The Bishop Tuff is a

welded tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

that formed 764,800 ± 600 years ago as a

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

during the approximately six day eruption that created the

Long Valley Caldera Long Valley Caldera is a depression in eastern California that is adjacent to Mammoth Mountain. The valley is one of the Earth's largest calderas, measuring about long (east-west), wide (north-south), and up to deep. Long Valley was formed 7 ...

. Large outcrops of the tuff are located in Inyo and Mono Counties,

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. Approximately 200 cubic kilometers of ash and tuff erupted outside the caldera.

Modern exposure

The Bishop Tuff caps a

volcanic plateau A volcanic plateau is a plateau produced by volcanic activity. There are two main types: lava plateaus and pyroclastic plateaus. Lava plateau Lava plateaus are formed by highly fluid basaltic lava during numerous successive eruptions throu ...

in the northern

Owens Valley Owens Valley (Numic Numic is a branch of the Uto-Aztecan language family. It includes seven languages spoken by Native American peoples traditionally living in the Great Basin, Colorado River basin, Snake River basin, and southern Great Pl ...

in eastern

. The tableland formation is located east of

U.S. Route 395 U.S. Route 395 (US 395) is a U.S. Route in the western United States. The southern terminus of the route is in the Mojave Desert at Interstate 15 near Hesperia. The northern terminus is at the Canada–US border near Laurier, where the road ...

and west of the

Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...

stateline, sitting northwest of

Bishop A bishop is an ordained clergy member who is entrusted with a position of authority and oversight in a religious institution. In Christianity, bishops are normally responsible for the governance of dioceses. The role or office of bishop is ca ...

and southeast of

Crowley Lake Crowley Lake is a reservoir on the upper Owens River in southern Mono County, California, in the United States. Crowley Lake is south of Mammoth Lakes. The lake was created in 1941 by the building of the Long Valley Dam by the Los Angeles Depa ...

and

Mammoth Lakes Mammoth Lakes is a town in Mono County, California, and is the county's only incorporated community. It is located immediately to the east of Mammoth Mountain, at an elevation of . As of the 2020 United States Census, the population was 7,191, r ...

. Another part of the flow is south of

Mono Lake Mono Lake ( ) is a saline soda lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in an endorheic basin. The lack of an outlet causes high levels of salts to accumulate in the lake which make its water alk ...

, and surrounding the Mono-Inyo Craters. Deposits of Bishop Tuff in this area cover nearly , and range in thickness from to . The

Owens River The Owens River is a river in eastern California in the United States, approximately long.U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map accessed March 17, 2011, It drains into and through the ...

cuts through the Volcanic Tableland, an

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

plateau that is a principal sector of the Bishop Tuff outflow sheet. Erosion of the plateau by the Owens River has created the

Owens River Gorge The Owens River Gorge is a steep 10 mi (16 km) canyon on the upper Owens River in eastern California in the United States. The canyon is located at the eastern edge of the Sierra Nevada mountains in southern Mono County, along the stre ...

Lithology

The Bishop Tuff is a high-silicate rhyolitic welded

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

, made up of ash and

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular vol ...

clasts. The main minerals found in the predominate pumice clasts are

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

, and

sanidine Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system ...

. The main composition is SiO₂ (73.4-77.9%), followed by Al₂O₃ (12.7%). The Bishop Tuff is compositionally zoned. The lower section, formed from ash fall, is notated by pyroxene-free high-silica rhyolite pumice. The upper section, formed by pyroclastic flow, is notated by pyroxene-bearing high-silica rhyolite pumice. The magma that formed the Bishop Tuff is suggested to be a "residual magma derived from some parental magma and not itself a primary or parental partial melt of common crustal rocks".

References

External links

Long Valley Observatory - Bishop Tuff

Tuff formations Pyroclastic plateaus Volcanism of California Geologic formations of California Geology of Inyo County, California Landforms of Inyo County, California Landforms of Mono County, California Inyo National Forest VEI-7 eruptions Pleistocene volcanism Volcanic eruptions in the United States Plinian eruptions {{US-geology-stub

Modern exposure

Lithology

See also

References

External links