San Andreas Fault
Tectonic plate boundaries, like the San Andreas Fault pictured here, can be the sites of mountain-building events, volcanoes, or valley or rift creation.
Photograph by Georg Gerster
Plate tectonics is a scientific theory that explains how major landforms are created as a result of Earth’s subterranean movements. The theory, which solidified in the 1960s, transformed the earth sciences by explaining many phenomena, including mountain building events, volcanoes, and earthquakes.
In plate tectonics, Earth’s outermost layer, or lithosphere—made up of the crust and upper mantle—is broken into large rocky plates. These plates lie on top of a partially molten layer of rock called the asthenosphere. Due to the convection of the asthenosphere and lithosphere, the plates move relative to each other at different rates, from two to 15 centimeters (one to six inches) per year. This interaction of tectonic plates is responsible for many different geological formations such as the Himalaya mountain range in Asia, the East African Rift, and the San Andreas Fault in California, United States.
The idea that continents moved over time had been proposed before the 20th century. However, the scientific community took notice in 1912 when a German scientist named Alfred Wegener published two articles about a concept called continental drift. He suggested that 200 million years ago, a supercontinent he called Pangaea began to break into pieces, its parts moving away from one another. The continents we see today are fragments of that supercontinent. To support his theory, Wegener pointed to matching rock formations and similar fossils in Brazil and West Africa. In addition, South America and Africa looked like they could fit together like puzzle pieces.
Despite being dismissed at first, the theory gained steam in the 1950s and 1960s as new data began to support the idea of continental drift. Maps of the ocean floor showed a massive undersea mountain range that almost circled the entire Earth. An American geologist named Harry Hess proposed that these ridges were the result of molten rock rising from the asthenosphere. As it came to the surface, the rock cooled, making new crust and spreading the seafloor away from the ridge in a conveyer-belt motion. Millions of years later, the crust would disappear into ocean trenches at places called subduction zones and cycle back into Earth. Magnetic data from the ocean floor and the relatively young age of oceanic crust supported Hess’s hypothesis of seafloor spreading.
There was one nagging question with the plate tectonics theory: Most volcanoes are found above subduction zones, but some form far away from these plate boundaries. How could this be explained? This question was finally answered in 1963 by a Canadian geologist, John Tuzo Wilson. He proposed that volcanic island chains, like the Hawaiian Islands, are created by fixed “hot spots” in the mantle. At those places, magma forces its way upward through the moving plate of the sea floor. As the plate moves over the hot spot, one volcanic island after another is formed. Wilson’s explanation gave further support to plate tectonics. Today, the theory is almost universally accepted.
layer in Earth's mantle between the lithosphere (above) and the upper mantle (below).
the movement of continents resulting from the motion of tectonic plates.
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
relationship between two or more forces, objects, or organisms.
outer, solid portion of the Earth. Also called the geosphere.
solid material turned to liquid by heat.
movement and interaction of the Earth's plates.
rift in underwater mountain range where new oceanic crust is formed.
massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.