Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 April 1998

Preserving the Valles Caldera:  A Foundation of Life & Culture

by Senator Jeff Bingaman
Sidebar on the geology of the Valles Caldera

It is often noted in natural-science literature that ecological systems, and even the human cultures that inhabit those systems, are largely shaped by their geophysical surroundings.  Nowhere is this relationship more evident than in the Jemez Mountain range in northern New Mexico, where one of the greatest geophysical formations in the world - the Valles Caldera - is found.  The Valles Caldera is a place of wonder rivaling Rocky Mountain National Park and Lake Tahoe for beauty.  Viewing the caldera from the volcanic rim, one cannot help but stare in amazement at the vast sweep of the rim.  Some 12-to-15 miles wide, it encircles several resurgent lava-dome mountains that sit within the caldera's basin, including 11,254-foot Redondo Peak.

Remarkably, this enormous caldera is privately owned.  Since the middle of this century the caldera, widely known as the Baca Ranch, has been fenced off, and only a few peopel hav been able to explore its mysteries.  The American public now has an incredible opportunity to purchase this land.  The owners have decided to sell, and have given the United States government the first option to buy the caldera.  I have introduced legislation in Congress, Senate bill 1210, to allow this historic transfer to occur.

Gaining support for this action is a matter of education.  Without access to the property, the American public currently has little knowledge of the Valles Caldera's existence, much less its value.  Even in northern New Mexico, many people are unaware of how much their culture has been shaped by the existence of this vast caldera in their own backyard.

Tucked away in the Jemez Mountain range above Los Alamos, the Valles Caldera was originally a closed basin and retained water, creating a high-altitude lake.  When the wall of the caldera was eventually breached by erosion, the lake drained and its long-accumulated sediments were exposed.  These sediments provided the initial soil for vast upland grasslands that are the premier characteristic of the caldera today.  The largest of these meadows, the Valle Grande, is more than six miles long and four miles wide.  The extent and the elevation of the greasslands, broken by forested mountains and protected by the volcanic rim, produced an ecosystem providing a rich rangeland for game animals and nesting places for birds of prey.

From the archaeological evidence, it appears that the original Native American residents of the Jemez Mountains area used the caldera principally as a hunting range, a source of obsidian for tools and trade, and possibly as a place of worship.  However, with subsequent European exploration and settlement in New Mexico, different land uses emerged.  The first Spanish explorers, struck by the grand vistas within the caldera, named the entire Jemez Mountain range, La Sierra de Los Valles, the Range of the Valleys.  Eventually, descendants of the Spanish Basque colonists used these valleys to support a tremendous sheep-grazing industry.  In this century, the Basque sheep gradually gave way to Anglo cattle.  Although the livestock industry has greatly diminished in recent decades and is no longer the principal sector of the economy, the tradition of making one's living from the land continues to permeate the culture of northern New Mexico.

Thirty years ago, the caldera was teetering on the verge of ecological disaster because of excessive logging and grazing practices.  Now, as a result of good forest and range management and the use of prescribed fire, the caldera is once again the fertile center of the Jemez Mountains.  Still a working ranch with about 6,000 head of cattle, the caldera is the cradle for once of the largest elk herds in the country and habitat for several threatened, endangered, and sensitive species - including the bald eagle and the Rocky Mountain lily.  Circling the basin of the caldera are 27 miles of some of the best trout streams in the Southwest.  The rejuvenation of this land in recent decades has made the caldera a prime example of long-term forest and rangeland restoration.

If we can bring this property into public ownership, it will quickly become a national treasure - providing innumerable recreational opportunities for hiking, photography, hunting, fishing, and even cross-country skiing.  However, even though the owners have given us the first opportunity to buy the property, the deal is by no means assured.  This property is estimated to cost between $75 million and $100 million, and the owners have indicated that we cannot allow their offer to remain on the table beyond a couple of years.

The federal government has taken some important steps since I introduced legislation last fall.  The U.S. Forest Service has identified the Valles Caldera as its top-priority land acquisition.  Toward this goal, President Clinton has already committed $20 million as a down payment, and had asked for $20 million more from Congress this year.  There is hope that as the American people discover this hidden treasure, Congress will move quickly to close the deal.  Land acquisitions of this magnitude are not unheard of.  The federal government has purchased several areas costing over $100 million before, including Fire Island National Seashore and Gateway National Recreation Area in New York; Assateague Island National Seashore in Maryland; and Sleeping Bear Dunes and Pictured Rocks National Lakeshores in Michigan.  I look forward to the day when you will read an open invitation in Geotimes to study and enjoy America's newest addition to this list - the magnificent Valles Caldera.

-Jeff Bingaman

 Jeff Bingaman is a senator (D) from New Mexico, and serves  on the Energy and Natural Resources Committee.  Aside from sponsoring the Valles Caldera Preservation Bill, S. 1210, he is the original sponsor of S. 231, a bill to establish a National Cave & Karst Institute in Carlsbad, New Mexico.  He is also an original co-sponsor of S. 1305, which would double U.S. investment in civilian science research and development over the next 10 years.

The Valles Caldera - a Geologic Wonder

Rick Aster and Philip Kyle

Beyond its exceptional scenic, wildlife, recreational, and archaeological values, there are many compelling reasons to preserve the Baca Land as public property accessible for study.  The Valles Caldera, resurgent Rodondo Peak, the rhyolite domes marking the ring fracture of the Valles Caldera, and the remnants of the domes that were associated with the earlier Toledo Caldera all reside mostly, or totally, on the Baca Land.

The immense volcanic edifice of the Jemez Mountains lies at the intersection of the western fault-bounded margin of the Rio Grande Rift and the Jemez Volcanic Lineament - a chain of Miocene-to-Quaternary volcanic centers stretching from Colorado to central Arizona that probably mark a deep basement weekness.  The 24-kilometer-wide Valles Caldera is one of the world's largest, best-exposed, and potentially most accessible examples of a young resurgent caldera.  The remarkable exposure makes it a geological field-study "Mecca" for volcanologists from around the world.  In many respects, it is the crown jewel of New Mexico's volcanic geology.

Volcanism began in the Jemez Mountains more than 16.5 million years ago and has been more or less continuous.  The catastrophic caldera-forming Bandelier Tuff eruptions at 1.6 and 1.2 Ma (with a volume equivalent to approximately 650 cubic kilometers of dense rock) have largely shaped the landscape we see today.  These eruptions formed two immense ignimbrite sheets, each underlain by think pumice fall units.  The Bandelier sheets dominate the geology of the adjacent Bandelier National Monument, which contains exceptional pre-Pueblo dwellings - many of which were carved into the tuff.  Following the upper Bandelier Tuff eruption and formation of the Valles Caldera, resurgence formed the classic domal uplift of Redondo Peak.  a series of eight rhyolitic domes were emplaced in a regular sequence from 1.1 Ma to 0.5 Ma along the ring fracture, formed during caldera collapse.  Radiometric dating and paleomagnetic studies of the domes helped define the Jaramillo subchron (named after Jaramillo Creek) and the Brunhes-Matuyama boundary.  New age determinations suggest the most recent volcanic features of the caldera - including a significant pumice fall deposit, the small but spectacular Battleship Rock ignimbrite, and the extensive Banco Bonito obsidian flow - are less than 60,000 years old.  Throughout the last million years, an extensive hydrothermal system with associated alteration has existed.

Numerous caldera-based geological and geophysical studies, many incorporating the talents of scientists at adjacent Los Alamos National Laboratory, have probed the present-day Valles Caldera - with the cooperation and assistance of the Dunigan family, the current owners of the Baca Ranch.  Geothermal studies by the Hot Dry Rock Project and the Continental Scientific Drilling Project have provided information on the thermal and geothermal history of the caldera.  The recent Jemez Topography Experiment (JTEX), funded by the Department of Energy and coordinated by Los Alamos National Laboratory and six major research universities, has provided seismic evidence suggestive of either residual magma from the Bandelier eruption or new magma in the mid-crust at a depth of 10 to 12 kilometers.

As a type locality of investigating structure and ongoing evolution of resurgent caldera geology, the Baca Land offers unexcelled opportunities for educational field activities.  An example of this is the Summer of Applied Geophysical Experience (SAGE) program (, which provides upper-level undergraduate and beginning graduate-level students with hands-on experience in exploration geophysical techniques.  In recent years, SAGE has occupied a number of magnetotelluric sites around the Jemez, with a total of 25 to 30 students taking part each year. Rick Aster is an associate professor of geophysics and Philip Kyle is a professor of geochemistry at New Mexico Institute of Mining and Technology.

Please send any comments or requests for information to the AGI Government Affairs Program at

Posted June 27, 2000.

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