During the evolution of our planet, the continents have moved about and changed shape, governed primarily by the opening and closing of oceans and seaways. Throughout this process, pieces of the continents have rifted away while other land masses have collided with their margins, welding themselves to the central cratons. These mobile pieces, known as exotic terranes, ride on the oceanic plates as they move from spreading zones to trenches.
Today, the continents are composed of central, "stable platforms" to which smaller terranes have been sutured. In North America, most of the southeastern Piedmont, all of the land west of eastern Utah, all of British Columbia and the entire State of Alaska have been added as a mosaic of exotic terranes. One of the more traveled land masses was the Alexander Terrane. After splitting from proto-Australia, 500 million years ago, it moved across the ocean, coupled with the Peruvian coast and then bounced along the western edge of North America before attaching to the northwest edge of Canada, 100 million years ago; it is now the southeast panhandle of Alaska. In like manner, Vancouver Island originated in the Southern Hemisphere, moved northward and, 50 million years ago, docked off the coast of British Columbia.
Today, new terranes are forming above oceanic hot spots and along subduction zones. Others are tearing away from their home continents; the opening of the Gulf of California is ripping the Baja away from Mexico and the East African Rift will eventually send a large chunk of that continent out to sea. Southern California is creeping northwestward along the San Andreas Fault and, after millions of years at sea, will eventually merge with western Alaska as the intervening oceanic plate disappears into the Aleutian Trench.
