The Volcanic Valley: You Are Driving Through the Aftermath of a Supervolcano

Seven hundred sixty thousand years ago, something happened here on a scale that makes Yellowstone the comparison people reach for.

The Long Valley Caldera erupted and discharged approximately one hundred forty four cubic miles of material. A hundred forty four cubic miles. The ash layer it deposited across this region, which geologists call the Bishop Tuff, is recognizable in rock outcroppings across hundreds of miles of the American West. The eruption was one of the largest volcanic events in North American geological history.

What you are driving through right now is the floor of that caldera, and the mountains around it are the results.

Mammoth Mountain, to your west, is not a classic volcano. It is a dome complex, a younger mound of volcanic material that formed on the western rim of the caldera beginning roughly one hundred ten thousand years ago, most recently active around fifty-seven thousand years ago. The United States Geological Survey has monitored the whole system as an active volcanic complex since the early nineteen eighties, when a swarm of earthquakes indicated that magma was still present. It is still present. Carbon dioxide venting from Mammoth Mountain's upper slopes has killed trees there; the dead forest is visible from the ski runs.

Ten miles north, the Mono Craters are the youngest chain of volcanic vents in the contiguous United States. The most recent eruption was approximately six hundred years ago, which in geological terms means this system is essentially still going. Before that eruption, someone was living in this valley. The Owens Valley Paiute have been here for at least four thousand years; volcanic events are part of their landscape history.

And then there is Devils Postpile, nine miles west of Mammoth. We will come to those columns in a separate track. But they are part of the same story: a landscape that is still being made.