Ch. 7 -2 Restless Continents Continental Drift Sea Floor Spreading

Wegner’s Continental Drift Hypothesis Alfred Wegener wrote a hypothesis about continental drift that states they were once a single landmass, broke up and then drifted to their present location. It explained why fossils of the same plant and animal species are found on continents on different sides of the Atlantic Ocean.

Pangaea Pangaea is Greek for “all earth.” It existed about 245 million years ago. We know that Pangaea further split into 2 huge continents: Laurasia and Gondwana.

Sea Floor Spreading From the calculated strength of rocks, it seem possible that crust could move this way – spread out! See p. 200 figure 3. Mid Ocean Ridges are underwater mountain chains that run through Earths ocean basins. This means that they are ALSO where sea floor spreading takes place! Sea floor spreading is the process by which new oceanic lithosphere forms as magma rises toward the surface and solidifies.

Sea Floor Spreading con’t. As the tectonic plates move away from each other the sea floor move away and the magma fills in the gap. The older crust is farther away from the mid ocean ridge that the younger crust is. Magnetic Reversals Evidence: when Earth’s magnetic poles change places this change, is called a magnetic reversal. Since the molten rock at the mid ocean ridges contain tiny grains of magnetic minerals (contain iron), they act like compasses. They will align when they cool and this creates a record or proof that sea floor spreading occurs.