Mammoth Mountain is a volcanic dome located in the southwestern part of Mono County, California. Mammoth Mountain has the highest skiable terrain in California with its summit rising to 11,053 feet above sea level. The mountain receives an average of over 400 inches of snow each year. The town of Mammoth Lakes is supported by tourism because of the top-notch skiing, swimming, fishing, biking, and hiking opportunities. The town was originally established by the Mammoth Mining Company; however the mining prospect was very short-lived. Logging supported the town in the early 1900s. The town of Mammoth Lakes has an elevation of 7,880 feet above sea level.

The Mountain

A volcano is formed when molten rock (about 2,000 degrees F) pushes up the surface, creating a hill or mountain. When the pressure becomes too great, the earth opens and lava explodes out. The name volcano comes from Volcan, the Roman god of fire.

Mammoth Mountain is actually several overlapping lava domes that formed in a series of eruptions, approximately every 5,000 years from 110,000 to 57,000 years ago. The resulting dome was carved and shaped by glacial activity.

Devil’s Postpile represents another part of volcanic history. Formed about 100,000 years ago, when the basaltic lava filled a valley and made a basalt lake some 400 feet deep, which then slowly cooled into solid rock. Contracting, cooling lava cracked into columns of rock. From then until the end of the last major glacial period, about 15,000 years ago, glaciers carved away the surrounding rock, exposing the columns we see today. Freezing and thawing, as well as earthquakes, continue the process of cracking between columns and cause columns to collapse.

Build it up, Wear it Down

Following the series of volcanic eruptions, glaciation further shaped Mammoth Mountain. The giant ice sheets pulverized and carved the volcano into steep cliffs, gentle ridges, and broad bowls or U-shaped valleys. Many surfaces are polished or striated, showing deep scratches and gouges from the passing ice. The formations resulting from glacial erosion are fun to learn and say! Check out a roche moutonnée, a glacial cirque, an arête, and paternoster lakes.

Although the Mammoth Mountain volcano is not considered “active” because it has not erupted in the past 10,000 years, continued geologic activity in the area (especially in the Long Valley Caldera to the north) is evident in the area surrounding the mountain:

The Mammoth Lakes Earthquake Fault, which is actually a fissure. Basically, the difference is that with a fissure there is no offset as with fault movement. The fissure, or fracture, is believed to have formed about 600 years ago.
Long Valley Caldera and the Inyo Craters formed by the interaction of cool groundwater and partially molten rock creating pressurized underground steam.
Seismic activity continues, if not in the form of an active volcano. Magma movement now leads to displacement and fractures that are evidenced by earthquakes, steam vents, hot springs, and volcanic gases. Mammoth Geothermal Complex generates up to 30 megawatt of geothermal power. Gas emissions high in carbon dioxide killed almost 20 acres of forest at Horseshoe Lake in the late 1980s and early 1990s.

The United States Geological Survey (USGS) manages a variety of field sensor equipment that monitor seismicity, gas emissions, and other geologic hazards for a better understanding of this unique area.

The Lakes

Mammoth Lakes Basin was carved by glacial ice and rocks. While many of the lakes are only accessible by hiking, there are still quite a few that can be reached by car and are very popular swimming and fishing destinations. In the lower basin, the lakes fill depression in glacial moraine (rock and debris deposits) left behind by a moving glacier. Upper basin lakes are mostly accessible only by hiking and are depressions created by glacial activity into the bedrock.

The Geology of Vegetation

The Mammoth Lakes basin varies in elevation, temperature extremes, and water availability, which in turn leads to a variety of ecosystems:

Above 10,000 feet the sub-alpine forest where harsh winters and intense winds keep the slopes barren except for a few hardy pines. Small mammals and some birds can be found at these high elevations.

From 10,000 to 7,000 feet the predominant lodgepole pine is joined by red fir, Jeffrey pine, and western juniper in the upper and lower montane forests. Most of the recreational activities take place here, and the forest supports large mammals, fish, and a variety of birds.

Between 7,000 and 5,000 feet the pinyon/juniper forest is dominated by short, hardy, drought-tolerant trees with a dense underbrush of mesquite and sagebrush. This ecosystem is terrific to support a variety of birds and makes for great bird-watching opportunities.

Riparian ecosystems are located adjacent to wetlands and streams, and support cottonwood, willow, quaking aspen, and alder trees; fields of wildflowers; and hundreds of other plant and animal species. The deciduous trees of this ecosystem change color in the fall and create a spectacular show that is a joy to visit.