The plates are in very slow, but constant, motion. When one plate is pushed under another plate, the top plate may scrape off pieces of the bottom plate. Over millions of years, this material piles up to form an island. Barbados in the West Indies and Kodiak Island near Alaska were formed this way. Movements of the earth's tectonic plates also cause land masses to drift apart.
Scientists believe that the present continents are parts of what was once a single large continent.
This land mass broke up about 200 million years ago, and ocean basins formed between the fragments. The smaller land fragments thus became tectonically formed islands. Greenland and Madagascar were formed this way. The tectonic plates are still slowly moving, and so new islands will continue to be formed. In several million years, southwestern California will probably have moved to the northwest to form an island off the west coast of North America.