* Continental drift was the theory proposed by Alfred Wegener in the early 20th century. It suggested that the continents were once joined together in a supercontinent called Pangaea and have since drifted apart.

* Sea-floor spreading was proposed in the 1960s, based on the discovery of mid-ocean ridges and magnetic striping patterns on the ocean floor. It explained how new oceanic crust is created at these ridges and moves away from them.

Plate tectonics combines these two ideas, explaining that the Earth's outer layer (the lithosphere) is broken into large plates that move and interact with each other, driving the movement of continents and the formation of new ocean floor.