volcanos in my back yard!

Geologist brainiacs:

I live in a southern Santa Clara county in CA. There’s a prominent small mountain (big hill?) (about 1500ft high) about 10 mi north of me known as El Toro, which is said to be a long-dead volcano. I live relatively near another hillock, ‘Lions Peak’ (about 1200ft), which I’d guess is another dead volcano. Behind my house is a conical hill about 200ft high – having seen some samples in Hawaii, this hill sure looks like a classic ‘cinder cone’.

Okay, so how can I know more about what I’m looking at in my neighborhood geologically? I’d guess I can look at local rocks to get a clue, but this area seems to be mostly dry & weathered formations, so I’m not sure what useful information an amatuer can gather. My immediate area seems to be mostly eroded flow from the hilltops or runoff, and the local hills are all private property.

What I’m most curious about: how much and what kind & when was local volcanism vs. faulting present in local area? Yes, it is an odd thing to be interested in, but I just want to know how the beautiful hills around my home got there.

I’ve searched the web in vain, so I come to you: how can I find out the geologic pedigree of my local neighborhood? How can anyone do this for thier locality? Is/are there online or other resource for someone curious about what is under thier feet?


I know litle about geology and virtually nothing about Santa Clara county, but a search of the internet revealed this and this, which might be helpful.

Happy (rock) hunting. :slight_smile:

If you’re going to check out the geologic maps in the links above, then you might want some help with reading them. Have a look here.

This more general site about San Francisco Bay Region geology might also be useful.

Check out the “Roadside Geology of _____” series [["of Washington, Oregon, Northern Calif., etc…]. Might not get down to specific sites in your county, but the series is a good general reference.

Thanks for the links, gang. I’ve been to the USGS sites, and the geology information is interesting, but doesn’t really (at least for me, the dumb newbie provincial geology enquirer) answer my question. The types of formations and thier age and type are listed, but I don’t know how they got that way. Remember, I’m interested in the neighborhood volcanism: was this hill over there a volcano?

For example, El Toro is shown on this map (unlabelled – it’s the big purple formation across the valley from Anderson reservior, near the lower-right of the map) as being composed of Mv-sp, or Mesozoic Serpentinite.

Okay, Mesozoic is the age, Serpentinite is the type of stone. Serpentinite is defined here as “Hydrothermal solutions concentrated during final stages of magma crystallization in batholiths or hot seawater solutions drawn down into subduction zones”, which sounds like 'this was once an underwater volcano that got sucked into a fault, later to be spit out for your viewing pleasure."

Which makes it seem that El Toro was not a volcano per se but remenants of a lava flow that got morphed. Hmmm. But everyone assures me that El Toro is the core of an old volcano.

Similarly, the long, high ridges (600-900ft) near Chez Squeegee are marked as mesozoic metavolcanics (greenstone). Okay, we’ve also got volcanoes in the mix, even if they’re the ‘meta’ kind. But how did these formations form? Lava flows? Or ancient lava squoosed around into ridges by the local faulting?

Yep, I’ve had “Roadside Geology of California” for years. There’s some great stuff in it, with many passing references to the formation of things like the Sierras and volcanism in the northern part of the state. It doesn’t really get into the Bay area very much, and like all geology references about SF, it dwells on faulting, with little mention of other processes in this area.

I just ordered “Assembling California” by John McPhee on Amazon; I don’t expect it will give me any deeper insight into my local geology, but it sounds like a great read on the broader geologic history of the state.

I checked out the California Federation of Mineralogical Soceities list of local websites and found The Santa Clara Valley Gem and Mineral Society Homepage. They’re mainly into collecting minerals and gems, but could probably answer your questions, or at least direct you to those who could. They also do some neat-sounding field trips.


I’ll start working on the OP while I’m waiting for the geologic map to load. From what you’ve said so far, it sounds like you’re living amongst typical Californian Accreted Terrane. All of California west of the Sierra Nevada, in fact are accreted terranes of one sort or another.

What this means is, following the emplacement of the Sierra Nevada (which formed in the Jurassic-Cretaceous as a contintnetal volcanic arc, like the Andes or Cascades), the North American continent “grew” westward as several microcontinents slammed into the trench that was responsible for creating the Nevadan arc, where too buoyant to be subducted, and instead stuck on and accreted. (Modern-day examples of microcontinents include New Zealand and Japan–islands underlain by continental rather than oceanic crust). In addition to these little microcontinents (like the Klamath terrane), bits of oceanic crust were sometimes squeezed upwards (obducted). The squeezing metamorphoses the volcanic rocks into the bluestones (blueschists) and greenstones (serpetenites) common in California. Essentially, oceanic sediments metamorphose into blueschists and the oceanic crust (which is volcanic in origin) metamorphoses into the serpetenites.