Jemez Mountain Trail
Valles Caldera National Preserve, NM

The remnants of the former Jemez Volcano is now known as Valles Caldera. The caldera is an almost circular, cliff-ringed pit 14 miles (23 kilometers) across, the upper slopes and canyons of which are forest covered. Only the base of the Jemez Volcano now remains. The volcano rose along faults that edge the west side of the Rio Grande Rift, now the Rio Grande Valley.

A composite volcano estimated to be about the size and shape of Mount St. Helens Volcano in Washington, Jemez Volcano reached its maximum height one million years ago; shortly thereafter, it burst forth with two extremely violent eruptions, spewing out incredible volumes of volcanic gases, ash, pumice, and broken rock.

The discharge of volcanic materials by these eruptions is thought to have been 100 times greater than the discharge of the 1980 eruption of Mount St. Helens Volcano. Ash clouds drifted as far east as Iowa, Missouri, Oklahoma, and eastern Texas. Other ash sped down the flanks of the volcano leaving great deposits at the base of the volcano and the surrounding region. These two massive explosions depleted the magma chamber far beneath the volcano. No longer supported from below, the volcano, ringed by fractures, collapsed, forming a vast caldera.

The great dome of Redondo Peak (11,254 feet or 3,430 meters), and numerous smaller domes that squeezed up through circular faults on the edge of the caldera now block the view of the caldera from one side of the rim to the other. The remaining level floor of the caldera is a vast grassland that is home to thousands of cattle. Los Alamos, established in 1942 by the United States Government for atomic research and development of the first atomic bombs, is located on the east slope of the former Jemez Volcano.