Geologists widely accept that continents are not fixed in place but slowly drift over geological time. This movement is explained by the theory of plate tectonics, which states that the lithosphere—Earth’s rigid outer layer that includes both continents and ocean floors—rests atop the asthenosphere, a softer, semi-fluid layer of partially molten rock within the upper mantle. Heat produced by the decay of radioactive elements deep in the mantle drives convection currents in this layer. These currents act like a conveyor belt, causing massive slabs of lithosphere, called tectonic plates, to move. Over millions of years, this process has rearranged continents, opened and closed oceans, and shaped Earth’s mountains, earthquakes, and volcanoes.