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The Long Valley-Glass Mountain volcanic field of eastern California was the site of numerous eruptions of rhyolitic tephra in late Pliocene to middle Pleistocene time. In the volcanic field, stratigraphic and isotopic age evidence clearly indicate that six pyroclastic eruptions occurred in the age span 2.1 to 1.0 m.y. Three layers of coarse-grained tephra near the Cowan mine and three similar coarse-grained tephra beds at Blind Spring Hill south of Benton Hot Springs, Calif., record episodes of pyroclastic volcanism about 2.1 million years ago. Another layer of coarse-grained tephra near the Cowan mine south of Benton Hot Springs, Calif., records a pyroclastic volcanic event that occurred about 1.0 million years ago. Two layers of volcanic ash interlayered in sediments along the Owens River north of Bishop, Calif. suggest additional pyroclastic volcanic eruptions in the volcanic field at about 1.0 m.y. Ten other pyroclastic eruptions are inferred to have occurred in the volcanic field from about 2.5 to 1.0 million years ago. This speculation is made because numerous ash beds in California, Nevada, and Utah have chemical and mineralogical properties.similar to the Bishop Tuff. Because of the protracted history of pyroclastic volcanism in the Long Valley-Glass Mountain region, it seems probable that more pyroclastic eruptive activity will occur in the future. The pyroclastic volcanism is the surface manifestation reflecting the buoyant rise into high levels of the earth's crust of a large granitic batholith that has been evolving beneath the Long Valley-Glass Mountain region in the last three million years. The pyroclastic materials formed during the many different episodes are united by a common set of chemical and mineralogical properties. These properties are considerably different than the properties of tephra generated at other late Pliocene and Pleistocene volcanic centers in the Western United States. Although the tephra and ash beds are united by overall chemical similarity, small but consistent differences in the amounts of Cs, Rb, U, La, Hf, Ta, Mn, and Fe termined by instrumental neutron activation analysis) allow their separation. Mineralogically the and ash beds are similar; they all contain quartz, sanidine, oligoclase, biotite, allanite, zircon, apatite, titanomagnetite, and ilmenite. In addition, the upper part of the Bishop Tuff contains orthopyroxene, clinopyroxene, and pyrrhotite. The catastrophic eruption of the Bishop Tuff 0.74 million years ago was the largest of all the eruptions generated in the Long Valley-Glass Mountain volcanic field. Other eruptions in the Long Valle Glass Mountain area were small, perhaps about the same volume (about 1.0 km) as that ejected during the May 18, 1980 eruption of Mount St. Helens volcano. About 500 km3 of tephra that now forms ashf lows of the Bishop Tuff was expelled from the vent area at Long Valley in a brief time. A large volume of volcanic ash that also was produced during the Bishop eruptive cycle was carried away from the vent by upper stratosphere winds and deposited as a thin blanketlike deposit that must have covered more than a million square kilometers of the Western United States. Remnants of the ash fall are found as far east as central Nebraska, as far south as southern New Mexico, and as far north as southern Idaho. Plinian pumice deposits are 3-4 m thick in the source region. Ash was as least 10 cm thick (compacted thickness) in western Utah, 500 km away from the vent and at least 6 cm thick in eastern Utah, 800 km downwind from the vent. The volume of ash carried away from the vent and distributed widely across the Western United States was enormous, perhaps as much as several hundred cubic kilometers.

The Bishop ash bed and some older compositionally similar ash beds in California, Nevada, and Utah