What Causes Land Masses to Shift ?

Continental Drift

• The idea the continents were in motion came long before anyone figured out how that could happen. Early in the 20th century, scientist Alfred Wegener amassed evidence for what he called continental drift. The evidence included the way the African and South American coasts fit together and similar fossil creatures found on both continents. He proposed that all the continents had once belonged to a single land mass. Later in the century, geologists found proof Wegener was correct. Their discoveries gave birth to the study of plate tectonics.

Magma Rises

• Plate tectonic theory sees the Earth's surface as multiple rock slabs floating on a giant sea of magma underneath. The magma rises and sinks as it heats and cools, creating circular currents in the molten rock that contribute to moving the plates. The boundaries between the plates are where the forces shifting the land masses make themselves known. At divergent plate boundaries, such as the Mid-Atlantic Ridge, magma rises to the surface, spreading the sea floor at a rate of 2.5 centimeters (0.98 inch) a year. The spreading floor moves the plates apart. Other divergent boundaries around the world have the same effect.

Rising and Sinking

• As the spread from the rifts pushes the tectonic plates around, they bump against each other. When they meet at a transform boundary, they slide past each other horizontally. At convergent boundaries, sometimes called subduction zones, one plate slides under another. The Andes Mountains in South America have risen because they lie along a convergent boundary where the Nazca Plate pushes below the South American Plate. Convergent boundaries in the Pacific Ocean have created the ocean's deep trenches. The speed of subduction matches the speed at which the sea floor spreads, so the Earth stays the same size.

When the Earth Shakes

• The interaction when plates meet isn't always smooth. Plates may lock in place with one unable to move under the other. One plate may fracture or break off smaller plates. When a plate locks, pressure builds up until it suddenly lurches past the plate on the other side of the boundary. The result is an earthquake. The release of the two plates into motion can lead to the surface lurching several meters upward during a quake.