Chapter 4 Plate Tectonics 4-2 Restless Continents

Essential Questions/Learning Goals: What was Wegener’s Theory of Continental Drift? How does seafloor spreading make room for the continents to move? How does new ocean floor form at the mid-ocean ridges? How are magnetic reversals evidence of seafloor spreading?

Science Terms: Continental Drift Sea-floor spreading

What can you say about the east coast of South America and the west Coast of Africa? They fit together almost perfectly like pieces of a giant jigsaw puzzle. Play the game!

Wegener’s Continental Drift Hypothesis: How did Wegener come up with this idea? He looked at the way the continents fit together and thought that they must have been joined together at one time.

What is the idea (hypothesis) of Continental Drift? Continental drift is the idea that the continents were all connected at one time in the past. At some point they broke apart and drifted away from each other.

What things did the Theory of Continental Drift help explain? How the continents seemed to fit together. The existence of identical fossils on continents separated by continents. Animation

What was Pangaea? It was the name Wegener gave to the super-continent that existed when the continents were all connected.

Why did he call Pangaea? (what does the word mean?) Pan = all Gaea = Earth

When did Pangaea exist? About 245 million years ago. What happened about 180 million years ago? Pangaea split into to huge chunks. Laurasia (North America, Europe, and Asia) Gondwana (Africa Australia, Antarctica and South America)

Sea-Floor Spreading: Did Scientists believe Wegener’s hypothesis for Continental Drift? No Why not? He couldn’t explain how it happened (neither could anybody else!) How could a continent move?

What are mid-ocean ridges? They are a extremely long “chains” of mountains found in the middle of the oceans. How long? All the way around the entire Earth! Like seams around a baseball

Why are these mountains found in the middle of the oceans? That’s where they are made. How? These are the thinnest parts of the crust. The mantle tries to poke through the crust. Any area where the mantle comes to the surface is called….. A VOLCANO! These volcanoes form chains of mountains on the bottom of the oceans.

Why are these volcanic mountain chains forming? The two tectonic plates are moving away from each other!!! As they move apart, magma comes up from the mantle and cools to form new crust

If new crust is being made at the mid-ocean ridges, is the Earth getting larger? NO! Explain: As new crust is being made, old crust is getting shoved under other pieces of crust.

Age of Oceanic Crust Where would you find the newest crust? At the middle of the mid-ocean ridges. Where would you find the oldest Oceanic crust? Far away from the mid-ocean ridges.

Evidence for Sea-Floor Spreading: Magnetic Reversals: Has the North pole always been where it is today? No. It actually switches places about every 300,000 years.

How is this evidence for Sea-floor spreading? Scientists can actually measure the magnetism in a rock and tell if it was made in a normal or reversed time period. This magnetism doesn’t change after the rock has cooled.

This forms a pattern……..an identical pattern on both sides of the mid-ocean ridges. The only way that they can be identical is if they formed at the same time and were then separated (pushed away from each other!) This magnetic “striping” was the final proof for Sea-floor Spreading!