California Faultlines

Continental Plates Map (credit: Scripps Institute of Oceanography)

It is not often that you can see where plate tectonics is unfolding. The globe-spanning process, created and still carries the continents on gigantic plates of the Earth's mantle that folds under or slides past each other mostly beneath the oceans. In a few places, a plate's faultlines are located on land. Iceland sits on the middle of the Mid-Atlantic ridge and is spreading apart from the inside of the island outward. Iceland's two sections move in opposite directions. An video explains the basics of the geologic process.

In California, the Pacific Plate and the North American Plate are grinding past each other on the West Coast. The friction of their slow but constant movement creates earthquakes as pressure is abruptly released from deep underground. Los Angeles and San Francisco sit on the edge of the continent and the San Andres Fault runs near both cities. In 1906, San Francisco dramatically experienced the deadly consequences of what can happen when the San Andres shifts.

The San Andres can be seen from space by Earth-monitoring satellites. It was photographed in a natural color image of Point Reyes National Seashore. A rift in the coastal landscape, where the two plates are grinding against each other in separate directions, is clearly visible at Tomales Bay north of San Francisco. As the rift continues to grow, from further movement of the plates, the bay will eventually become open to the ocean at the southern end. The western section of the coast will become an island. A similar result will occur on a portion of Southern California that will including the Baja peninsula to become a second island carried along on Pacific Plate as it moves moves north. The process is predicted to take between 3-5 million years for both rifts to become islands. WHB