Plate tectonics is a theory that scientists have made to explain how the continental plates move. In 1912 a German scientist, Alfred Wegener, first talked about continental drift and seafloor spreading. According to the plate tectonics theory the surface fo the earth is made up of 12 giant seventy-mile thick rock plates. The 12 plates fit together a little bit like a puzzle does. They are constantly moving and grinding against each other. Continental plates float on the aesthenosphere, a liquidy plastic-like layer below the lithosphere, and it is also called the mantle of the earth. The plates never stop moving. They crunch against each other where they meet. Volcanoes and mountain ranges are also found where the continental plates meet.

Since the lithosphere covers the entire surface of the earth, all of the Earth's plates are part of the theory of continental drift. There are some plates above the surface of the ocean and others that are below the surface of the ocean. Under the surface of the ocean there are mountain ranges called the mid-ocean ridge. The tallest mountains in the world are found at the bottom of the Pacific Ocean.

Millions of years ago the continents were not in the same place that they are today. They are now far apart. Because of what is called continental drift, where the continents move, they have moved apart to where they are today.

	Another part of the continental drift theory tells us how volcanic activity causes the seafloor to expand and swallow up parts of the earth that are around it. The earth is recycled down into the hot core and some of it comes back up through the vents in volcanoes after it has melted.