Along much of the boundary, the bulk of the motion occurs along the San Andreas Fault. The plate boundary is a broad zone of deformation with a width of about 60 miles (100 kilometers). The San Andreas Fault is just one of several faults that accommodate the transform motion between the Pacific and North American plates. The San Andreas Transform Plate Boundary developed within the past 40 million years as a large portion of the Farallon Plate was subducted and the Pacific Plate made contact with the North American Plate in the California region. In central and southern California, for example, the volcanoes have largely eroded away and massive areas of granite from the cooled magma chambers form portions of the Sierra Nevada Mountains, including Yosemite National Park. Remnants of the ancient volcanic mountain chain remain. Over time, the San Andreas transform plate boundary has grown longer as the Farallon Plate split into two separate plates-the Juan de Fuca Plate on the north, and the Cocos Plate on the south. Beginning about 30 million years ago, so much of the Farallon Plate was consumed by subduction that the Pacific and North American plates were in contact, forming the San Andreas transform plate boundary in western California. This resulted in a line of volcanoes stretching all the way from what is now Alaska to Central America.
About 200 million years ago, a large tectonic plate (called the Farallon Plate) started to subduct beneath the western edge of North America. The transform plate boundary between the Pacific and North American Plates in western California formed fairly recently. Cabrillo National Monument south of San Diego also lies within the broad zone of deformation between the two plates. Channel Islands National Park, Santa Monica Mountains National Recreation Area and Joshua Tree National Park are within the Transverse Ranges, a block of crust that rotated as a result of the shearing motion. Point Reyes National Seashore, Golden Gate National Recreation Area, and Pinnacles National Park present landscapes affected by the main line of movement, the San Andreas Fault. Modified from “Earth: Portrait of a Planet, by S. Letters in ovals are abbreviations for NPS sites listed above. In Mexico, a combinatiion of divergent and transform plate boundary motion is opening the Gulf of California, causing the Baja Peninsula to separate from the rest of Mexico. The San Andreas Fault is responsible for most of the movement in western California, causing a sliver of the state to slide past the rest of the continent. The Pacific Plate slides north-northwestward past the North American Plate along the San Andreas Transform Plate Boundary. They were lifted out of the ocean as part of the accretionary wedge of an ancient subduction zone. The sedimentary and metamorphic rocks across the fault line are similar to those found in Redwood National and State Parks on the North Coast of California. They have been transported about 300 miles (500 kilometers) in a north-northwestward direction along the transform plate boundary. The granite rocks in the foreground are similar to those found in Yosemite National Park in the Sierra Nevada Mountains. Tomales Bay is the surface expression of the San Andreas Fault, seen in the photo below. Point Reyes National Seashore, California.
Lateral Movement along a Transform Plate Boundary Virgin Islands is located on another transform plate boundary, where the Caribbean Plate is sliding past the oceanic part of the North American Plate. The landscapes of Channel Islands National Park, Pinnacles National Park, Point Reyes National Seashore and many other NPS sites in California are products of such a broad zone of deformation, where the Pacific Plate moves north-northwestward past the rest of North America. Perhaps nowhere on Earth is such a landscape more dramatically displayed than along the San Andreas Fault in western California. The grinding action between the plates at a transform plate boundary results in shallow earthquakes, large lateral displacement of rock, and a broad zone of crustal deformation. Such boundaries are called transform plate boundaries because they connect other plate boundaries in various combinations, transforming the site of plate motion. Instead, blocks of crust are torn apart in a broad zone of shearing between the two plates. Where tectonic plates slip horizontally past one another, lithosphere is neither created nor destroyed.
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