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38-435: The Pajarito Plateau is primarily composed of Bandelier Tuff , a voluminous deposit of volcanic tuff laid down in an explosive eruption — in this case, a pair of eruptions from the nearby Valles Caldera. The two ignimbrite-forming eruptions occurred about 1.6 million and 1.2 million years ago and ejected about 300 cubic kilometers of rock each. The orange-pink rock formations constituting the resulting ignimbrites are known as

76-517: A blue iridescence ( chatoyance ). The member contains abundant fragments of country rock . Ar/ Ar radiometric ages for the member range from 1.61 ± 0.01 to 1.62 ± 0.04 Ma. The maximum exposed thickness is about 120 meters (390 feet). This member was erupted in the Toledo event, which was named after the Toledo Embayment, a structural feature in the northeast caldera wall which was long thought to be

114-432: A few have been used for ultramarathons and endurance rides . Popular uses include walking, dog walking, running, orienteering and rogaining , mountain biking, and horse riding. Due to constraints of geography and land ownership, both Los Alamos and White Rock are compact populated places. Outside of these, the plateau is sparsely populated and wildlife is abundant and diverse. A large elk herd that spends summers in

152-419: A large and growing magma pool until the crust is unable to contain the pressure and ruptures. This can occur at hotspots (for example, Yellowstone Caldera ) or at subduction zones (for example, Toba ). Large-volume supervolcanic eruptions are also often associated with large igneous provinces , which can cover huge areas with lava and volcanic ash . These can cause long-lasting climate change (such as

190-507: A maximum thickness of over 900 meters (3,000 feet). It was emplaced by the Valles event, which took place 1.256 million years ago and created the Valles caldera. The Tshirege Member is described as a compound cooling unit, composed of distinct pulses of deposition, and two schemes have been developed to label its beds. The Rogers classification divides the member into lettered zones A through F based purely on mappable lithological criteria, while

228-550: A significant migratory flyway that brings many other birds (notably sandhill cranes ) to the plateau during migration seasons, and three species of hummingbirds are abundant during summer. White Rock Canyon , the major canyon containing the Rio Grande into which the plateau's canyons empty, is notorious for rattlesnakes . The Pajarito Plateau has been home to Native American communities for at least 11,000 years. Archaeological evidence indicates both seasonal and permanent use of

266-657: Is interpreted by Broxton and Reneau as a devitrification front rather than a cooling unit boundary. The beds below the vapor phase notch are glassy tuffs while those above are devitrified; the beds are otherwise chemically and petrologically indistinguishable. In many locations, the Tshirege Member is separated from the Otowi Member by the Pueblo Canyon Member of the Cerro Toledo Formation . Ash matching

304-476: Is popular here with locals and visitors alike, and is done in the canyons on basalt cliffs that lie within and below the tuff, which itself is too soft to be climbed safely. The canyons and mesas are highly scenic and well endowed with trails , about which numerous guidebooks have been written. Some trails reach the Rio Grande; others reach the rim of the Valles Caldera. Some are long distance trails , of which

342-555: Is sometimes used for caldera supervolcanoes, such as the Blake River Megacaldera Complex in the Abitibi greenstone belt of Ontario and Quebec , Canada. Though there is no well-defined minimum explosive size for a "supervolcano", there are at least two types of volcanic eruptions that have been identified as supervolcanoes: large igneous provinces and massive eruptions. Large igneous provinces, such as Iceland ,

380-522: The Jemez Mountains of northern New Mexico . It has a radiometric age of 1.85 to 1.25 million years, corresponding to the Pleistocene epoch . The tuff was erupted in a series of at least three caldera eruptions in the central Jemez Mountains. The Bandelier Tuff was one of the first ignimbrites recognized in the geologic record, and has been extensively studied by geologists seeking to understand

418-511: The Laki fissure , which is approximately 40 km (25 mi) long. An estimated 14 km (3.4 cu mi) of basaltic lava was poured out during the eruption (VEI 4). The Ontong Java Plateau has an area of about 2,000,000 km (770,000 sq mi), and the province was at least 50% larger before the Manihiki and Hikurangi Plateaus broke away. Volcanic eruptions are classified using

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456-569: The Siberian Traps , Deccan Traps , and the Ontong Java Plateau , are extensive regions of basalts on a continental scale resulting from flood basalt eruptions. When created, these regions often occupy several thousand square kilometres and have volumes on the order of millions of cubic kilometers. In most cases, the lavas are normally laid down over several million years. They release large amounts of gases. The Réunion hotspot produced

494-486: The triggering of a small ice age ) and threaten species with extinction . The Oruanui eruption of New Zealand's Taupō Volcano (about 25,600 years ago) was the world's most recent VEI-8 eruption. The term "supervolcano" was first used in a volcanic context in 1949. Its origins lie in an early 20th-century scientific debate about the geological history and features of the Three Sisters volcanic region of Oregon in

532-406: The tuff show compositional zoning, in which the lower pyroclastic flows are more silicic and contain less mafic (magnesium- and iron-rich) minerals than the upper flows. This is interpreted as progressive eruption of a gravitationally zoned magma chamber in which volatiles are concentrated at the top of the chamber and mafic minerals have partially settled into the lower, hotter portions of

570-497: The Bandelier Tuff. Supereruption A supervolcano is a volcano that has had an eruption with a volcanic explosivity index (VEI) of 8, the largest recorded value on the index. This means the volume of deposits for such an eruption is greater than 1,000 cubic kilometers (240 cubic miles). Supervolcanoes occur when magma in the mantle rises into the crust but is unable to break through it. Pressure builds in

608-520: The Broxton and Reneau classification divides the member into numbered Qbt 1g through Qbt 4 zones based on interpretation as cooling units. The two schemes can be placed in close correspondence across most of the Pajarito Plateau. The division between the A unit (Qbt 1g) and B unit (Qbt 1vc) is particularly striking and is described as a vapor phase notch. This is recognizable across the Pajarito Plateau but

646-617: The Deccan Traps about 66 million years ago, coincident with the Cretaceous–Paleogene extinction event . The scientific consensus is that an asteroid impact was the cause of the extinction event, but the volcanic activity may have caused environmental stresses on extant species up to the Cretaceous–Paleogene boundary . Additionally, the largest flood basalt event (the Siberian Traps) occurred around 250 million years ago and

684-642: The Guaje Pumice Bed on the east flanks of the Jemez Mountains. Production was high enough in 1994 to help make New Mexico the second largest producer of pumice among the United States. The pumice itself is unconsolidated and easily removed once the overburden (typically Otowi Member ignimbrite) is removed. Much of the pumice was strip mined from public lands before reclamation bonds were required, leaving mining scars that are slowly revegetating. The formation

722-520: The Jemez Mountains descends to the Pajarito Plateau during the winter, creating a significant driving hazard. Deer , black bear and coyote are common, and the plateau and mountains support a small but stable population of mountain lions , which typically flee from humans. Smaller mammals such as raccoons , skunks , and gophers are common. Parts of the Bandelier backcountry have been closed seasonally due to nesting bald eagles . The Rio Grande supports

760-613: The La Cueva caldera. On the other hand, the presence of lithic breccia in this member in the La Cueva area suggests the caldera was located to the southwest. The Otowi Member consists of a basal air fall pumice bed (the Guaje Pumice ) and a massive, typically unwelded, ignimbrite, though this is densely welded in a few locations. The upper ignimbrite is a rhyolitic ash-flow tuff containing abundant phenocrysts of sanidine and quartz, and sparse mafic microphenocrysts. The sanidine may display

798-677: The Otowi Member may have covered the Española and Santo Domingo basins. These have since been mostly eroded away. Ash matching the Otowi Member in age and chemistry has been found as far away as Mount Blanco , Texas, where it forms a bed 30 centimeters (12 in) thick. The Tshirege Member has been described as "arguably New Mexico's most famous rock". It consists of multiple flows of densely welded to nonwelded rhyolitic ash-flow tuff. These contain abundant phenocrysts of sanidine and quartz, sparse microphenocrysts of clinopyroxene and orthopyroxene, and extremely rare microphenocrysts of fayalite . In

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836-608: The Otowi and Tshirege Members of the Bandelier tuff. The tuff lies on top of a volcanic field , exposed to the east, which used to be continuous with the Caja del Rio (now across the Rio Grande). Erosion has created a maze of canyons up to 800 feet (240 meters) deep that dissect the plateau into mesas . Many of these mesas climb on the western side rather than descending on all sides, thus sometimes are referred to as potreros . Rock climbing

874-556: The Plateau throughout the Paleoindian, Archaic, Developmental, Coalition, Classic, and historic periods. Ancestral Puebloans who used the Plateau have ties to modern Puebloan tribes in New Mexico. 35°51′N 106°18′W  /  35.850°N 106.300°W  / 35.850; -106.300 Bandelier Tuff The Bandelier Tuff is a geologic formation exposed in and around

912-461: The Tsankawi Pumice in age and composition has been found as far away as Utah and may have reached western Canada. The distant dispersal is likely a result of the eruption column penetrating the jet stream . Much of the material in these deposits now forms the Pajarito Plateau, a scenic region of canyons and mesas on which Los Alamos is situated. Pumice has been extensively mined from

950-479: The United States. In 1925, Edwin T. Hodge suggested that a very large volcano, which he named Mount Multnomah , had existed in that region. He believed that several peaks in the Three Sisters area were remnants of Mount Multnomah after it had been largely destroyed by violent volcanic explosions, similarly to Mount Mazama . In his 1948 book The Ancient Volcanoes of Oregon , volcanologist Howel Williams ignored

988-547: The intersection of the western margin of the Rio Grande Rift and the Jemez Lineament . Here magma produced from the fertile rock of an ancient subduction zone has repeatedly found its way to the surface along faults produced by rifting. This has produced a long-lived volcanic field , with the earliest eruptions beginning at least 13 million years ago and continuing almost to the present day. Both upper members of

1026-567: The low end of the supereruption range ( VEI 8). The member is exposed over the entire Jemez area, except within the Valles caldera itself, where it is present only in the subsurface. It is particularly extensively exposed in the Jemez Plateau west of the caldera, but is also exposed across much of the Pajarito Plateau east of the caldera at the bases of its characteristic finger mesas . Distant isolated outcrops suggest that thin ash flows of

1064-405: The magma chamber. The tuff contains up to 30% lithic fragments, which in the Otowi Member are estimated to have a total volume of 10 km and to be sufficient to quench welding through their cooling effect. The lithic fragments are 90% earlier volcanic rock, 10% Paleozoic sedimentary rock , and only traces of Precambrian rock, implying considerable flaring of the eruption vents. Some of

1102-511: The more densely welded portions of the member, the sanidine is chatoyant. The member typically contains fragments of country rock, and locally has a thin (less than 2 meters (6.6 foot)) basal pumice and surge deposit bed, the Tsankawi Pumice Bed . This bed contains roughly 1% of hornblende dacite pumice. The member is exposed throughout the Jemez region and within the Valles Caldera, and has

1140-499: The possible existence of Mount Multnomah, but in 1949 another volcanologist, F. M. Byers Jr., reviewed the book, and in the review, Byers refers to Mount Multnomah as a "supervolcano". More than fifty years after Byers' review was published, the term supervolcano was popularised by the BBC popular science television program Horizon in 2000, referring to eruptions that produce extremely large amounts of ejecta . The term megacaldera

1178-493: The processes involved in volcanic supereruptions . The formation is composed of ignimbrites produced by a series of at least three Quaternary caldera eruptions that culminated in the Valles Caldera eruption 1.256 million years before the present ( Mya ). The Valles Caldera is the type location for resurgent caldera eruptions , and the Bandelier Tuff was one of the earliest recognized ignimbrites. The caldera lies on

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1216-414: The remains of the Toledo caldera. However, more recent work has demonstrated that the Toledo caldera was likely more or less coincident with the Valles caldera. The total dense-rock equivalent volume of the eruption, including pyroclastic flows and ash fall, was between 216 cubic kilometers (52 cu mi) and 550 cubic kilometers (130 cu mi), with the larger estimate placing the eruption in

1254-416: The rock shows indication of contact metamorphism in the magma chamber walls with a magma rich in water and fluorine . The Bandelier Tuff consists of three members corresponding to at least three distinct caldera eruptions. The La Cueva Member is an unwelded to poorly welded tuff with phenocrysts of quartz and sanidine and traces of pyroxene and magnetite . It has been divided into two units;

1292-519: The stratigraphic framework for the Jemez volcanic field in 1969, R.L. Smith, R.A. Bailey, and C.S. Ross adopted Grigg's unit names and added the name Tsankawi Pumice for the basal pumice bed of the Tsherige Member. In their 2011 map of the Valles Caldera , Fraser Goff and his coinvestigators formally added the La Cueva Member, informally known until then as the ignimbrite of San Diego Canyon, to

1330-476: The upper unit is nonwelded to slightly welded and contains large pumice clasts, while the lower unit is nonwelded and includes abundant lithic fragments. Separating the two units is a bed of reworked pumice and debris flows . However, the Ar/ Ar ages are indistinguishable, at 1.85 ± 0.07 and 1.85 ± 0.04 Ma for the upper and lower units, respectively. The maximum observed thickness is 80 meters (260 feet). This member

1368-467: Was coincident with the largest mass extinction in history, the Permian–Triassic extinction event , although it is unknown whether it was solely responsible for the extinction event. Such outpourings are not explosive, though lava fountains may occur. Many volcanologists consider Iceland to be a large igneous province that is currently being formed. The last major outpouring occurred in 1783–84 from

1406-413: Was emplaced by the first and smallest (but still enormous) known caldera eruption of the Jemez volcanic field. It is exposed in only a few locations, including San Diego Canyon, the southwestern caldera wall, and in scattered locations on the Pajarito Plateau . It is possible that the Toledo Embayment, a structural feature of the northwest rim of the caldera coincident with a gravity low , is the remnant of

1444-414: Was given its name by H.T.U. Smith in 1938. The formation was divided into upper and lower units, which were recognized almost at once to correspond to separate caldera eruptions. In 1964, R.L. Griggs assigned the formal member names of Otowi Member to the lower unit and Tshirege Member to the upper unit, and gave the name Guaje Pumice to the basal pumice bed of the Otowi Member. In their paper establishing

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