• 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)

• 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!

• 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?

• 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 have “ switched ” many times – about once every 600,000 years.
• Let ’ s put these facts together – magnetized iron particles and magnetic reversals .. .

• We do know that the ocean floor is not striped – right?
• We use the stripes as a model!

• 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?

• The area where the ocean floor sinks back into the mantle is re-melted.
• Can you tell why ocean crust is so much younger than continental crust?

• Since the ocean floor is being re-melted, why isnt the earth getting smaller?

• Let ’ s draw an illustration to help us with the answer
• There is one in our book, page 41 and one here.