The Plate Tectonics Theory

the plate tectonics theory

The plate tectonics theory suggested that Earth's surface is divided into large plates of rigid rock. Each plate moves over Earth's hot semi-plastic mantle.

plate tectonics

The theory of plate tectonics states that Earth's surface is made of rigid slabs of rock, or plates, that move with respect to each other.

tectonic plates

Earth's tectonic plates are large pieces of lithosphere. These lithospheric plates fit together like the pieces of a giant jigsaw puzzle.

lithosphere

the cold and rigid outermost rock layer

plate boundaries

Put two books side by side and imagine each book represents a tectonic plate. You can pull the books apart, push together, and slide the books past one another. Earth's tectonic plates move in much the same way.

divergent plate boundary

forms where two plates separate.

divergent plate boundaries

Mid-ocean ridges are located along divergent plate boundaries or in the middle of a continent. They pull continents apart and firm rift valleys.

transform plate boundary

forms where two plates slide past each other.

transform plate boundaries

As they move past each other, the plates can get stuck and stop moving. Stress builds up when the plates are stuck. Eventually, the stress is too great and the rocks break, suddenly moving apart.

convergent plate boundaries

form where two plates collide.

subduction

the process where a denser plate sinks below a more buoyant one.

convergent boundaries

When an oceanic plate and a continental plate collide, the denser oceanic plate subducts under the edge of the continent. This creates a deep ocean trench. A lone of volcanoes forms above the subducting plate in the edge of the continent.

evidence for plate techtonics

The theory of plate tectonics also provides an explanation for why earthquakes and volcanoes occur in different places. When plates collide, separate, or slide past each other along a plate boundary, stress builds.

plate motion

The main objection to Wegner's continental drift hypothesis was that he could not explain why or how continents move. Scientists now understand that continents move because the asthenosphere moves underneath the lithosphere.

convection

the circulation of material caused by differences in temperature and density.

convection currents

Plate tectonic activity is related to convection in the mantle. Radioactive elements, such as uranium, thorium, and potassium, heat the interior.

forces causing plate motion

Convection occurs in the mantle underneath Earth's tectonic plates. Three forces act on plates to make them move: basal drag from convection currents, ridge push at mid-ocean ridges, and slab pull from subducting planes.

basal drag

Convection currents in the mantle produce a force that causes motion called a basal drag. The convection currents in the asthenosphere circulate and drag the lithosphere similar to the way a conveyer belt moves items along a supermarket checkout.

ridge push

a force where rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge.

ridge push

Because mid-ocean ridges are higher, gravity pulls the surrounding rock down and away from the ridge.

slab pull

the force that happens when a slab sinks and it pulls on the rest of the plate.

slab pull

Because the slab is old and cold, it is denser than the surrounding mantle and will sink. Scientists are still uncertain about which force has the greatest influence in plate motion.

a theory in progress

Plate tectonics explains the connection between continental drift and the formation and destruction of crust along plate boundaries. It also helps to explain the occurrence of earthquakes, volcanoes, and mountains.