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Geologic history of the Long Valley, Mono Basin region

The Long Valley-Mono Basin Region

Persistent earthquake and volcanic activity over the past 4 million years has formed the spectacular eastern Sierra landscape in the vicinity of Long Valley caldera and the Mono Basin. Beginning about 3 million years ago (3 Ma), the Sierra Nevada and White Mountains fault systems became active with repeated episodes of fault movement (earthquakes) gradually producing the impressive relief of the eastern Sierra Nevada and White Mountain escarpments that bound the northern Owens Valley-Mono Basin region. Two distinct, but interrelated, magmatic systems have dominated the volcanic evolution of the Long Valley - Mono Basin region over this time interval. The compositions of lava produced by both systems have evolved similarly, becoming more silica-rich with time from early, predominantly basaltic eruptions to later, predominantly rhyolitic eruptions.

The older magmatic system is centered on Long Valley Caldera and covers a 4,000 km 3 (1,544 mi3) area straddling the eastern Sierra Nevada escarpment at the northern end of Owens Valley. This system produced widespread eruptions of basalt and andesite between 3.8 and 2.8 Ma over much of the Long Valley-Mono Basin region. The Sierra Nevada fault system has offset some of these early lava flows as much as 1,000 m (3,280 ft), leaving one side high on the Sierra crest and the other in the valley below. Volcanic activity became concentrated in the vicinity of the present site of Long Valley caldera 3.1 to 2.5 Ma with eruptions of rhyodacite followed by high-silica rhyolite from 2.1 to 0.8 Ma. Lava from the latter eruptions form Glass Mountain on the northeast rim of the present caldera.

Long Valley Caldera

The Glass Mountain eruptions, which were fed by a large, chemically evolving magma chamber in the shallow crust, culminated in the cataclysmic eruption of 600 km3 (144 mi3) of high-silica rhyolite 760,000 years ago. This massive eruption resulted in the widespread deposition of the Bishop Tuff and the simultaneous 2 to 3 km (1.2 to 1.7 mi) subsidence of the magma chamber roof to form the present 17 by 32 km (10 by 20 mi), oval depression of Long Valley Caldera. Subsequent eruptions from the Long Valley magma chamber were confined within the caldera with extrusions of relatively hot (crystal-free) rhyolite 750,000 to 640,000 years ago as the caldera floor was upwarped to form the resurgent dome followed by extrusions of cooler, crystal-rich moat rhyolite at 200,000-year intervals (500,000, 300,000, and 100,000 years ago) in clockwise succession around the resurgent dome. Repeated eruption of dacite and rhyodacite from vents on the southwest rim of the caldera 220,000 to 50,000 years ago formed Mammoth Mountain, a dome complex.

Mono-Inyo Craters Volcanic Chain

The younger system, the Mono-Inyo Craters volcanic chain, is localized along a narrow, north-trending fissure system that extends from north of Mammoth Mountain through the western moat of Long Valley caldera to the north shore of Mono Lake. This system began by erupting basalt and andesite first in the west moat of Long Valley caldera 400,000 to 60,000 years ago and then in the Mono Basin 40,000 to 13,000 years ago. Dacite and rhyodacite also were erupted in the Mono Basin 100,000 to 6,000 years ago. The Mono Craters were formed by multiple eruptions of high-silica rhyolite 40,000 to 600 years ago, and the Inyo Craters were formed by eruptions of low- silica rhyolite 5,000 to 500 years ago.

During the past 3,000 years the Mono-Inyo Craters have erupted at intervals of 700 to 250 years, the most recent eruptions being from Panum Crater and the Inyo Craters in 1350 C.E., and Paoha Island about 300 years ago. Evidence from both seismic soundings of the crust and studies of the fabric and composition of the lava indicate that these eruptions probably originated from small, discrete magma bodies rather than from a single, large magma chamber of the sort that produced the caldera-forming eruption 760,000 years ago.

This geologically recent volcanic activity, together with unrest in Long Valley Caldera that began in 1980 and the frequently felt earthquakes in the region, are reminders the processes that have sculpted the eastern Sierra landscape over the past 4 million years continue today.