Yellowstone: a deep dive into America's restless supervolcano
Yellowstone is the most studied caldera on the planet. Beneath its geysers, hot springs and bison meadows lies a magma reservoir roughly the size of greater Los Angeles and powerful enough, in the deep past, to have produced three of the largest eruptions in the geological record. The current park sits inside the most recent of those calderas.
A hotspot under a continent
The Yellowstone magmatic system is fed by a mantle plume — the "Yellowstone hotspot" — that the North American plate has drifted across over millions of years. The track of the hotspot is visible across southern Idaho as a chain of older calderas. The current position has been beneath northwest Wyoming for the last 2 million years.
Three caldera-forming eruptions
The system has produced three super-eruptions: Huckleberry Ridge (2.1 million years ago, ~2,500 km³), Mesa Falls (1.3 Ma, ~280 km³), and Lava Creek (640,000 years ago, ~1,000 km³). The Lava Creek eruption produced the present Yellowstone Caldera, which still defines the park's central plateau. Each of these was a true supervolcanic event (VEI 8 or close).
Smaller post-caldera eruptions
Since the Lava Creek event, the system has erupted many times on a much smaller scale — large lava flows like the Pitchstone Plateau rhyolite (70,000 years ago) reshaped much of the caldera floor. The most recent significant eruption was about 70,000 years ago. Steam explosions have continued in historical time.
The geyser basins
The park's famous hydrothermal features — Old Faithful, Mammoth Hot Springs, the Grand Prismatic — are the surface expression of the magma below. Roughly half of all the world's geysers are within a few square kilometres of the caldera. The systems are exquisitely sensitive: a single earthquake can change geyser intervals.
Ground deformation
The caldera floor breathes. GPS networks measure centimetres of uplift and subsidence over months and years as magma and hydrothermal fluids move beneath the surface. Episodes of uplift have made headlines repeatedly; none has so far been a precursor to eruption, only to changes in the hydrothermal system.
The hazard nobody plans for
A repeat of Lava Creek would be civilisation-altering. Its more likely modern hazards, however, are much smaller: hydrothermal steam explosions on the order of the 1989 Pork Chop Geyser event, or large rhyolite lava flows confined to the caldera. The USGS Yellowstone Volcano Observatory keeps the public probability of an imminent super-eruption near zero.
Monitoring
Yellowstone is monitored by the USGS, the University of Utah and several partners. The network includes dozens of seismometers, GPS stations, gas sensors and stream-chemistry probes. The data feed is public, and the observatory publishes a monthly update.
Visiting the caldera
Most park visitors never realise they are standing inside a caldera 80 kilometres across. The boardwalks at Old Faithful and Grand Prismatic, the climb up Mount Washburn for the rim view, and the overlooks at the Grand Canyon of the Yellowstone are the classic introductions. Bears and bison do not care which caldera you are in.
Why Yellowstone matters
Yellowstone is the world's working example of a continental hotspot. It is the place where modern hazard communication around very-low- probability, very-high-consequence events has been most carefully practised. And it is a reminder that "national park" and "future volcanic disaster" can be the same set of coordinates.
On the map
Open the map and find Yellowstone Caldera in northwest Wyoming. The track of the Yellowstone hotspot extends southwest across Idaho through older calderas like Heise and Bruneau-Jarbidge.