Winter 2010 THE V ALLES CALDERA : NEW MEXICO ’S SU P ER V OLCA N O New Mexico contains one of the three larg- viscous, high-temperature foam. Much like near the vent and smaller ash far from the est young volcanoes in the United States: a shaken champagne bottle when the cork vent. Pyroclastic-fall deposits can be found the Valles caldera, in the central Jemez is removed, rapid formation of free gas, throughout the Jemez Mountains but are Mountains west of Santa Fe. Calderas are mostly steam, eventually causes a violent best exposed in the southern Valles caldera large volcanic depressions, more or less explosion, but at eruption temperatures of area along NM–4 between the junctions circular, the diameter of which is many roughly 1,700° F. When the bubble-rich with NM–501 and NM–126. times greater than that of subsequent, post- magma fragments are blasted into the cold When supervolcanoes explode, they caldera vents. The other two large calderas atmosphere, they “freeze,” forming pum- form huge, hot clouds of pumice, ash, crys- are Yellowstone, Wyoming, and Long ice. Pumice is nothing more than solidified tals, and hot gas, called pyroclastic flows, Valley, California. Of the three, the Valles magmatic foam. which radiate outward from the caldera caldera is both the oldest and at speeds of 50 to 200 miles per the smallest. But when it formed hour. Depending on volume, 1.25 million years ago, the distance from the volcanic vent, explosions were anything but and other factors, temperatures small. Approximately 95 cubic may easily exceed 1,000° F when miles of high-viscosity molten the flows come to rest. In recent rock (magma) was blasted years pyroclastic flows have into the stratosphere and onto become known to the general the surrounding landscape, public through movies such as forming what is arguably New Dante’s Peak and a multitude of Mexico’s most famous rock, the documentary films. Geologists call picturesque Tshirege Member of large pyroclastic-flow deposits ash- the Bandelier Tuff. As the tuff flow tuffs or ignimbrites. These (consolidated volcanic ash) was rocks are mixtures of pumice, erupted, the caldera floor col- fine ash, crystal fragments, and lapsed, forming a broad circular pre-eruption rock debris (lithic depression roughly 12 x 14 miles fragments). The lithics are reamed across. In today’s jargon, caldera- out of the volcanic vents during Redondo Peak (11,254 feet) looking west across Valle Grande. This producing eruptions of this size broad mountain is a resurgent dome formed by structural uplift of eruption or picked up as the are called supervolcanoes. over 3,000 feet of intracaldera Bandelier Tuff. Recent research shows pyroclastic flows race over the Magmas of the Bandelier type, that Redondo grew during the 30,000-year period after the caldera countryside. called rhyolite, are exceptionally formed, an average uplift rate of about one inch per year. Although If emplacement temperatures hot, viscous liquids rich in sili- uplift is caused by upwelling magma at depth, Redondo Peak is not are hot enough, the ash-flow a volcanic dome. con, aluminum, potassium, and deposits may “weld,” a process sodium. The explosive power of in which enough heat is retained such magma derives from its high content If pumice and ash rain out of the in the deposit to flatten pumice and glass of dissolved water and other volatile com- atmosphere during volcanic explosions, shards and fuse them together. In most ponents, as much as 5 to 6 percent water distinctive beds called pyroclastic-fall deposits viewed outside the Valles caldera by weight. As this magma rises toward the deposits accumulate on the landscape. (e.g., at Bandelier National Monument), surface of the earth, a depth is reached at Such beds are typically composed of the Tshirege (or upper) Member of the which the pressure of overlying cold rocks light-colored, well-sorted layers of pumice. Bandelier Tuff is a nonwelded to moder- is exceeded by the pressure of dissolved The layers display uniform bed thickness ately welded ash-flow tuff. However, some volatiles in the magma. The magma is then and mantle existing topography. Fragment deposits of Tshirege inside the caldera are a mixture of melt, crystals, and bubbles, sizes are generally dependent on distance so densely welded they have formed glassy which can be thought of as an extremely from the volcanic vent, with larger pumice layers of rock resembling obsidian. Glassy, Published by the New Mexico Bureau of Geology and Mineral Resources • A Division of New Mexico Tech welded ash-flow tuffs are sometimes called collapse into the chamber, forming a vitrophyres. circular caldera depression. Much of the Volcanic ash from supervolcano explosions exploded pyroclastic material falls back is often carried long distances by wind, into the collapse depression, but much is but it can be distinguished by geochemical dispersed around the caldera and into the composition and by age. Because of our abil- atmosphere. ity to obtain radiometric dates on volcanic After the caldera forms, several events materials, ash layers can provide important take place almost simultaneously: Caldera indicators of age in deposits that are other- walls slough inward forming an apron wise difficult to date. Thus, we know that ash of sediments and landslides adjacent to from the Valles caldera explosion was carried steep slopes; a lake (or lakes) develop in by winds as far away as central Utah and the caldera depression from rain and snow eastern Kansas and was rafted by the ancestral melt; small volcanic flows and domes erupt Rio Grande as far south as El Paso. If ash into the lake, becoming interlayered with of known age is found in river, lake, and the sediments; and the caldera floor begins ocean sediments, it can provide accurate time to lift from continued rise of magma from constraints on sedimentation rates and other below. Rapid uplift of the caldera floor earth processes. soon after caldera formation is called resur- gence. After resurgence, more magma erupts along the ring of buried collapse faults, forming lava domes and flows between the resurgent dome and the caldera walls. These later eruptions, called ring-fracture or moat eruptions, become interlayered with yet more sedi- ments and lake deposits. The interior of a well-developed, mature caldera eventually becomes a complicated maze of uplifted pyroclastic flows, lava flows, and sediments flanked by a ring of younger moat lava domes and sediments. The Valles caldera forms an almost perfect circular depres- sion containing a resurgent Minimum distribution of Valles caldera ash deposits; the prevailing wind direction at the time of the explosion was dome with more than 3,000 generally to the north. Rafts of pumice that floated down feet of central uplift above the the Rio Grande formed deposits still observable as far south caldera floor and a nearly com- as El Paso. plete ring of moat eruptions. It is no wonder that the Valles caldera is the type example of a resurgent Caldera Formation caldera and the place where volcanologists General model of caldera formation. From research published in the 1960s come to study caldera timing, deposits, earth’s crust, calderas tend to form in by Robert L. Smith and Roy A. Bailey and morphology. Most other calderas clusters. Thus, the Valles caldera (1.25 mil- of the U.S. Geological Survey, the Valles are not as perfect in form. For example, lion years old) was preceded in this region caldera became famous as the location Yellowstone displays an elongate, elliptical by the comparably sized Toledo caldera where the first model of large caldera shape with two resurgent domes. Ring- (1.62 million years old). Much of Toledo formation was devised: As large volumes fracture eruptions were so voluminous that caldera was obliterated during formation of magma accumulate a few miles beneath one resurgent dome and the west caldera of the Valles caldera. In Wyoming the the earth’s surface, there may be swelling wall are buried in younger lava flows. As a Yellowstone caldera (0.64 million years of the ground surface, rupture of faults result, Yellowstone was not recognized as a old) was preceded by the smaller Henry’s and fractures, seismic activity, and heating resurgent caldera until many years after the Fork caldera (1.3 million years old) and the of overlying ground water. Eventually Valles model was studied, published, and larger Big Bend Ridge caldera (2.1 million gas pressure exceeds confining pressure of applied to other suspected calderas. years old). The San Juan Mountains of overlying rocks, and the magma explodes, Because they commonly occur along southwest Colorado contain more than producing immense pyroclastic falls and hot spots, tectonic boundaries, and other fifteen calderas that formed between 24 flows. Emptying of the magma chamber locations where anomalously large amounts and 30 million years ago, and the Gila destabilizes pre-existing rock masses, which of heat rise from the mantle through the Wilderness–Black Range region of south- new mexico earth matters 2 winter 2010 west New Mexico contains another ten (albedo) causing climatic cooling. calderas that formed between 25 and Such eruptions would presumably 35 million years ago. affect oscillatory climate patterns such as El Niño. Climate disruptions Predicting the Next Eruption could continue for decades while the People often ask if the Valles caldera ash blanket is slowly eroded. Many is dormant or extinct. The youngest researchers also have concluded that eruption within the caldera is the such eruptions have detrimental effects Banco Bonito lava flow dated at on animal populations; thus, forma- roughly 40,000 years ago, which is tion of the Toba caldera, Indonesia partially traversed by NM–4 in the (74,000 years ago), produced a huge southern caldera.