Ever heard of the Bruneau-Jarbridge event?
If not, it's unsurprising; neither had I. Plus, it happened twelve million years ago, during the mid-Miocene Epoch. It's a supervolcano eruption of the Yellowstone Hotspot, which was at the time under what is now southwestern Idaho. Between then and now, the hotspot has stayed pretty much where it was, but the North American Plate has moved, resulting in its current location underneath northwestern Wyoming,
The Bruneau-Jarbridge event was enormous. It created monstrous pyroclastic flows that traveled 150 kilometers from the caldera, incinerating everything in their path. The winds at the time of the eruption were from the west; we know this because the ash produced by the eruption traveled at least 1,600 kilometers to the east, creating meters-thick layers including the ones at the amazing Ashfall Fossil Beds in northeastern Nebraska.
In fact, it's the Ashfall Fossil Beds -- now an official National Natural Landmark and State Historical Park -- that's why the topic comes up. A friend and frequent contributor of topics for Skeptophilia sent me a photograph of the site, and asked me if I'd heard of it:
I hadn't, so naturally I had to look into it.
The whole thing is staggering, if grim. Ashfall contains the skeletons of thousands of animals killed, more or less simultaneously, by the Bruneau-Jarbridge ash cloud. The remains of the rhinoceros species Teloceras are so common there that one part of the fossil bed has been nicknamed "the Rhino Barn." But there are lots of other species represented as well; five different kinds of prehistoric horses, including both three-toed and one-toed; three species of camels; two canids, the fox-sized Leptocyon and the wolf-sized Cynarctus; a saber-toothed (!) deer species, Longirostromeryx; three species of turtles; and three species of birds -- a crane, a hawk, and a vulture.
Despite the size of the eruption and resulting ash cloud, everything in the area didn't die during the ashfall. Some of the bones show signs of scavenging, and some have breaks and tooth marks consistent with the dentition of the hyena-like canid Aelurodon. So even a horrific catastrophe like Bruneau-Jarbridge didn't extinguish life completely; there were still scavengers around to chow down on the victims.
When looking at this sort of event, the question inevitably comes up of whether it could happen again. The facile answer is: of course it could. The Earth is still very much tectonically active, and more specifically, the Yellowstone Hotspot is a live volcano, as the frequent earthquakes and boiling-hot geysers and lakes should indicate. It's likely to erupt again -- whether a monumental cataclysm like Bruneau-Jarbridge, or something smaller, isn't certain.
But despite the prevalence of clickbait-y YouTube videos about how "Yellowstone is about to erupt!" and "Scientists fear the Earth will crack wide open!" (both direct quotes from video titles), there is no imminent danger from the Yellowstone Hotspot. What the geologists are actually saying is that a major eruption is likely some time in the next hundred thousand years, which puts it well outside the realm of what most of us should be worried about.
However, there's no doubt the the Ashfall Fossil Beds are a sobering reminder of what the Earth is capable of. They're the fingerprint of a twelve-million-year-old catastrophe that makes any recent eruption look like a wet firecracker. But as horrible as it was for the Miocene animals in the path of the ash cloud, it's provided us with a snapshot of what life was like back then, when Nebraska had a climate more like modern Kenya -- and the Great Plains was home to rhinos, camels, horses, and wild dogs.