Plate tectonics is the geological theory that the Earth's outermost layer, the lithosphere, is broken into tectonic plates that move across the mantle. The theory was developed in the 1960s-1970s from evidence of seafloor spreading, paleomagnetism, and the distribution of fossils, earthquakes, and volcanoes. The Earth's interior is divided into layers - the crust, mantle, and core. The crust consists of continental and oceanic plates that move due to convection currents in the mantle, causing phenomena like subduction and seafloor spreading.