Plate tectonics is the scientific theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared to Earth's mantle.
The theory was developed from the 1950s through the 1970s, building on the earlier concept of continental drift.
It is the unifying theory of geology, explaining phenomena like earthquakes, volcanic eruptions, and the formation of mountains.