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Regional Geology - New Mexico


Background

Beginning about 30 million years ago, tension caused by movement in the earth's mantle created a huge valley, and immense tear that runs across New Mexico from Colorado to northern Mexico. Now known as the Rio Grande Rift, this pulling apart of the earth's crust resulted from separation along two parallel fault zones. ... Bandelier National Monument is located in north-central New Mexico on the eastern side of the geologically young Jemez Mountains. Situated on the gently sloping Pajarito Plateau, Bandelier is bordered on the south by the Rio Grande and to the west by the San Miguel Mountains.

Pajarito Plateau
The Pajarito Plateau is of interest geologically as well as archaeologically. It is constituted largely of tuff (consolidated volcanic ash) and basaltic lava ejected thousands of years ago by a great volcano. The caldera (saucer-shaped depression) created by the collapsed summit of the volcano is among the world's largest calderas; its rim forms the Jemez Mountains. Through this highland, running water has cut many steep-walled canyons down to the Rio Grande.

Jemez Mountains
The Jemez Mountains are located at the junction of the western boundary of the Rio Grande Rift and the Jemez Lineament. The Jemez Mountains are best known for two major volcanic eruptions, the first of which occurred more than 1.4 million years ago, and the second, a little over one million years ago. Together these eruptions were 600 times more powerful than the 1980 eruption of Mount St. Helens. Over 100 cubic miles of material was spewed out, covering over 1,500 square miles with volcanic ash that was up to 1,000 feet deep in some areas.

Jemez Lineament
The area near Bandelier is also crossed by the Jemez Lineament. The lineament is a line of young volcanoes that represent a weakness in the earth's crust running from east-central Arizona to northeastern New Mexico. These volcanoes include Mount Taylor, the Jemez Mountains, and Capulin Volcano.

St. Peter's Dome and Boundary Peak
Bandelier is bordered on the west by St. Peter's Dome and Boundary Peak. These peaks, part of the San Miguel Mountains, are the eroded remains of a group of 8 to 13 million year-old volcanoes.

Cerros Del Rio Volcanic Field
Across White Rock Canyon, the monument's southern boundary, lies the exposed portion of the Cerros Del Rio Volcanic Field. Active one to three million years ago, this volcanic field features cinder cones, basaltic lava flows, and maar cones. Maar cones form when rising magma encounters water. The resulting steam produces violent eruptions which blow out rock fragments that settle into thin layers. This creates a cone characterized by having many thin layers, a low rim, and a large diameter.