Oh man there's some sort of volcanic park north of Berkely, CA that we visited once, and up on some mountain was a firewatch tower with a dude just chilling in there looking for smoke. He worked up there in 2 week shifts, just up on a mountain reading books and watching for smoke. Seemed like an awesome job, especially if you could get a data tether (he did) and do some programming on the side.
I think you're thinking of Mount Diablo. It's not quite volcanic anymore though (however when California was just the ocean it was), but is instead caused by uplift to the earth's plates: https://en.wikipedia.org/wiki/Mount_Diablo#Geology.
I've thought about this ever since visiting the Needles fire watch tower and meeting Margee while climbing there. Sadly, that tower burned down in 2011 [1].
