During a gigantic eruption about 760,000 years ago, an area bordered by what is now Mammoth Mountain, the Glass Mountains and Crowley Lake (approximately 12 miles wide by 18 miles long), collapsed to form the Long Valley Caldera. The eruption produced the Bishop Tuff, a pinkish-red rock that can be seen along US 395 on the Sherwin Grade. Three hundred and fifty square miles were buried beneath 500 feet of Bishop Tuff. Ash from the eruption has been found as far east as Nebraska. Later, other volcanoes erupted along the rim of this gigantic Caldera, one of which was Mammoth Mountain, which grew on the rim of the Caldera over 50,000 years ago.
Volcanic eruptions have continued within the Caldera and north along the Inyo-Mono volcanic chain. Eruptions at Obsidian Dome, South Deadman Dome and Panum Crater all occurred as recently as 500 to 800 years ago. The bottom of the Mono Lake was pushed up above the lake surface by an injection of magma to form Paoha (the white) Island approximately 300 years ago.
At a Glance
||Mammoth Lakes, CA
The long valley Caldera is the massive, depressed area with Mammoth Lakes to the West, Crowley Lake to the Southeast and the Glass Mountains to the North.