Viewed from space, one of the remarkable features of our planet is that it has continents and oceans. The changing pattern of the continents and oceans is a very important characteristic of the earth, and the discovery of how this movement occurs is one of the most exciting scientific stories of modern times. The key to this understanding lay not in the continents at all, but in the oceans. The oceans turned out to have a record of their history as precise as tree rings or a magnetic bar code, and with it came the history of relative motion between the continents. These motions are not fast by our standards, but by geological standards they are very rapid. As a description of what is happening in the oceans, plate tectonics is so simple, powerful, and accurate that it has come to dominate our view of the earth’s behavior. (57 minutes)