Geology of Valles Caldera National Preserve, New Mexico

Valles Caldera National Preserve lies in the Jemez Mountains of northern New Mexico, in Rio Arriba and Sandoval Counties. It protects 36,017 hectares (89,000 acres) of valley meadows, forested volcanic domes, meandering streams and old-growth ponderosa pine groves inside the 22-km (13.6-mi)-wide Valles Caldera. The caldera formed about 1.23 million years ago, in the Pleistocene, in one of only three supervolcano eruptions in the United States in the last 2 million years.

Image from the National Park Service's Geodiversity Atlas page on Valles Caldera National Preserve.

The contrast of broad grassy meadows — valles in Spanish — against forested domes gives the preserve its look. Plentiful rain, rich soils and varied terrain support many kinds of animals, plants and fungi, including several thousand elk and healthy populations of mountain lions, bears and coyotes. American Indian tribes used the caldera for thousands of years to hunt, fish, gather plants for food, medicine and ceremonies, and collect obsidian for tools.

A textbook caldera

Valles Caldera is the premier example on Earth of a resurgent caldera: a giant circular volcano with an uplifted central floor. Its collapse depression holds a central elliptical dome, Redondo Peak, and is ringed by evidence of about 15 later lava-dome and lava-flow eruptions. Those small domes around the edge let a visitor see an entire volcanic dome at a scale that's easy to read, and the caldera has long served as an outdoor classroom for large explosive volcanism.

The caldera sits near the summit of the Jemez Mountains, a larger volcanic complex, where the Rio Grande Rift meets the Jemez Lineament — known weak zones in Earth's crust. Since the great eruption, the crater floor has risen, smaller and younger domes have erupted, and at times the caldera has filled with water, forming lakes with their own record.

Three packages of rock

Hot water and geothermal drilling

Beneath the caldera is a classic liquid-dominated hydrothermal system at 575°F. Hot springs and fumaroles reach up to 195°F; the best known are the acid solfatara at Sulphur Springs and the bathing spots at Spence, McCauley and San Antonio Springs.

• Commercial drilling: from the mid-1960s to 1983, before it joined the park system, the caldera was heavily explored for geothermal power. The deepest well, Baca-12, reaches about 10,500 ft (3,200 m) and seems to end in largely impermeable Precambrian quartz monzonite at 650°F beneath Redondo Creek. The Baca project never produced electricity.
• Scientific drilling: three research holes drilled from 1984 to 1988 yielded a wealth of information on the rock layers inside the caldera, the shape of the hydrothermal system and techniques for coring and logging in narrow, very hot holes.
• Hot dry rock: at Fenton Hill, on the caldera's southwest edge, engineers built the world's first hot dry rock geothermal system, trying to mine heat from hot, impermeable rock. Four holes went as deep as 2.8 mi (4.5 km) into Precambrian crystalline rock at 615°F. The experiments proved the method technically feasible, but a commercial system was never developed there.

Helping prove plate tectonics

In the 1960s, rocks from Valles Caldera helped confirm plate tectonics. Sea-floor rocks show symmetrical stripes of alternating normal and reversed magnetic polarity, and some of the young reversals were also found in similarly aged rocks at three domes in the northern caldera:

That match supported sea-floor spreading and helped win wide acceptance for the theory that unified geology.

Image from the National Park Service's Geodiversity Atlas page on Valles Caldera National Preserve.

Sources

Based on "NPS Geodiversity Atlas—Valles Caldera National Preserve, New Mexico," National Park Service, which cites Nasholds (2020), Dunbar (2010), Goff (2011), Doell et al. (1968) and other studies; a work of the United States government in the public domain.