Plate tectonics

Earth's lithosphere consists of the crust and the rigid uppermost mantle. It is divided into a set of large and smaller plates that move slowly over the weaker, deformable mantle beneath. Plates carry both oceanic and continental regions; continents are not independent rafts drifting through oceanic crust. Their motion is measured over geologic time and can also be tracked today with satellite geodesy. The exact contribution of forces such as gravitational sinking of old oceanic lithosphere and mantle flow is an active area of geophysical research.

At divergent boundaries, plates move apart. Magma rises and cools to form new oceanic lithosphere along mid-ocean ridges, while continental rifting can begin the separation of landmasses. At convergent boundaries, plates move together. Dense oceanic lithosphere commonly descends beneath another plate in a subduction zone, generating trenches, earthquakes and, in many settings, volcanoes; collision between continents can build mountain belts. At transform boundaries, plates slide horizontally past each other, producing faults and earthquakes without creating or consuming lithosphere at the boundary. Some interactions occur across broad plate-boundary zones rather than a single narrow line.

The modern theory developed through the synthesis of several lines of evidence in the mid-twentieth century. Matching fossils and rock formations across now-separated continents, together with the fit of continental margins, supported earlier proposals of continental drift. Mapping of the ocean floor revealed mid-ocean ridges and deep trenches. The pattern of magnetic stripes recorded in oceanic crust and the age distribution of the seafloor supported seafloor spreading and the recycling of oceanic lithosphere at trenches. Together, these observations established a global, dynamic system rather than isolated continental movement.

Plate interactions help explain the global distribution of many earthquakes and volcanoes and the long-term growth and destruction of oceanic crust. They also shape mountain ranges, island arcs and ocean basins. The framework does not make every earthquake or volcanic eruption predictable: local geology and the complex behavior of faults and magma systems matter. Geologists use seismic observations, rock records and precise satellite measurements to reconstruct past plate motions and monitor present deformation. Research continues on how plate motion is driven and how plate tectonics began early in Earth's history.