Geology
Mammoth Mountain lies on the South end of the Mono-Inyo chain of volcanic craters. Mammoth Mountain is also on the southwestern edge of Long Valley Caldera, a large area that subsided after an enormous eruption 760,000 years ago. The Mammoth Mountain magmatic system is considered distinct from that of Long Valley caldera and Inyo Craters.
The volcano was formed from a long series of eruptions that started about 110,000 years ago and lasted to approximately 57,000 years ago. During this time, massive dacite eruptions occurred roughly every 5000 years. The volcano is still active with minor eruptions, the largest of which was a minor phreatic (steam) eruption 700 years ago.
Mammoth Mountain is composed primarily of dacite and rhyolite, some of which have been altered by hydrothermal activity from fumaroles (steam vents).
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