Hi All. Do you know that there is an active volcano in the Bay Area? According to Dr. Terry Wright there is. It is located in Sonoma County. It is Mount Konocti. There are a few more ancient volcanos in Sonoma County, but this is the only one that is classed as being active.
The Sonoma Volcanics are "shield' volcanos, low and spread out because the lava flowed so fast.
Mount Konocti has the more classic rugged cone shape and is still on the active list because it erupted "recently in geologic time, that is about 10,000 years ago. Relics of the lava flow's scorch have become one of Sonoma County's tourist attractions the "Petrified Forest'' off of Calistoga Road near the Napa County line. There you can see trees that were felled and preserved by the lava.
An earthquake fault indirectly gets credit for those petrified trees. Wright explained that the volcanos erupted when the earth's plate dragged deep down into the core of the earth, where it melted and turned to magma, which in turned returned to the surface as lava.
Plate movement also explains why a vast area of western Marin and Sonoma Counties are now dry instead of underwater like they were five million years ago. One plate moved beneath another, lifting the earth, causing inland areas to rise, draining the inland sea. This is the main reason that when any major earthquake centers in this area it will be highly destructive.
I tell people right off the bat that Sonoma County geology is schizophrenic. There are two distinct geologies, and the San Andreas is the dividing line,'' said Wright.
The San Andreas is the major dividing point between the North American Plate, which extends from the West Coast across the continent to the Atlantic, and the Pacific Plate, under the Pacific Ocean.
Bodega Head -- a big chunk of 100 million year old granite -- is on the Pacific Plate, on the west side of the San Andreas, moving north.
It has moved up the 320 miles from the Tehachapis, earthquake by earthquake. The last big move was 15 feet in the 1906 quake, Wright said. Take Care…Don in creepy town.