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Valles caldera is located in north-central New Mexico in the central Jemez Mountains west of Santa Fe. It is the oldest of three young caldera-type volcanoes in the United States; the other two are Yellowstone in Wyoming and Long Valley in California.


The Valles caldera-forming eruption occurred approximately 1.25 million years ago when huge volumes of Bandelier tuff were explosively evacuated from a underground magma storage region. As a result of the "supereruption," the ground subsided along a network of nearly circular faults, called a ring fracture, leaving the caldera, a 20 x 23 km (12 x 14 mi) depression. Resurgent doming of the central caldera floor occurred between 1.25 and 1.22 million years ago, which was immediately followed by additional dome eruptions around the caldera margins up until about 40,000 years ago. These resurgent and ring-fracture eruptions are interlayered with lake deposits, indicating the caldera has been at least partially filled by lakes since its formation. The youngest volcanism occurred in the southwest sector, which coincides with the present-day Valles geothermal system, which is fueled by a magma storage region residing beneath the southwestern caldera.

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