Bandelier Geology 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. The Geologic Story Bandelier National Monument is located in north-central New Mexico on the eastern side of the geologically young Jemez (HAY--mez) Mountains. Situated on the gently sloping Pajarito (Pah-hah- REE-toe "little bird") Plateau, Bandelier is bordered on the south by the Rio Grande and to the west by the San Miguel Mountains. The monument's current serenity hides its turbulent past. In order to understand the forces that created the current landscape, let's go back in time and look below the surface. Beginning about 30 million years ago, tension caused by movement in the earth's mantle created a huge valley, an 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. The area near Bandelier is also crossed by the Jemez Lineament. The lineament is a line of young volcanos that represent a weakness in the earth's crust running from east-central Arizona to northeastern New Mexico. These volcanos include Mt. Taylor, the Jemez Mountains, and Capulin Volcano. THE VOLCANOS 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. These mountains were slowly built by numerous eruptions along the Jemez Lineament. Across White Rock Canyon, the monument's southern boundary, lies the exposed portion of the Cerros Del Rio (SER-os del REE--oh "Mountains of the River") 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. One maar cone that formed during these eruptions is visible along the Falls Trail approximately 1.5 miles from monument headquarters. Inside the monument, most of the Cerros Dei Rio volcanics are covered by subsequent eruptions originating in the Jemez Mountains to the west. 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 1500 square miles with volcanic ash that was up to 1,000 feet deep in some areas. THAT'S TUFF Hot ash from these eruptions settled and through a process of welding, it consolidated to form tuff, the pinkish-orange rock now visible in Frijoles (free-HOH-leez "Beans") Canyon. These welded layers of ash may have taken up to 10 years to cool. The upper and lower surfaces of a flow cool much more quickly than the center. As a result, the center of a sheet is the most densely welded, while the top and bottom are usually non-welded. Welding also decreases away from the vent due to heat loss. Frijoles Canyon is 10 miles from the vent, too far for the tuff to be densely welded. The hot ash that flowed down the mountain's side filled in existing canyons, creating a series of gently sloping plateaus around the volcano. Some streams continued to follow their old courses and have re-cut their ancient channels. Some canyons were not re-cut, and one of these can be seen on the Falls Trail. SWISS CHEESE Across the Pajarito Plateau, the soft top of the tuff sheet was quickly removed by erosion. Streams cut into the tuff, producing the characteristic mesa and canyon topography of the area. Under-cutting of the softer tuff gives the canyons their steep walls. Variations in welding led to the stepped appearance of the cliff walls; each step marks the top of a resistant layer. Vertical columns in these layers are due to contraction and cracking of the cooling tuff. The "Swiss cheese" appearance of canyon walls is due to weathering-out of loosely consolidated pockets of tuff and pumice. These pockets are more frequent at the bottom of flows and between ash flow layers. TENT ROCKS Cone shaped rocks, know as "tent rocks," occur in great numbers throughout this area. These eroded remnants of tuff cliffs show the position of canyon walls at a previous time. There are several possible reasons why they form. Some may have been near an escape route of hot gases, where the additional heat welded the ash more tightly. Some still bear a capstone of harder material, which has slowed their erosion. Others may be protected from erosion by a resistant rind created by minerals deposited by ground water. WILL IT HAPPEN AGAIN? With a past this violent, the natural question is "will it happen again?" The presence of hot springs and hot dry rock in the area tell us magma chambers are still active. The Rio Grande Rift continues to widen. This evidence suggests the answer is "YES!" Occupation Period Outstanding Features Location Discovery