Plate Tectonics
Plate tectonics is the theory that Earth's lithosphere is divided into moving plates whose interactions cause earthquakes, volcanoes, and mountains.
Definition
Plate tectonics is the idea that Earth's outer shell is broken into big pieces called plates that slowly move, and where the plates bump, pull apart, or slide past each other, we get earthquakes, volcanoes, and mountains. It is the unifying theory that the lithosphere is divided into about a dozen major plates and several minor ones that move over the asthenosphere at rates of $2$ to $10\ \text{cm/yr}$, interacting at divergent (moving apart), convergent (coming together), and transform (sliding past) boundaries, with mantle convection, ridge push, and slab pull driving the motion. The theory explains the distribution of earthquakes, volcanoes, mountain belts, and ocean basins. Rigid plate motions are rotations about Euler poles on a sphere, so linear velocity varies with angular distance from the pole. Driving forces are dominated by slab pull (the negative buoyancy of cold subducting lithosphere), with ridge push and basal drag secondary; the fastest plates such as the Pacific are those with the longest subducting margins. Plate boundaries are zones of strain release while plate interiors are comparatively stable, which is why intraplate earthquakes such as the 1886 Charleston event are rare and puzzling, and the Wilson cycle describes the opening and closing of ocean basins over hundreds of millions of years.
Example
The plates move about as fast as your fingernails grow, and over millions of years that adds up: the Atlantic Ocean is getting wider by a few centimeters every year. The Mid-Atlantic Ridge is a divergent boundary, the Andes mark a convergent boundary where the Nazca Plate subducts beneath South America, the San Andreas Fault is a transform boundary, and the Appalachians formed by continental collision when Pangaea assembled. GPS geodesy now measures plate motion directly: the Pacific Plate moves northwest relative to North America at about $5\ \text{cm/yr}$ across the San Andreas system, and the North American and Eurasian Plates separate at about $2.5\ \text{cm/yr}$ at the Mid-Atlantic Ridge. Hot spot tracks like the Hawaiian-Emperor chain record plate motion over a fixed mantle plume.
Key Insight
Almost all earthquakes and volcanoes happen along plate edges, so if you draw the earthquakes on a map you have drawn the plates. Plate tectonics was accepted only in the 1960s, after seafloor mapping revealed mid-ocean ridges and magnetic stripes; it is a young theory that explained decades of puzzling evidence at once. It operates on Earth and, as far as we know, nowhere else in the solar system today. Its existence depends on water weakening the lithosphere and on a mantle hot enough to convect but cool enough to form rigid plates, a narrow window that may be essential for a habitable planet.