Geologic history of Mammoth Mountain
Mammoth Mountain was built by a series of approximately 25 separate eruptive episodes between 110,000 and 57,000 years ago. The eruptions were fed by a magma body located somewhere beneath the mountain and has a source separate from the magma body beneath Long Valley caldera.
During the time interval when Mammoth Mountain was active, a series of effusive (Hawaiian type) eruptions from vents in the west moat of the Long Valley caldera fed fluid, basaltic lava flows that covered the west moat. These lavas flowed around the caldera's resurgent dome into the north and south moat.