From Glaciers to Gondolas: How Mammoth Mountain Got Its Shape

The giant lava dome complex, which is located deep within the heart of California’s legendary Sierra Nevada Mountains, is a broad volcanic cone positioned along the southwest rim of the giant Long Valley Caldera. This geologically young volcano reaches to the dizzying heights of 11,053 feet above sea level and looms above the outdoor playground and year-round scenic destination of Mammoth Lakes.

Erupting just over 100,000 years ago, this extinct volcanic complex has had several phases of mountain growth and expansion. Multiple eruptions, followed by tens of thousands of years of destructive glaciation and erosion, have created one-of-a-kind landscapes, slopes and geologic features just waiting to be explored on foot, skis, snowboard or mountain bike.

The Geological Composition of Mammoth Mountain

Mammoth Mountain is composed of Pleistocene rhyolite and dacite (gray to brown silica-rich volcanic rocks). It is believed 25 separate eruptive events occurred and persisted to around 55,000 years ago, when the final eruption concluded the growth of Mammoth Mountain. These eruptions created at least 12 overlapping volcanic domes, many of which are geographically distinct from the others. The youngest volcanic rocks of Mammoth Mountain comprise the summit, which is also known by geologists as the summit dome.

The Mammoth Mountain volcanic complex lies along the rim of the massive Long Valley Caldera, which erupted and collapsed 760,000 years ago. Based on the geochemistry of the rocks produced by each eruption, geologists believe the two had separate magma chambers as sources for volcanic material. Like the caldera, Mammoth Mountain is extinct, as active volcanoes must have erupted within past 10,000 years (Holocene time division).

The Shaping of Mammoth Mountain: Destruction by Ice

The impressive pulverizing force of high-alpine glacial ice has greatly carved the rocks of Mammoth Mountain. From the knife-edge ridge of Dragon's Back to the steep face of Chair 1 out of Main Lodge, evidence of a glacially influenced landscape is everywhere. If you look closely, many of the rocks of Mammoth Mountain are striated (deep gouges from passing ice), polished and are carved into broad bowls and cirques from the interaction of glacial ice versus rock.

Chair 1 passes above a perfect example of a glacially formed roche moutonnée. These glacially-specific land forms are created by advancing sheets of ice (glaciers) as they grow and migrate down slope. As glaciers advance, they often pulverize the rock on the downhill side by a combination of gravity and the freeze-to-thaw cycle. This creates steep cliffs on the leeward (downslope) side and gentle ridges on the upslope side, similar to that of a breaking wave in the ocean.

The backside of Mammoth Mountain is a broad, glacially carved basin. Red Lake, located just below the bases of chairs 13 and 14, marks the final resting place of a massive sheet of ice that extended into the San Joaquin River drainage 11,000 years ago. Dragon's Back is a long, thin ridge carved by glaciation and known to geologists as a glacial arête. The eastern edge of the Dragon's Back is truncated by a series of huge cliffs created by thick Pleistocene glaciers that likely extended to the edge of downtown Mammoth Lakes during their peak.

Lincoln Peak Landslide

Lincoln Mountain is a volcanic dome located to the northeast of the Mammoth Mountain summit. The precipitous Avalanche Chutes on the northern face of Lincoln Mountain are the remnants of a 500-meter by 350-meter landslide that left a horseshoe-shaped bowl in its aftermath. This massive landslide and debris avalanche occurred when the weathering and decay of an overly steepened volcanic slope led to collapse of the mountainside. The tipping point is postulated to have occurred around 20,000 years ago as a response to possible nearby steam (phreatic) explosions. Today, the Canyon Lodge neighborhood is built upon the unconsolidated debris and sediments of this flow.

Mammoth Mountain Today

Although Mammoth Mountain itself is considered extinct by definition, several small volcanic events have recently occurred in its vicinity. Eruptions to the southwest at Red Cone are radiometrically dated as 9,000 years old. Within the past 1,000 years, geographic features such as the Inyo Craters formed when cool ground water met partially molten rock and created pressurized underground steam.

In the 1980s, the shallow underground intrusion of magma under the southern flanks of Mammoth Mountain created earthquake swarms and released volcanic gases. High concentrations of carbon dioxide gas killed almost 20 acres of forest and the source of this CO2 is believed to be from the remaining magma beneath Mammoth Mountain. Today, visitors are able to see the “ghost forest” at Horseshoe Lake in the Mammoth Lakes Basin.

To date, the United States Geological Survey monitors a variety of field data collection sensors that record seismicity, gas emissions and other geologic hazards around Mammoth Mountain. Through this constant monitoring, geologists are able to develop a better understanding of the unique geologic environment of Mammoth Mountain.

View Mammoth Mountain’s Unique Shape from the Gondola

To put the area into geologic perspective, take a ride to the summit on the Mammoth Mountain Ski Area gondola. From its peak, Mammoth Mountain offers a breathtaking panorama of the Long Valley Caldera to the east, Inyo Craters to the north and the glacially carved Lakes Basin to the south. On the ride up you should expect to see a diverse array of land forms, many of which record the separate and distinct volcanic events. While at the top, visit the Eleven 53 Interpretive Center in the gondola station for more information and displays on the amazing geologic history of Mammoth Mountain.