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Geologic History of Volcan Mountain
Geologically speaking, Volcan Mountain has a
relatively short history. The oldest rocks identified in San Diego County
date from the middle of the Mesozoic era - the age of dinosaurs - 190-150
million years ago. The land in the southwest formed an island arc, similar
to Japan. To the west was a deep sea trench and to the east a shallow sea.
This geological pattern forms what is called a "subduction zone",
where one plate of the earth's crust began to slide beneath another. This
action created long periods of great volcanic activity, followed by erosion
processes. About 22 million years ago, the Pacific coast of Mexico and Southern
California started to break up. The Pacific Plate began to move northwest
relative to the North American Plate. This caused a renewal of volcanic
activity. The stress and tension of this movement formed the San Andreas
Fault. The flat plain that had been the west coastal area tipped up like
a giant wedge forming the Peninsular Ranges from south of Los Angeles to
the center of Baja California, of which Volcan Mountain is a part.
Today, driving east from San Diego, you climb the gentle slope of this tipped wedge, to the top of Volcan, where you drop quickly down the gulf escarpment on the east face to the Salton Sea depression. Tectonic activity, or movement of the land, is still in process as can be seen by the slanting vertical patterns on watersheds and smaller canyons on the northern flank of Banner Canyon. The Elsinore Fault, a sub-parallel fault of the San Andreas Fault, traverses Volcan Mountain in a northwest-southeast direction. To date, there has been little recorded seismic activity, though the fault is capable of generating a 7.3+ earthquake.
The entire Peninsular Range is underlain by Jurassic/Cretaceous rocks of the Southern California Batholith which are composed of undifferentiated granites and tonalites. Also found is Triassic metamorphic rock composed of micaceous schist, gneiss and quartzite, known as Julian schist. The Julian schist that became exposed through erosion was found to be layered with rich ore-bearing quartz containing gold of unusually high grade (940-980 fine). This discovery, of course, brought about the extensive mining activities in and around Julian.
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