[Geology2] History of the San Andreas Fault

History of the San Andreas Fault The San Andreas fault is about 28 million years old. Back then, California didn't exist, at least not recognizably so. Bits and pieces could be found scattered around the western edge of North American, which at the time ended at the Pacific ocean lapping the shores of central Utah and Arizona. Other parts were volcanic islands in the eastern Pacific. Most of what was to become California was either under the ocean or still being created beneath western North America. Thirty millions years ago seems like a long time but in geological time, it is like 'yesterday', being less than 1% of the age of the Earth (4.6 billion years). Like all land forms and topographic features, California's are much younger than the rock from which they are made. As a mountain range, the Sierras are less than five million years old, even though some of their granite and diorite solidified deep underground about 115 million years ago. The most recent uplift of the mountains began about 4 million years ago and continues today. The SAF was born when the Pacific Plate first touched the North American Plate. Before this time the Farallon Plate was subducting under North America, and most of it is now scraping along the bottom of the NA plate, ultimately to descend into the mantle, melt and be recycled. Pieces of the Farallon Plate are still around as the Juan de Fuca, Cocos and Rivera plates. The boundary between the Pacific and Farallon plates is called the East Pacific Rise (EPR). The EPR still exists as a series of transform faults and spreading centers that run the length of the Sea of Cortez; indeed, it was the EPR the split Baja from mainland Mexico.

Like countless others, Alfred Wegener noticed that the west coast of Europe and Africa had more-or-less the same shape as eastern North and South America. It was as though they were once together and had been pulled apart, the gap being filled by the Atlantic Ocean. Though trained as an astronomer and working as a meteorologist, Wegener visited the Atlantic shores and found that rocks and fossils on one side of the ocean matched those on the other side. Wegener proposed that the continents were drifting apart. He was roundly damned by geologists as an outsider and ignored. But the idea took root and evidence of shifting continents slowly accumulated. Of particular note was the discovery of alternating magnetic field alignments in sea floor rocks near what are now called spreading centers, like the Mid-Atlantic Ridge. In 1960, Harry Hess and Robert Dietz put the pieces together and showed that Wegener was right, placing the idea of plate tectonics on firm ground. Hess (and others) unifying work proved to be one of the most important concepts in geology because it explained so many things: sea floor spreading, mountain building, the distribution of volcanoes and earthquakes and how they are related, fault motions and much more. The San Andreas Fault received its name from Andrew Lawson after the 1906 earthquake. He named it for San Andreas Lake, a (now) man-modified sag pond in San Mateo county through which the fault passes. The lake was named by Gaspar de Portola in 1769 who camped near the (then) small body of water (See etymology link below). The earliest documented record of an earthquake comes from China, in 1831 BC from the Shandong province. The most complete historical records also come from China, starting in 780 BC during the Zhou Dynasty in China.

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