Plate Tectonics

Alfred Wegener (1912) proposed the idea of continental drift, not widely accepted at the time.

Based on the shape of the continents, and on the biological record, Wegener proposed that the continents once fitted together, and had been slowly moving since. But how would the continents move across the Earth's crust?

In the 1960s, evidence was found for the spreading of the sea floor of the Atlantic Ocean. It was realized that the continents don't move over the crust, but rather that the crust itself is in motion. This was the birth of the modern theory of plate tectonics.

The lithosphere is the solid crust and upper mantle of the Earth. It "floats" upon the plastic, fluidlike, and convective part of the mantle. Giant crustal plates move due to convective "currents'' in the mantle.

What happens where the plates come into contact? How can they move against one another?

Look at the map.

Where are earthquakes common?
Where are volcanos common?
Where are the mountain ranges?

Where plates slide by each other we find transform faults.

Example: San Andreas Fault -- interface between the North American and Pacific plates

Plates move slowly (few cm/yr): LA will be next door to SF in 20 million years...

Motion not smooth. Plates "snag" against one another, then suddenly release: Earthquake!

Where plates ram into one another we have subduction zones, where ocean plates are pulled back into the mantle. Volcanoes are common here. On land, mountain ranges can form.

In the sea floors, we have active rifts, where new magma rises up from the mantle reforming the sea floor.

Plate tectonics is essentially a continual recycling of material between the Earth's crust and mantle:

Not all volcanoes occur at plate boundaries. For example, the Hawaiian islands formed from a hot spot volcano. As the Pacific plate moves across a fixed hot spot (mantle plume), new volcanoes (and islands) form:

Continental Drift

The continents are merely along for the ride on the crustal plates. 225 million years ago, all the continents were part of a single large mass known as Pangaea: