Hi Petra. After doing some more research into Donnalee’s statement about the Castle Peak volcano and the description of the caldera that she gave me that it just might be a Rhyolit caldera.

I think the light colored rocks you found are Rhyolit. I gave the description to Tracy and he said that most likely that is what the material we found is.

Rhyolite caldera complexes are the most explosive of Earth's volcanoes and more often as not they don't even look like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure. That may account for why I can find the same kind of rocks in South County. The collapsed depressions are large calderas, and they indicate that the magma chambers associated with the eruptions are huge. In fact, layers of ash (either ash falls or ash flows) often extend over thousands of square kilometers in all directions from these calderas. Many rhyolite caldera complexes, however, are the scenes of small-scale eruptions during the long reposes between big explosive events. This appears to be the case with the one we were told about.

The site you showed me last night concerning the Mt. Diablo range volcanics here in San Benito County and the much smaller one in Parkfield may be the source of past volcanic activity in this area. The one under the Mt. Diablo range in San Benito County could be the source of the heat for our hot springs both of which are about a 45 minute drive from where I live. Take Care…Don in creepy town