To understand the basics of this theory, we need to know a little about the structure of the planet on which we live. The outer crust is not uniform, however. Underneath the oceans it consists of very heavy basalt and is only two to five miles thick. The much thicker continental crust is made up mostly of granite, a much lighter rock. Several lines of evidence support the theory of Plate Tectonics.
Most plates consist of oceanic crust and a larger or smaller chunk of continental crust.
Other breaks occur where plates collide and old oceanic material of one plate plunges below another and back into the Earth's interior. A long time ago, the east coast of North America was the scene of plate subductions and mountain-building. A series of three such orogenies started 500 million years ago and lasted about 300 million years. During each of the first two orogenies, island arcs, maybe similar to the Japanese islands in the Pacific today, were added to the North American continent from the east. By 250 million years ago, Africa had bumped into North America.