Plate Tectonics Chapter 10

CONTINENTAL DRIFT In 1912, Alfred Wegener proposed a hypothesis: the continents moved across the ocean floor.

Evidence for Continental Drift The maps of South America and Africa matched like a jigsaw puzzle. Rare fossils were found on the coasts of South America and Africa. The rock structures and ages of North America and Europe were also similar. There were signs of glaciers in warm climates. There were swamp plant fossils in cold climates. Evidence for Continental Drift

Scientists rejected the idea Scientists rejected the idea. They could not find a mechanism that would cause this drift. It was considered impossible.

In 1947, scientists mapped undersea mountain ranges.

They discovered that the continental crust is older than the oceanic crust. And then, they learned that the crust closest to the ridge is newer than the rock further out.

How do we measure the ages of rocks?

Amount of sediment on top Amount of radioactive isotope left Magnetic reversals

The oceanic crust is newer than the continental crust The oceanic crust is newer than the continental crust. The crust closest to the ridge is newer than the rock further out. New rock is forming in the ridge.

Sea Floor Spreading If hot magma rises in the middle, colder rock must sink on the outside.

Causes of Spreading: Convection currents, Ridge push, and Slab pull.

There are many ocean ridges There are many ocean ridges. They are called divergent boundaries because the sea floor separates there.

The lithosphere is divided into plates The lithosphere is divided into plates. The plates are in motion atop the plastic asthenosphere.

Plates have ocean crust and, often, lighter continental crust.

If the sea floor spreads, the crust has to go somewhere.

The planet is covered with very slooooooowww crashes.

One plate must sink under the other, in a process called subduction One plate must sink under the other, in a process called subduction. As it reaches hotter levels, this plate often melts.

Volcanoes & earthquakes happen where plates meet.

Continental crust is less dense, and ends up on top of the oceanic crust.

When continental crust hits ocean crust, mountains with volcanoes may form. An ocean trench occurs alongside.

Where two ocean plates meet, volcanic islands may form.

Where two continental plates meet, very large mountains form.

The continents are believed to have once been a supercontinent called Pangaea.

Florida was part of the African continent before Pangaea formed Florida was part of the African continent before Pangaea formed. Afterwards, it joined North America.

CONTINENTAL DRIFT Alfred Wegener’s hypothesis has become the theory of plate tectonics.