The above three images (click to enlarge) are varying combinations of Google Earth and One Geology for the Lassen Peak area. All are looking to the northwest; all show Mt. Shasta in the far background to the right of Lassen Peak. The first image is just a plain Google Earth image, the second uses a 50% setting for One Geology, and the third uses a 75% setting (at 100% the image is all color and no background earth surface). The blue in the far distance is not the ocean but some Paleozoic rocks, the purple is probably serpentinite - see the Geologic Map of California on Andrew's site for comparison.
This last image, above, shows the Lassen Peak area looking straight down. The pink area on the right is underlain by a bunch of "older" volcanic rocks (Pliocene on the Westwood sheet - somewhere in the 2 to 5 million-year-old range, although they could possibly include some younger flows); the yellow area with pink triangles is underlain by younger volcanic rocks (Quaternary for sure - that is, less than 2 million years old - and some have only been cooling since 1915!). You can see more detail on the Geologic map of the Westwood sheet (and compare the geology) and a lot more detail on the Geologic map of Lassen Peak, Chaos Crags and Upper Hat Creek area, California (see below).
You can find geologic timescales here and here.
Some References:
Lydon, P. A., Gay, T. E., and Jennings, C. W., 1960: Geologic map of California: Westwood sheet: California Division of Mines and Geology, scale 1:250,000.
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