The Supervolcano Beneath Your Feet

From the summit of Mount Washburn, ten thousand two hundred forty-three feet above sea level, the view opens in every direction. To the south and west, the land drops into a broad depression. This is what you came up here to see: the Yellowstone caldera, thirty miles wide and forty-five miles long, the scar left by one of the largest volcanic eruptions in the history of this continent.

Six hundred forty thousand years ago, the eruption that formed this caldera expelled roughly two hundred forty cubic miles of material. Ash fell as far east as the Mississippi Valley and as far south as the Gulf Coast. The explosion was not a mountain blowing apart; it was the ground itself collapsing into an evacuated magma chamber. The caldera is not a crater. It is an absence.

Smaller eruptions have occurred since. The most recent, approximately seventy thousand years ago, resurfaced parts of the caldera floor. The magma chamber is still present, still active, still measured. The United States Geological Survey monitors the park continuously with seismometers, GPS stations, and ground-tilt sensors. The caldera floor rises and falls by inches, year to year, as magma moves beneath it. USGS scientists do not ask whether Yellowstone will erupt again. They ask on what timescale.

Every geyser, every hot spring, every fumarole and mud pot in this park, all ten thousand of them, the greatest concentration of hydrothermal features on Earth, is a surface expression of what you are looking down at from this summit. The same heat that makes Old Faithful predictable makes the ground itself temporary.