Sketch of Long Valley Caldera and the Mono-Inyo Craters Volcanic Chain in central California, viewed from the southeast. (Sketch by Tau Rho Alpha.)
Long Valley Caldera, California. Long Valley Caldera lies on the eastern front of the Sierra Nevada, about 300 kilometers east of San Francisco. A huge explosive eruption about 700,000 years ago formed the caldera and produced pyroclastic flows that traveled 65 kilometers from the vent and covered an area of about 1,500 square kilometers. Ash from the caldera-forming eruption fell as far east as Nebraska. Within the past 40,000 years, eruptions have been restricted to a linear zone of vents, including the Mono-Inyo Craters Volcanic Chain, that extends about 50 kilometers north from the northwest part of the caldera.
This volcanic chain consists of many vents that
have erupted in the past several thousand years. Eruptions from vents as
recently as 550 years ago produced lava flows, pyroclastic flows, and ash,
all of rhyolitic composition. Geologic mapping shows that some eruptions
were preceded by ground cracking, suggesting that the ground was pulled
apart or stretched as magma neared the surface.