The weird thing about volcanic activity in the Western United States is that it’s actually quite difficult to explain. The Cascade volcanoes of the Pacific Northwest are one thing—standard tectonic plate boundary volcanoes like the rest of the Pacific Ring of Fire—but they are far from alone.
There is Yellowstone, of course, which has a history of frighteningly massive eruptions stretching across Idaho and into Wyoming. And neighboring the Cascades, a fair share of Washington and Oregon are blanketed by tremendous lava flows that erupted around 15 million years ago, while southeastern Oregon is home to Newberry Caldera and a line of related volcanoes. Nevada, meanwhile, is dotted by a string of smaller eruptions. (And we’re leaving out the Sierra Nevada Mountains and the Southwest entirely.)
There’s a lot of talk about a plume of hot mantle rock rising up beneath Yellowstone, which is what explains volcanic chains like the Hawaiian Islands. (The mantle plume stays in one place while the tectonic plate slides overhead.) But there is actually real disagreement about Yellowstone. Ultimately, this is because the last 50 million years in this region have been geologically wild, from the building of the Rocky Mountains to the stretching out of Nevada like an inhaling accordion and the creation of the San Andreas Fault. It’s… complicated.