Geology (cont. 8)


About 20 million years ago, the Farallon Plate disappeared completely, and a large fracture zone, known today as the San Andreas Fault Zone, appeared in the Pacific floor, and cut through part of the North American continent. A slice of land began to move to the northwest with the ocean floor.  Thus was part of Southern California ripped off the North American Plate, becoming part of the Pacific Ocean Plate, and carried away from the Mexican mainland at the rate of about two inches a year. The fan deposits of the Poway conglomerate came with us. Today, the cementing muds and sands have been eroded away, and the cobbles are on our beaches, testifying to our Mexican past. We are headed to Alaska, where a trench leads the Pacific Plate back into the Earth’s mantle. At the present rate of movement, we won’t get there for another 40 million years, or so. The movement of the plate, however, shakes the many faults of the San Andreas Fault Zone, and we feel many small earthquakes in the Batiquitos Lagoon area.

Page 8

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