Hi Petra. The volcanics in the Oakland, Orinda areas are most likely the Quien Sabe Volcanics that start in the Hollister area and go north into Sonoma County and possibly Lake County.

The day we went to the Olompali State Historic Park and you and Jen took a walk to the Indian village you most likely were very close to the Burdell Mountain Fault Zone. If you remember the fountain that looked like a volcano the rock that was used to make it was most likely Miocene (25 to 13 million years ago) andesite, which is typical of the Burdell Mountain Volcanics.

The following is from: “MCLAUGHLIN, Robert J.1, SARNA-WOJCICKI, Andrei1, FLECK, Robert J.1, LANGENHEIM, Victoria E.1, MCPHEE, Darcy1, GRAYMER, Russell1, ROBERTS, Carter W.1, MCCABE, Craig1, VALIN, Zenon1, and WAGNER, David L.2, (1) U.S. Geol Survey, 345 Middlefield Road, Menlo Park, CA 94025, rjmcl@usgs.gov, (2) California Geological Survey, 801 K St., MS1232, Sacramento, CA 95814”

“The distribution of Sonoma Volcanics that erupted in Napa Valley between 5 and 2.8 Ma follows a younging trend toward the NW, seemingly parallel to the subduction-related fault system. Earlier work by Fox on dike orientations suggests W-NW to E-SE extension during volcanism, compatible with present-day strike-slip. So although volcanic venting in the wake of the Mendocino Triple junction was in response to the stress field of the transform margin, volcanism has migrated northwestward along pre-San Andreas structures.”

“Another W-NW-trending, NE- and SW-vergent imbricate zone of inactive reverse and thrust faults splays from the southern Rodgers Creek Fault and extends to the San Andreas Fault near Jenner. Faults of this system have been proposed to have taken up strike-slip between the volcanics of Burdell Mountain and the coeval Quien Sabe Volcanics near Hollister between ~12 and 3.5 Ma (a proto-Hayward fault system). Most strike-slip along this fault system pre-dates the 6.3 Ma Roblar tuff and may be masked by the younger thusting.”

Past volcanic activity can be seen in the Diablo Mountain Range in the Hollister area in the form of andesite and basalt, which can be seen in the Pacheco Pass area. There are two active hot springs that are most likely associated with the Quien Sabe Volcanics. These are the Gilroy Hot Springs in Santa Clara County and the Mercey Hot Spring in Fresno County just south of San Benito County. What makes this place so interesting is that a compass will go bonkers in some areas.

I have been told that one can see cinder cones from the Panoche fire lookout station on Pacheco Peak. I have never seen them so can’t say for sure that they are there, but don’t doubt that there are some there as we have just about everything else in this area including some ancient lava flows which can be seen on my Aunts ranch.

Wouldn’t it be something if all of the ancient volcanics in an around the area decided to become active again? Boy oh boy what a show that would be. Take Care…Don in creepy town

field trip

another field trip

Volcanics