Geologic History of Long
Valley Caldera and
the Mono-Inyo Craters volcanic chain, California
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 slip (earthquakes) gradually producing the impressive relief
of the eastern Sierra Nevada and White Mountain escarpments
that bound the northern Owens Valley-Mono Basin region (see
the geologic map). Two
distinct, but interrelated, magmatic systems
have dominated the volcanic evolution of the Long Valley -
Mono Basin region over this time interval (Bailey and others,
1976; Bailey, 1989; see the reading list ). 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 of the two magmatic systems is centered on Long Valley Caldera and covers a 4000-square-km 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, 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.
The older of the two magmatic systems is centered on Long Valley Caldera and covers a 4000-square-km 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, 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 cubic kilometers 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 subsidence of the magma
chamber roof to form the present 17 by 32 km, 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 700,000 to 600,000
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.
Mono-Inyo Craters
The younger system, the
Mono-Inyo Craters volcanic chain, is localized along a narrow,
north-trending fissure system that extends from south
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 - 60,000 years ago
and then in the Mono Basin 40,000 -13,000 years ago. Repeated
eruption of dacite and
rhyodacite from vents on the southwest
rim of the caldera 220,000 - 50,000 years ago formed Mammoth
Mountain, a cumulo-volcano. Dacite and rhyodacite
also were erupted in the Mono Basin 100,000 - 6,000 years ago.
The Mono Craters were formed by multiple eruptions of high-silica
rhyolite 40,000 - 600 years
ago, and the Inyo Craters were formed by eruptions of
low-silica rhyolite 5,000 - 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 500
to 600 years ago (Miller, 1985; Bursik
and Sieh, 1986), and
Paoha Island about
250 years ago (Stine, 1990). 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.
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.
URL http://lvo.wr.usgs.gov/History.html
Contact: Long Valley Web Team
