Hi Don - Interesting speculations -

The Farallon plate was to blame for most of our volcanics in California - and the Sierra Nevada mountains while it was still being subducted.

It appears that the plate was also responsible for thinning the crust above it as it subducted and that appears to be still continuing to some degree today (continuing extension of Basin and Range). Both the Long Valley Caldera and this Medicine Lake Volcano appear to be in areas of extension. (and both calderas) - I have heard speculation that Long Valley might be a hot spot - I think the jury may still be out on that one.

I also find it interesting that Mt St Helen's and Long Valley were active at the same time - maybe the plumbing under there is somehow connected?

Canie