Valles Caldera National Preserve

The Valles Caldera National Preserve is one of the most geologically unique and significant areas in North America. The preserve encompasses much of the Valles caldera, a huge volcanic crater that formed 1.2 million years ago during an enormous volcanic eruption that spread ash over large parts of New Mexico. The caldera is located near the summit of the Jemez Mountains, a large volcanic complex in north-central New Mexico. The Valles caldera exhibits world-class examples of the landforms produced by a very large, explosive volcano, and the preservation and exposure of geological features within the Valles is spectacular. Much of what geologists know about large-scale explosive volcanism began with detailed studies of the rocks in the Valles caldera and Jemez Mountains, and the area continues to draw geologists from around the world. Since the eruption 1.2 million years ago, there has been uplift of the crater floor, followed by the eruption of smaller, younger volcanoes called “domes” within the crater left by the large eruption. Since then, the caldera has from time to time been home to a series of large lakes. This dynamic geological history is responsible for the beautiful and unique landscape that we see in the region today.

Location

Regional Setting

The Jemez Mountains, in which the Valles Caldera National Preserve is located, are situated at the intersection of the Rio Grande rift, a major zone of crustal weakness formed by stretching of the continental crust, and the Jemez lineament, which is a second, more poorly defined crustal zone along which volcanism has occurred. The large and long-lived nature of the Jemez Mountains volcanic field may be related to its location at the intersection of these two major crustal anomalies.

The Rock Record

Three main packages of rocks are present in the Valles Caldera National Preserve. The first group includes rocks related to the eruption of the upper member of the Bandelier Tuff, which occurred 1.2 million years ago and formed the large volcanic crater that frames the preserve. These rocks are mainly exposed in the area around Redondo Peak, which is a part of the crater floor that was uplifted following the major eruption of the Bandelier Tuff. These rocks are dense and hard, and typically grayish to purple. They contain small glassy-looking crystals of quartz and feldspar and in some cases exhibit faint layering, which is caused by the rock being compressed while still hot after eruption.

The second package of rocks in the preserve is formed by the eruption of a number of volcanic domes, which form the rounded hills around the periphery of the preserve. These domes include Cerro del Medio, Cerro Abrigo, Cerro Santa Rosa, Cerro la Jara, and South Mountain. These volcanic domes, which consist of several rock types, erupted after the large 1.2 million year old eruption. Most of the domes are composed primarily of a fine-grained, gray-to-pinkish crystalline rhyolite. A second, very notable rock type associated with the domes is rhyolitic obsidian. This black, glassy rock is present in parts of some of the domes, and represents the same chemical rock composition as the crystalline rhyolite. The obsidian cooled quickly upon eruption, producing the glassy texture, whereas the crystalline rhyolite cooled more slowly, allowing time for the crystals to grow. Obsidian is typically found on the outer part or “carapace” of the domes. Obsidian from the Valles caldera domes was widely used as tool-making material by local prehistoric Puebloan cultures, and a number of quarries and tool-making sites have been identified on the flanks of the volcanic domes. Another rock type associated with the Valles caldera domes is rhyolitic pumice. The whitish-colored, frothy rock can best be observed around South Mountain. Roadcuts exposing thick deposits of pumice can be seen along NM 4 on the western approach to the Valles Caldera National Preserve.

The third package of rocks found in the Valles caldera is the most difficult to see and includes remnants of lakes that once occupied the floor of the caldera. These lake sediments are being studied to help us understand the climate history during the time that they were deposited. Although the sediments are exposed only in a few places, the evidence of these lakes is clearly visible in the notable flatness of the caldera floor between the volcanic domes. Additional evidence for the presence of lakes in the caldera includes wave-cut terraces and the presence of water-deposited gravels on the flanks of some of the volcanic domes. In addition to these three main packages of rocks, older rocks are exposed in a few places in the preserve. On the northern wall of the caldera, older volcanic rocks and sediments associated with the Jemez Mountains volcanic field can be seen. In several places on the Redondo Peak resurgent dome, large blocks of much older, Permian sedimentary rocks and older volcanic andesites are found. These rocks are thought to have slid off the crater walls during the final phase of the 1.2 millionyear- old eruption of Bandelier Tuff.

Geologic History

Thanks to many years of geological mapping, aided by geochemical and geochronological analysis, the fascinating geologic history of the Valles Caldera National Preserve is well understood. The formation of the large crater or caldera that dominates the preserve occurred 1.2 million years ago with the enormous volcanic eruption that produced the upper member of the Bandelier Tuff (stages 1 and 2 in the illustration on this page). This eruption was an order of magnitude larger than any eruption that has ever been recorded by humans, and ejected at least 50 cubic miles of magma, most of which was deposited outside the caldera. This material may be seen today in the buff-to-salmon-colored cliffs in many parts of the Jemez Mountains. The caldera formed roughly on top of the crater that formed during the previous large eruption, 1.6 million years ago. Calderas form during these huge eruptions when such a large amount of magma is withdrawn from the chamber during the eruption that the overlying crust can no longer support itself, and it collapses into the magma chamber below. The eruption of the upper Bandelier Tuff would have caused total devastation in a large area around the Jemez Mountains, as well as a moderate amount of destruction further afield. Volcanic ash 5 inches thick from this eruption is found 130 miles to the south, in Socorro. This ash is found interbedded with river sediments, which indicates to geologists that the ancestral Rio Grande was well established 1.2 million years ago.

Following eruption of the upper member of the Bandelier Tuff, the crater filled with a shallow lake (stage 3). Shortly thereafter, a structural upheaval of the crater floor occurred, resulting in formation of Redondo Peak. Redondo Peak is a resurgent dome (stage 4), very distinct in origin from the other hills inside the caldera, which are volcanic domes (stage 5). The volcanic domes form by eruptive processes, whereas a structural dome forms by being pushed up from below, by processes that, although not fully understood, are probably related to the presence of magma at depth. Redondo Peak is largely composed of Bandelier Tuff, which formed the floor of the caldera after the 1.2 million-year-old eruption (which produced the Bandelier Tuff) and prior to the uplift of Redondo Peak. The type of resurgent dome represented by Redondo Peak is present on many other large volcanoes around the world, but few are as well exposed or preserved as Redondo Peak. Recent detailed geochronological investigations within the Valles caldera suggest that the uplift of Redondo Peak may have occurred in as little as 40,000 years.

Shortly after the formation of the Redondo Peak resurgent dome, the eruption of the family of volcanic domes inside the Valles caldera began. These eruptive domes are quite different in nature from Redondo Peak. The first dome that erupted, 1.2 million years ago, was Cerro del Medio, which is also one of the largest of the volcanic domes. This dome is visible as a series of low hills in the eastern part of the Valles Caldera National Preserve. Following the eruption of Cerro del Medio, eruption of the domes continued in an orderly, counterclockwise, pattern, ending with the eruption of the tiny Cerro la Jara approximately half a million years ago. Cerro la Jara is the very small, tree-covered hill that is visible to the north of NM 4 and to the west of the main access road to the Valles Caldera National Preserve headquarters. The volcanic domes within the caldera would have been formed by passive eruptions that squeezed the viscous magma out of the magma chamber, allowing it to accumulate near the volcanic vent. These eruptions would have been similar to the eruption of the Mount St. Helens dome that began building shortly after the explosive 1980 eruption. Each dome probably took many tens of years to accumulate, and some, particularly Cerro del Medio, show evidence of having formed in a series of eruptive events, each forming a distinct lobe. Some of the dome eruptions may have begun with an explosive phase that formed pumice beds underlying the dome lavas, but these are not well exposed in most of the Valles caldera domes. Some of that pumice ended up in Los Alamos, many miles to the east. The distinctive arrangement of the domes along an apparently circular trace is probably due to the presence of a large fault or ring fracture that formed when the crater floor collapsed to form the caldera. The dome-forming magma rose along this fractured, weakened area on the crater floor.

Following the eruption of Cerro la Jara peak approximately half a million years ago, there was a long period of quiescence in eruptive activity. Then 68,000 years ago the volcano came back to life, producing a series of small, rhyolitic eruptions. The one of these that can be seen within the Valles Caldera National Preserve is the El Cajete pumice deposit, formed by an explosive eruption. El Cajete pumice can be seen in the southernmost part of the preserve, to the west of and overlying the 0.5 million-year-old South Mountain dome. The crater from which the pumice was erupted formed a distinctive, flat, circular meadow in which the previous owners of the Valles caldera used to hold Fourth of July celebrations. Some geologists who have studied the El Cajete pumice suggest that this eruption represents the beginning of a new phase of eruptive activity in the Valles caldera.

The large, circular depression of the Valles caldera has, from time to time, been filled by large lakes. The history of these lakes in not well known, but they were probably best developed during glacial times, when the climate of New Mexico was cooler and wetter than it is today. In these lakes accumulated the sediment that underlies the relatively flat areas seen between the volcanic domes. There may have been several episodes of lake formation and emptying in the 1.2 million-year life of the caldera. Some of these events may have been related to water flow out of the caldera being dammed by eruption of domes, and there is evidence that episodes of damming followed by dam failure may have occurred. A recent scientific drilling project in the Valles caldera indicates that a lake formed around 0.5 million years ago, probably as a result of obstruction of drainage through San Diego Canyon. Lake sedimentation probably occurred for several tens of thousands of years following that damming event. A number of areas of the caldera floor today are distinctly marshy, and some small lakes exist, even in today’s warm, dry climate.

Geologic Features

The most remarkable geological aspect of the Valles Caldera National Preserve is the enormous, well-preserved crater itself. Among the many fascinating geological features of the preserve is the beautifully exposed example of a resurgent, structural dome (Redondo Peak) as well as the series of distinctive volcanic domes arrayed around the outer edge of the caldera. Of these, the small, and very visible Cerro la Jara offers a good opportunity to observe an entire volcanic dome at a scale that is easily interpretable. The geometry of the other, larger domes, although more difficult to observe directly, is basically the same as Cerro la Jara, although the shape of some, particularly Cerro del Medio and South Mountain, has been complicated by multiple eruptive events.

Another geological feature of interest within the Valles caldera is the presence of beautiful, black, glassy obsidian associated with formation of the domes. This material is not only geologically interesting but also has cultural significance because of its value to the prehistoric Puebloan societies who frequented the Valles caldera.

The Valles obsidian was likely a valuable commodity that was not only used to form tools but probably also widely traded throughout the region. Obsidian, as well as white pumice deposits, can be seen along several miles of NM 4 just west of where the road crosses the East Fork of the Jemez River. Other obsidian deposits and quarries are present on the flanks of the intracaldera domes.

Additional Reading

Geologic map of the Valles Caldera
— Fraser Goff, Jamie N. Gardner, Steven L. Reneau, Shari A. Kelley, Kirt A. Kempter, and John R. Lawrence, 2011, scale 1:50,000

Valles Caldera: New Mexico's supervolcano, New Mexico Earth Matters, v.10, n.1.
— Fraser Goff