The Theory of Plate Tectonics, first proposed by Alfred Wegener, is the idea that the Earth's crust is made up of plates that float on a liquid layer of magma. These plates move constantly, sometimes bumping into each other or sliding under each other. We have a great deal of evidence for this theory, such as the shape of the continents, the location of fossils of similar species on different continents, and the distance of the continents from one another.