Articles from Digital Geology of Idaho site of interest –
Neogene Snake River Plain-Yellowstone Volcanic Province < has good map
Challis Magmatic Episode < has map < yeah, that Challis!
That place, Challis, Idaho which has so many quakes lately is full of old volcanoes!
[snip] “…The Challis Volcanic Group covers approximately 25,000 square kilometers in Idaho , making it the largest of the Eocene volcanic fields (Figure 2). The Eocene geologic setting of the Challis volcanics is commonly accepted as an extensional basin related to the subduction of the oceanic Farallon Plate beneath the continental North American Plate. Prior to Eocene time, before ~60 Ma, the convergence rate of the Farallon and North American Plates was very fast.”
Fun Fact:The caldron under Yellowstone started out to the right, under Idaho and moved left.
“The Yellowstone hot spot first appears about 17 million years ago with rhyolite eruptions on the Owyhee Plateau of southeast Oregon and northern Nevada. The hot spot slowly migrates across southern Idaho to its present-day position at Yellowstone National Park.” – Idaho history