The Mono-Inyo Craters
chain of volcanoes on the east side of the Sierra Nevada in California stretches about 30 miles from Mammoth Mountain at the east side of the Long Valley Caldera north to Mono Lake. The Inyo Craters form the southern part of that chain with the Mono Craters extending to the north. Volcanic activity in the Long Valley area has been confined to the Mono-Inyo Craters during the past 35,000 years. About 20 small to moderate eruptions have occurred along the chain in the last 5,000 years. The intervals between eruptions range from 250 to 750 years. The most recent eruption was at Mono Lake between 1720 and 1850 which led to the emerging Paoha Island.
A long narrow body of molten rock beneath the ground (called a “dike”) fed eruptions that formed the Inyo lava flows and several explosion craters around Deer Mountain. When magma began rising beneath the southern end of the Mono-Inyo Craters volcanic chain about 600 years ago, a series of eruptions formed lava flows, craters, layers of pumice and ash, and cracks between Mammoth Mountain and Obsidian flow, all familiar geological features of that area today. When molten rock broke through existing rock and spreaded horizontally and upward, reaching about 7 miles long and 30 feet wide, the ground above cracked and faulted. A series of explosive eruptions followed ejecting pieces of molten and solid rock and creating small craters. The eruptions started at the South Deadman vent. Pumice and ash fell to the ground and covered extensive areas with a layer of rock debris downwind. The explosive activity also produced a pyroclastic flow that traveled about 4 miles from South Deadman. Explosive eruptions followed at the Obsidian vent and then at the Glass Creek vent. This explosive activity was followed by smaller steam-driven (phreatic) explosions around Deer Mountain. Heat from the rising magma superheated groundwater which eventually released the pressure by a massive steam explosion, while magma never reached the surface. These phreatic explosions formed the Inyo Craters and a crater atop Deer Mountain.
The Mono Crater chain of volcanoes is located to the north at the edge of Pumice Valley and consists of at least 30 coalesced domes, flows, and craters. Most domes and flows are less than 10,000 years old. They stretch from Panum Crater south of Mono Lake in the north to Devils Punchbowl in the south. Some peaks rise 2,000 feet above Mono Lake. The Mono Craters were formed by volcanic activity that began about 40,000 years ago and has continued to the present time. Paoha and Negit islands in Mono Lake are the northern extensions of this volcanic chain. Most eruptions began when stiff rhyolite magma forced its way to the surface along a crack in earth’s crust. Ash and pumice were then ejected from the vents. A large explosion creating a crater with a rim of pumice, rock debris, and fragments of bedrock followed. Stiff rhyolite magma then oozed from the vent, leaving a small rhyolite plug in the center of the crater. If the eruption continued the rhyolite magma would slowly fill the crater and form a circular steep-sided dome. On some domes the magma flowed over the crater wall and enveloped the original crater. In some places the flow continued as a stream of stiff molten glass that extended for up to two miles. These glass flows are known as coulees. Some domes have a secondary crater when gas that built up under a plug was released in an explosion.
List of Craters and Domes
Devils Punch Bowl (~8,030’)
Wilson Butte (8,488’)
Obsidian Dome (8,601’)
South Glass Creek Dome (~8,610’)
North Deadman Dome
South Deadman Dome (~ 8,460’)
Deer Mountain (8,786’)
(crater rim ~ 8,250’)
The Mono-Inyo Craters chain of volcanoes can easily be visited and accessed from Highway 395 around the Mammoth Lakes/Mono Lake region.
The trailhead to the southernmost Inyo Craters can be reached from the Mammoth Lakes Scenic Loop Highway. Deadman Creek Road and Glass Flow Road are both dirt roads from Highway 395 leading to the Deadman Domes and Glass Creek Dome. Wilson Butte lies directly on Highway 395.
Devils Punchbowl can be accessed by the Pumice Mine Road from Highway 395 south of Highway 158 (June Lake Loop). Highway 120 south of Mono Lake leads to the turn-off to Panum Crater. A dirt road leads to the Panum’s trailhead. A short drive further east on Highway 120 gets you close to the North Coulee. Access to the other Coulees is mostly by four-wheel drive dirt roads.
All sorts of accomodations (motels, hotels, resort cabins, etc.) can be found in Mammoth Lakes. June Lake north of Mammoth Lakes also provides several types of accomodations for exploring this area. Campgrounds are scattered all around the Mammoth Mountain/Mono Lake region.
guide to HIGHWAY 395 los angeles to reno
by Ginny Clark, Western Trails Publications, 1990
Geological Trips Sierra Nevada
by Ted Konigsmark, GeoPress, 2002
United States Geological Survey