The Valles caldera, also known as the Jemez caldera, is a large volcanic caldera located west of Los Alamos, New Mexico. It last erupted a little over one million years ago. That eruption ejected enough material to cover the state of New Mexico to a depth of six feet, corresponding roughly to a cube six miles across. The caldera is easily visible from space, and from the ground it appears as a large, bowl-shaped depression in the Jemez Mountains of north central New Mexico. It was (at the time of my last visit) known as the Baca Ranch, and was used for cattle, horses, and sheep. The area also has a large number of hot springs. Volcanism in the Jemez range, in conjunction with erosion by the Rio Grande, is responsible for the complex and beautiful topography and geology of the Los Alamos and surrounding mesas.

The Jemez volcano is just one of a number of volcanic structures throughout the state of New Mexico. The last eruption occurred in the state a thousand years ago, forming what is now the Valley of Fires, a recreation area on Highway 380, west of Carrizozo, New Mexico. Not reassuringly for Los Alamos, the Jemez volcano is considered dormant rather than extinct, and may erupt in the (albeit distant) future. The area is geologically active, and suffers occasional, very minor earthquakes.

The caldera is an impressive sight, particularly in the summer. Not only did they raise animals on the ranch, but they raise thunderstorms over the caldera as well, which billow up from the warm, moist pastures and move eastward towards Los Alamos and the Rio Grande valley. Shortly after I left New Mexico, the government purchased the land to convert it to a monument and it is now known as the Valles Caldera National Preserve. It can be reached from Los Alamos by heading west on West Jemez Road, and turning north on Highway 4, into the hills. (Turning south on Highway 4 takes you to Bandelier National Monument.) The caldera itself is not yet accessible, but is still an impressive sight even from the side of the road.

Some of this information was adapted from and exhibits at the Los Alamos Historical Society museum.

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