Continents rest on massive plates known as tectonic plates, which are fluid and able to move due to the mantle and magma underneath, and as the tectonic plates move about, they cause continental drift to happen. The concept of continental drift was first described in the early 20th century by German scientist Alfred Wegener, who explained that continental landmasses were actually drifting across the Earth. This is opposed to the original theory that landmasses are fixed and immobile.
Wegener believed that all of the landmasses were originally joined together in a single landmass known as Pangaea. Pangaea is thought to have existed 240 million years ago; however, this large landmass or continent started breaking up, approximately 200 million years ago, into the continents that are known today.
Underwater exploration by scientists has determined that continental drift is indeed an accurate theory. The seafloor is spreading at various places, and the new crust that forms pushes the two nearby plates further apart from one another. This is caused by the molten rock from within the earth that has been steadily rising. For example, North America and Europe are thought to be moving further away from each other at a rate of 1 inch per year.