Plate Tectonics

Plate Tectonic Theory

The theory of plate tectonics tells us that the Earth's lithosphere (hard outer rock layer) is comprised of several rigid pieces, called plates (See Figure 1). These plates float on the asthenosphere (a layer of rock that is not liquid but can flow, kind of like silly putty). The plates move around with respect to one another. Along the edges of the plates is where most of the geologic action on Earth takes place. Plates scrape past each other, pull away from each other, or one plate dives beneath another into the asthenosphere. All this turmoil causes stuff like earthquakes and volcanoes. Rocks are created and rocks are destroyed, and continents drift around on the surface of the globe.

Here are some examples of the types plate boundaries observed around the globe

• Convergent plate boundaries
• Divergent plate boundaries
• Transform fault boundaries

Seafloor Spreading

This is the process by which new oceanic crust is produced along divergent (spreading apart) plate boundaries. As continents rift and the plates move apart, magma moves upward from the asthenosphere and fills in the spaces. The process is repeated over time. Imagine a conveyor belt moving outward on both sides of the spreading center.

Hot Spots

Most volcanic activity occurs along plate margins, however there are cases of intraplate volcanism. These are caused by upwelling of hot rock moving through the mantle. This results in what is called a hot spot. Yellowstone National Park sits atop a hot spot, this is what causes the hot springs, geysers, and other phenomena the park is famous for. Another hot spot, beneath the Pacific plate, is responsible for the volcanism that created the Hawaiian Islands and continues to change the landscape there. Hot spots stay in one place as plates move over them. Volcanoes grow out of the water and form islands. As the plate continues to move, the new island moves away from the hot spot and begins to subside into the ocean.

See the "Dynamic Earth" an On-line publication by U. S. Geological Survey about Plate Tectonics

CLICK HERE to view reconstructed plate motion histories for

• Pacific Plate 85 my ago - Present
• NE Pacific and North America
• Southern California

Return to Current Schedule Page