Yellowstone's geology: a hotspot, a caldera and 10,000 hot springs

Lion Geyser.

Yellowstone was set aside in 1872 — the first national park in the country — for what its geology does on the surface: more than 10,000 thermal features, the largest and most active collection of geysers, hot springs, mud pots and steam vents on Earth. Almost everything about the park, from its plateaus to its canyon, comes from the volcanic hotspot underneath and the ice that later scoured it.

The park in brief

What made it

• The hotspot. Repeated cycles of volcanism from the Yellowstone hotspot, over the last few million years, buried large parts of the park in lava and ash. Its three great caldera-forming eruptions, the last about 631,000 years ago, shaped the central plateau.
• The ice. Pleistocene glaciers covered the park and left their deposits over much of it.
• The older rock. Where erosion and ice have cut through, much older rocks show: Precambrian to Late Cretaceous layers in the Gallatin Range, Jurassic and thick Cretaceous strata on Mount Everts, Precambrian and Paleozoic rock in the northeast corner, and Paleozoic rock in the Birch Hills and along the Snake River on the southern boundary.

What the park protects

The park's own foundation document names two geological values:

1. The thermal features — fed by hot water and steam systems below ground, and home to heat-loving microorganisms found almost nowhere else, which makes them as valuable for research as for sightseeing.
2. Active landscapes — the Grand Canyon of the Yellowstone, Overhanging Cliff and Obsidian Cliff, made by volcanism, glaciation, erosion and earthquakes, and still being made.

Falls on the Yellowstone River.

Yellowstone belongs to the Middle Rocky Mountains physiographic province. The Park Service's Geologic Resources Inventory has published geologic maps and a report for it, free on the NPS DataStore; a soil survey has not yet been done.

Sources

Written from "NPS Geodiversity Atlas—Yellowstone National Park, Wyoming, Montana, and Idaho", National Park Service Geologic Resources Division, a work of the United States government in the public domain, which cites Anderson (2017), Santucci (1998) and the park's Foundation Document (2014).