – During WWII, the navy used sound waves (sonar) to detect submarines and map the topography* of the ocean floor. – Naval officer Harry Hess used sonar to detect the enemy and to map the ocean floor (to help prepare for landings on various islands). – After the war, this technology continued and eventually vast areas of the ocean floor were mapped. – Let’s look at what their research found
  • 3. Evidence • The bottom of the ocean was not flat, as many scientists believed. • There was an underwater mountain range that circled the globe, much like the seams of a baseball. • At this mid ocean ridge system, molten material erupted from the ocean floor, pushing older material out of the way. • The sea floor was spreading!
  • 4. Evidence
  • 5. Evidence in Support of Sea Floor Spreading • Pillow lava – a type of rock that forms when lava hardens very quickly under water. • These rocks have been found at the mid ocean ridge. • What can you infer from this evidence?
  • 6. Evidence in Support of SFS • The earth is like a giant magnet. • Basalt, the type of rock that forms oceanic crust, contains a lot of iron. • What do you think happens to the iron particles in the molten material as it hardens and forms a rock? • Interesting factoid: The north and south poles of the earth has “switched” many times – about once every 600,000 years. • Let’s put these facts together – magnetized iron particles and magnetic reversals.
  • 7. Magnetized Particles and Switching Poles • We do know that the ocean floor is not striped – right? • We use the stripes as a model!
  • 8. Evidence in Support of SFS • Scientists can calculate the age of rocks. • Scientists have found out that – the oldest continental rock is almost 3 billion years old – the oldest ocean crust rock is about 200 million years old – the youngest ocean rock is nearest the mid ocean ridge – the oldest ocean rock is nearest the continents • How can that be?
  • 9. Subduction Zones • The area where the ocean floor sinks back into the mantle, is re-melted. • Can you now tell why ocean crust is so much younger than continental crust?
  • 10. Sea Floor Spreading • Since the ocean floor is being re-melted, why isn’t the earth getting smaller?
  • 11. Questions?