Tectonics

Tectonics is the science of the large-scale deformation of a planet's outer shell, especially the processes that shape the Earth's lithosphere. On Earth, tectonics focuses on the movement and interaction of rigid plates that ride on the underlying, partially molten mantle.

Earth's plate tectonics posits that the lithosphere is divided into about a dozen major and several minor tectonic plates that move relative to each other. This motion is driven largely by convection in the mantle—circulating rock beneath the plates—and by gravitational forces acting on cooling, subducting slabs (slab pull) and at spreading centers (ridge push).

At plate boundaries, three principal types occur: divergent boundaries where plates pull apart, creating new lithosphere at mid-ocean ridges; convergent boundaries where plates collide, generating mountain belts or volcanic arcs and often subduction of one plate beneath another; and transform boundaries where plates slide horizontally past one another, producing shallow earthquakes along faults such as the San Andreas Fault.

Tectonic activity explains many surface features and hazards: seafloor spreading at ridges forms ocean basins; subduction zones produce deep earthquakes and volcanoes; continental collision builds mountain ranges; and transform boundaries link the system as plates move. Over geologic time, plate tectonics drives the Wilson cycle of supercontinent assembly and breakup, reshaping continents and oceans.

Evidence for plate tectonics includes ocean floor magnetic anomalies, earthquake distribution, age patterns of seafloor, and direct measurements of plate motions using GPS. The theory was developed in the 20th century, combining Wegener's continental drift concept with data from seafloor mapping, paleomagnetism, and seismic observations, and remains central to modern geology.