Yellowstone is located above a blank the collapse of a crater after volcanic eruption

Starting near the Oregon/Nevada/Idaho juncture 16 million years ago, a line of rhyolite magma centers—supervolcanoes—formed across what is now the Snake River Plain of southern Idaho. Yellowstone National Park today lies directly over the hotspot.

Those parts of the surrounding states of Montana, Idaho, and Wyoming that are closest to Yellowstone would be affected by pyroclastic flows, while other places in the United States would be impacted by falling ash (the amount of ash would decrease with distance from the eruption site).