Recently, when I was going to read up on New Mexico (USA) intra-plate volcanism, I stumbled first upon an area designed National Monument where the Native Americans had carved their homes into cliffs of pumice and tuff – now this sounded exciting, there must be a volcano nearby! Indeed there was, and everything about it was highly interesting, both the Pueblo history and the geology, so that I started to read all I could get (including Bandelier’s book The Delight Makers, link at the bottom), and taking notes. I would like to introduce you now to the Valles Caldera which provided the very rocks for people to live IN, and to the Bandelier National Monument.
We are here:
Bandelier National Monument >
comprises 50 square miles (130 km²) of the Pajarito Plateau >
from the eastern slopes of the Jemez Mountains, along the Rio Grande >
to the rim of the Valles Caldera (or Jemez Caldera) >
in the Jemez Mountains (comprising the Valles Caldera rim) >
in Rio Arriba, Sandoval, and Los Alamos counties >
in New Mexico, United States.
RIO GRANDE RIFT – JEMEZ MOUNTAINS – VALLES CALDERA
Rio Grande Rift
The Jemez Mountains lie to the north of the Albuquerque Basin in the Rio Grande rift – a north-trending continental rift zone. This rift separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift is said to be nearly tectonically quiescent at present, but significant deformation and faulting with offsets of many kilometers were responsible for its formation starting about 35 Ma ago. The largest-scale manifestation of rifting involves a pure-shear rifting mechanism, in which both sides of the rift pull apart evenly and slowly, with the lower crust and upper mantle (the lithosphere) stretching like taffy.
This extension is associated with very low seismic velocities in the upper mantle above approximately 400 km depth associated with relatively hot mantle and low degrees of partial melting. This intrusion of the asthenosphere into the lithosphere and continental crust is thought to be responsible for nearly all of the volcanism associated with the Rio Grande rift. Volcanism along the rift increased slowly after the middle Miocene lull, which occurred around 20 to 13 million years ago. Most of the activity at this time was concentrated initially in the Jemez Mountains and in the Socorro area, where the rift transsects major northeast trending lineaments. The southern Rocky Mountains and adjacent areas were strongly uplifted between about 7 and 4 million years ago, and total uplift since the middle Miocene has been about 1100 meters, with much of this uplift occurring during this brief interval (Chapin, 1979).
The Rio Grande Rift is intersected in northern New Mexico by the NE-SW trending Jemez Lineament, a line of young volcanos that represent a weakness in the earth’s crust running from east-central Arizona to northeastern New Mexico. The Jemez Lineament is defined by aligned volcanic fields and several calderas in the area, including Mt. Taylor, Capulin Volcano and Valles Caldera National Preserve in the Jemez Mountains. The significance of the Jemez Lineament is unclear; it is not obviously related to the formation of the Rio Grande rift, but may be a hotspot trail.
The thickness of the crust beneath the rift increases to the north, where it may be as much as 5 km thicker than it is in the south. The crustal thickness underneath the rift is on average 30–35 km, thinner by 10–15 km than the Colorado Plateau on the west and the Great Plains to the east
It has been suggested that the Colorado Plateau acts as a semi-independent microplate and one way of explaining the creation of the Rio Grande Rift is by the simple rotation of the Colorado Plateau 1-1.5 degrees in a clockwise direction relative to the North American craton. Unfortunately I haven’t found more on that interesting idea yet, but I’ll keep looking.
The Jemez Mountains
They are a classic example of intracontinental volcanism and consist of a broadly circular ridge surrounding the famous Valles Caldera. It is one of the largest young calderas on Earth – and the type area for resurgent ash-flow calderas!
Valles, or Jemez, is a volcano which is considered a smaller one of the seven or so “supervolcanoes” on earth, within an area of great crustal tension with a long history of volcanism, caused by the Rio Grande Rift.
The Jemez Mountains are a complex volcanic pile at the intersection of the two regional geologic features: the eastern rim of the Colorado Plateau to the west, and the Rio Grande rift to the east. They are the remnants of the large, collapsed volcano; its two ignimbrite-forming eruptions occurred about 1.6 million and 1.2 million years ago. Valles Caldera is the younger of two calderas known at this location, having collapsed over and buried the older Toledo Caldera, which in turn may have collapsed over yet older calderas.
The Valle Grande (the central crater) was formed by an explosion believed to have been 600 times as powerful as the Mt. Saint Helens eruption. As the magma erupted along a ring fracture, water dissolved in it flashed to steam, spraying the lava as huge clouds of tephra and steam. This fiery cloud surged downhill like water, and deposited its ash in layers as ash-flow tuff; the Bandelier tuff has a volume of ~300 km^3! During the eruption, the plug of rock bounded by the ring fracture settled down into the rapidly diminishing body of magma. Steep walls surrounded where the plug had been, and they collapsed as massive landslides into the hole left by the sinking plug. The heads of those landslides are now the topographic rim of the caldera, which is 20-24 km in diameter and about 300 m deep.
Resurgent Lava Domes inside Caldera
Rain water and snow melt then filled up the caldera, and one can imagine that lake to be steaming hot and sulfurous from the eruptions just finished and ongoing. Following the formation of the caldera, a resurgent dome, Redondo Peak (3431 m) rose above the caldera floor. A resurgent dome is a structural feature rather than an eruptive one; usually it is formed by magma rising under the caldera but not erupting. Redondo Peak resurgent dome formed by uplift of the intra-caldera Bandelier Tuff. Recent research shows that Redondo grew in hight during the 30,000-year period after the caldera formed, an average uplift rate of about 25 mm per year. Although this uplift is caused by upwelling magma at depth, Redondo Peak is not a volcanic (eruptive) dome.
While the pre-caldera eruptions produced mainly basalts (now underlying the ignimbrites of the caldera-forming events), the post-caldera lavas consist of rhyolite. As many as ~10 post-resurgence eruptive rhyolite domes have been pushed up along the ring fracture around Redondo Peak within the caldera. The most recent known eruption was an obsidian flow dated 50 ka to 60 ka BP, but most of the volume of the range is now composed of rhyolite containing abundant phenocrysts of quartz and sanidine. The youngest rocks in the Valles Caldera probably erupted from El Cajete crater on the south side. The Battleship Rock, on the southern slope, is a non-welded to partly welded ash-flow tuff composed of rhyolite ash and pumice, the El Cajete Member (older than 42k yrs BP) is composed of well-to-crudely bedded air-fall deposits of rhyolitic pumice blocks and lapilli.
PAJARITO PLATEAU AND FAULT SYSTEM
As the Valles Caldera formed, air-fall ash and pumice must have covered most of New Mexico, and neighboring states, and been identified as far east as Kansas. The Pajarito Plateau is constituted largely of tuff (consolidated volcanic ash) and basaltic lava. Across it, the soft top of the tuff sheet was quickly removed by erosion. Pyroclastic flows moving away from the caldera formed what is today known as the Pajarito Plateau, and the sites of Los Alamos and White Rock.
The plateau, extending E of the Jemez Mountains, is bounded on the west by the Valles Caldera and on the east by the impressive White Rock Canyon of the Rio Grande.
Streams draining from topographic hights dissected the relatively soft tuffs and formed a maze of narrow canyons and valleys – the characteristic mesa-and-canyon topography of the area. The elongated mesas, the surfaces of which share a common elevation and slope, are remnants of the once continuous lava-flow. Flowing water cut up to 330 m into the tuff, and in places, even up to 65 m into the underlying basalt and sedimentary strata. Major faults along the western side resulted from crustal adjustments associated with the Rio Grande rift. A series of normal faults are part of the Pajarito fault system, a system of over 105 km of mapped faults. This concentrated fault zone is 400 m wide and has over 130 m of displacement, with the down-dropped wall to the east. Seismic hazard studies indicate the Pajarito fault system could produce maximum earthquakes with a Richter magnitude of about 7.
Welding of tephra layers
During eruption phases tens of meters thick layers of very hot materials were deposited by ash emission, pyroclastic flows and dome collapses. These layers of ash may have taken up to 10 years to cool. As the upper and lower surfaces of a flow cool much more quickly than the center, 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. Such variations in welding also led to the stepped appearance of the cliff walls; each step marks the top of a more resistant (better welded) layer, and under-cutting of the softer tuff gives the canyons their steep walls. 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 (poorly welded material).
The layer into which the historical dwellings of the indigenous people were carved is referred to as the Bandelier Tuff. Most of the Monument’s elongated mesas consist of this 1000 ft thick tuff in the western part of the Plateau which thins to about 260 feet eastward near the Rio Grande. The yellow-orange-pink rock formations constituting these ignimbrites are known as the Lower and Upper, or Otowi (from the earlier Toledo caldera event) and Tshirege Members of the Bandelier tuff. They are the rocks now visible in Frijoles Canyon. Frijoles Canyon is 16 km from the vent, too far for the tuff to be densely welded; therefore the rock can be carved easily but is friable and prone to erosion in many places.
WILL IT HAPPEN AGAIN?
With a past this violent, the natural question is “will it happen again?” The Rio Grande Rift continues to widen. Seismic investigations show that a low-velocity zone lies beneath the caldera, and an active geothermal system with hot springs, hot dry rock and fumaroles exists today. Roughly 25 eruptions have occurred since the Valles caldera formed, about one eruption every 50,000 years. Thus, on this crude statistical basis, we might expect the next eruption in another 10,000 years or so. But the length of time between these 25 eruptions is highly irregular, so predicting the next eruption on frequency is not realistic. However, an eruption sometime in the future is likely; the Jemez volcanic field has produced hundreds of eruptions during its 14-million-year history, and the Valles caldera still retains immense quantities of heat within its interior. Most volcanologists would probably classify Valles as dormant. So, all this evidence and modelling suggests the answer to the above question is a resounding “YES”.
BANDELIER NATIONAL MONUMENT
Now I am back to what hooked me in the first place: here, along the Frijoles Canyon on the eastern slopes of Valles Caldera, on, or rather in the Pajarito Plateau, early permanent settlements by ancestors of the Puebloan peoples have been dated back to 1150 CE and later had moved closer to the Rio Grande by 1550. The distribution of basalt and obsidian artifacts from the area, along with other traded goods, rock markings, and construction techniques, indicate that its inhabitants were part of a regional trade network that included what is now Mexico. On the banks of the Rio Rito ancestral Pueblo People broke up the firmer materials to use as bricks, while they carved out dwellings from the softer material. The place was abandoned by the 17th century.
The Pueblo Tribe consists of twenty-one separate Native American groups that lived in the southwestern area of the United States, primarily in Arizona and New Mexico. They get their name from the Spanish who called their towns “pueblos” which means village or little town in Spanish.
Adolph F. A. Bandelier (1840-1914) was an American archaeologist and ethnologist of Swiss origin, who became the leading authority on the history of the Native Americans of the southwestern United States. One Pueblo, Jose Montoya, brought Adolph Bandelier to visit the area for the first time in 1880 and, looking over the cliff dwellings, Bandelier said, “It is the grandest thing I ever saw!”
Bandelier studied and lived with the Natives for a long time. To make his research more “palatable” for the general public he wrote a book in shape of a novel about the Ceres people’s life in the canon, which has since become a must-read for anyone interested, public and researchers alike: “The Delight Makers” (see download link below).
Based on documentation and research by Bandelier, there was much support for preserving the area and President Woodrow Wilson signed the legislation creating the monument in 1916. The monument was closed to the public for several years during World War II, since the lodge was being used to house personnel working on the Manhattan Project to develop an atomic bomb.
Frijoles Canyon contains a number of ancestral pueblo homes, kivas (ceremonial structures), rock paintings, and petroglyphs. Some of the dwellings were rock structures built on the canyon floor; others were cavates produced by voids in the volcanic tuff of the canyon wall and carved out further by humans. A 1.2-mile (1.6 km), predominantly paved, “Main Loop Trail” from the visitor center affords access to these features. A trail extending beyond this loop leads to Alcove House, a shelter cave produced by erosion of the soft rock and containing a small, reconstructed kiva that hikers may enter via ladder. – Click on the first image in the Gallery to view them in full size.
Enjoy! – GRANYIA
SOURCES, FURTHER READING & Videos
– U.S. National Park Service
– The Raton-Clayton Volcanic Field (by agimarc)
– Valles Caldera, GVP Smithsonian Institute
– The Valles caldera: New Mexico’s Supervolcano (Fraser Goff, USGS, 2010)
– New Mexico Bureau of Geology & Mineral Resources
– New Mexico Volcanoes and Volcanics (album)
– Valles Grande Caldera – Geologic History
– […] geology, petrography, and chemistry within the resurgent
dome area, Valles Caldera, […]. NMGS (PDF)
– Valles Caldera, Magma Cum Laude, J. Ball. (Blog)
– Valles Caldera, Jemez Volcanic Field (NM Museum of…)
– Wikipedia, various topics
– Bandelier, The Delight Makers (Download)
– Native Americans – Pueblo Tribe
– Video 1: Valles Caldera Formation
– Video 2: Sky Island – New Mexico’s Jemez Mountains (26 min)
– Video 3: Bandelier National Monument, New Mexico, USA (images only) in HD (6 min)