Caldera

Long Valley Caldera a 15- by 30-km oval-shaped depression situated 20 km south of Mono Lake the length of the east side of the Sierra Nevada in east-central California. This area of eastern California has shaped numerous volcanic eruptions in excess of the past 3 million years, including the massive caldera-forming eruption 760,000 years ago. The majority recent eruption occurred immediately 250 years ago in Mono Lake at the north end of Mono-Inyo Craters volcanic chain.

Calderas are found at the tops of volcanoes, where the unique peak has collapsed into an unfilled chamber beneath. The basin, many times better than the original volcanic vent, may be flooded, producing a crater lake, or the flat floor may have a number of small volcanic cones, produced by volcanic activity behind the collapse.