The plates are rigid, and move about. Some plates are comprised only of continents, while others are part continent and part ocean floor. As the plates move, the continents move with them. New ocean floor is created at certain ridges in the oceans.
Old crust is destroyed in ocean trenches and along some continental edges.
Geologists and geophysicists use paleomagnetism to work out the movements of the plates, and consequently, of the continents. Every million years or so the magnetic poles of the earth flip. The earth keeps spinning on its axis as it does now, but the directions compass needles point is reversed. Rocks of the ocean crust acquire a very weak magnetic field when they cool from the molten state at the midocean ridge. The ocean crust, then, acts like a tape recorder, recording the direction of magnetic polarization at the time each portion of the oceanic crust was created.