The Eastern California landscape near Mammoth Lakes is dominated by a major active volcanic area. The question of whether Mammoth is a volcano is complex, as the term refers both to a specific mountain peak and a much larger, interconnected system. This region is situated on one of the Earth’s largest volcanic depressions and is actively monitored by geologists. The ongoing geological events are a direct result of magma remaining relatively close to the surface.
Differentiating Mammoth Mountain and the Long Valley Caldera
The relationship between Mammoth Mountain and the Long Valley Caldera often confuses visitors. Mammoth Mountain is a distinct, large lava dome complex that grew on the southwestern rim of the much older and larger Long Valley Caldera. Reaching an elevation of over 11,000 feet, the mountain is essentially a younger volcano built upon the edge of a massive, ancient one.
A caldera is a giant, bowl-shaped depression. It forms when a volcano’s magma chamber is emptied during a massive eruption, causing the ground above it to collapse. The Long Valley Caldera measures approximately 20 miles long and 11 miles wide, making it one of the largest calderas globally. Mammoth Mountain’s position on the rim means it is closely associated with the caldera’s underlying magma systems.
Geological History of Major Eruptions
The Long Valley Caldera was created by a cataclysmic eruption about 760,000 years ago, one of the largest such events in North America. This eruption released an estimated 150 cubic miles of material, over 2,000 times the volume of the 1980 Mount St. Helens eruption. The sheer volume of ejected ash, which formed a rock layer known as the Bishop Tuff, caused the land to subside by thousands of feet, creating the giant caldera depression.
Following this collapse, subsequent volcanic activity occurred within the caldera, causing the floor to uplift and form a central resurgent dome. Mammoth Mountain itself formed much later, between about 110,000 and 57,000 years ago, from a series of eruptions on the caldera’s edge. The most recent volcanic activity in the immediate area occurred about 700 years ago, involving steam-driven or phreatic eruptions on the north side of Mammoth Mountain.
Current Activity and Monitoring Status
The Long Valley volcanic system is currently considered restless, showing signs of ongoing geologic activity that began to escalate in 1980. The unrest includes persistent seismic swarms, which are sequences of numerous small earthquakes indicating magma movement at depth. These events often occur beneath the southern margin of the caldera and under Mammoth Mountain.
Another indicator of activity is ground uplift, or inflation, within the central part of the caldera, known as the resurgent dome. This dome has been slowly rising since 1980, with a maximum uplift of around 75 centimeters. Continuous GPS and other instruments track this deformation, suggesting an accumulation of magma or magmatic fluids beneath the surface.
A specific sign of this ongoing unrest is the release of magmatic carbon dioxide (CO2) gas, particularly near Horseshoe Lake on the flanks of Mammoth Mountain. Since the early 1990s, high concentrations of this invisible, odorless gas have seeped from the ground, killing over 100 acres of trees by suffocating their roots. The total rate of CO2 emission is estimated to be hundreds of tons per day.
The U.S. Geological Survey (USGS) and the California Volcano Observatory (CalVO) maintain an extensive network of instruments to monitor the area. This monitoring system includes seismometers, GPS receivers, and gas sensors. This system tracks changes in earthquake activity, ground deformation, and gas emissions in real-time. While the current alert level is typically “green/normal,” this detailed monitoring provides timely warning should unrest escalate toward a potential eruption.