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 • (the study of the shape of the Earth’s • surface)
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!
Evidence in Support of SFS• 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?
Evidence in Support ofSFS • 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.
Magnetized Particles and Switching Poles• We do know that the ocean floor is not striped – right?• We use the stripes as a model!
Evidence in Support ofSFS• 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?
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?
Sea Floor Spreading• Since the ocean floor is being re-melted, why isn’t the earth getting smaller?
At Last! Wegener’s Answer• Let’s make a model to help us with the answer• Good – the one in our book, page 41 – this is the one we will construct• Better – maybe for extra credit• Best - maybe for a science fair