Mammoth Mountain straddles the southwest rim of Long Valley Caldera. It is the largest post-caldera volcanic feature in the Long Valley area. Mammoth Mountain was built by a series of approximately 25 separate eruptive episodes between 100,000 and 51,000 years ago.
Mammoth Mountain was built by a series of approximately 25 separate eruptive episodes between 100,000 and 51,000 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.