Plate Tectonics
The theory that Earth's outer shell consists of moving plates whose interactions build mountains, open oceans, and trigger earthquakes and volcanoes.
Plate tectonics is the unifying theory of geology: Earth's rigid outer shell, the lithosphere, is broken into a dozen or so large plates and many smaller ones that move relative to one another over the hotter, ductile asthenosphere beneath. The theory explains the global distribution of earthquakes, volcanoes, mountain ranges, and ocean basins, and it developed in the 1960s from earlier ideas about continental drift and seafloor spreading.
Plates move at rates of a few centimeters per year — comparable to the growth rate of fingernails. Their interactions occur at three types of boundaries. At divergent boundaries, plates move apart and new oceanic crust is created at mid-ocean ridges, where upwelling mantle melts and fills the gap; the Mid-Atlantic Ridge is the largest example. At convergent boundaries, plates collide: the denser plate subducts beneath the other, generating deep ocean trenches, volcanic arcs (such as the Andes and Japan), and, when two continental plates collide, mountain ranges such as the Himalayas. At transform boundaries, plates slide past one another horizontally, as along the San Andreas Fault in California.
The forces that drive the plates are debated in detail, but the major contributors are slab pull — the weight of cold, dense subducting lithosphere dragging the rest of the plate behind it — and ridge push, the gravitational sliding of elevated oceanic ridges. Mantle convection provides the larger context in which plates move, though plate motion is not simply a conveyor belt riding on convection cells.
The evidence for the theory is extensive and mutually reinforcing: the jigsaw fit of continents; matching fossils and rock sequences across oceans (for example, Mesosaurus on both sides of the South Atlantic); paleomagnetic stripes on the seafloor recording reversals of Earth's magnetic field as crust formed at ridges; the systematic aging of seafloor away from ridges; and direct measurement of plate motion by GPS.
The theory also explains Earth's deep-time record: continents assemble into supercontinents and break apart in cycles lasting hundreds of millions of years, most recently exemplified by Pangaea, which began to split about 200 million years ago. Earthquakes and volcanic eruptions concentrate at plate boundaries because that is where stress accumulates and magma rises — which is why mapping plate boundaries is also a practical tool for hazard assessment.