The Theory of Plate Tectonics starts with another idea… Continental Drift. The Earth once had a single land mass called Pangaea that broke apart into pieces that drifted away to become the major continents of today.
The continents are moving because of sea floor spreading.
Running down the middle of the oceans are long mountain chains with valleys in the middle called mid ocean ridges. Lava erupts from the center of the valley and pushes the ocean floor away on the other side. The Mid-Atlantic Ridge is separating at approximately 4 cms per year.