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Long Valley Caldera Field Guide - Lookout Mountain

A good vantage point for viewing the region. 

Geologic Summary

Obsidian on top of Lookout Mountain, Long Valley Caldera....
Obsidian on top of Lookout Mountain, Long Valley Caldera.(Public domain.)

Lookout Mountain is a volcano and high point in the NW Moat of the caldera and outside the area of uplift associated with the Resurgent Dome. The Resurgent Dome is an area inside the caldera that was built up by volcanism and uplift. Most of the 400 m (1300 feet) of uplift occurred within the first 100,000 years of the caldera formation as magma intruded the underlying crust (Hildreth, 2004). The volcanic deposits, part of a suite of erupted materials called the Early Rhyolites, were also erupted in the first 100,000 years after the formation of Long Valley Caldera.

Lookout Mountain was formed by eruptions about 677-692,000 years ago and is an Early Rhyolite just outside of the area of uplift (Hildreth, 2004). The summit crater is 1 km (0.6 mi) in diameter. The west side of the summit of Lookout Mountain provides an excellent view of the western edge of the caldera, Mammoth Mountain, and the Inyo Craters chain to the north.

The top of Lookout Mountain is covered with obsidian, a dark-colored rhyolite (high silica volcanic rock) formed when lava cools rapidly to a glass. The composition of the Lookout Mountain obsidian is similar to other rhyolite eruptions in the area. Obsidian from Lookout Mountain does not contain vesicles (holes) or visible minerals, which makes it a good a source for arrowhead material. Obsidian was very important to the Native American tribes throughout the area for tools and trading.

View From Summit

View from Lookout Mountain of Obsidian Dome and surrounding area....
View from Lookout Mountain of Obsidian Dome and surrounding area, Long Valley Caldera. (Public domain.)

See the panorama images below. The west and northwest walls of the caldera (opposite parking area) are underlain by Sierran granite (high silica rocks that cooled beneath the surface) and metamorphic rocks (mixed composition rocks that were altered by high temperatures and or pressures). The caldera rim is capped by lavas that erupted between 3.8 and 2.6 million years ago, prior to the caldera-forming eruption.

Between Lookout Mountain and the Caldera Rim are the barren rhyolite dome- flows of the Inyo Craters chain. Facing west (away from the parking area and towards the domes) you can see from south to north (left to right): Deadman Creek Dome, Glass Creek Dome, Obsidian Dome, and Wilson Butte. Activity at the southern three younger vents occurred within a short period of time in 1350 CE (Millar, et. al., 2006). The eruptive sequence began with steam explosions, followed by explosive ash eruptions, and concluded with viscous lava flows, which form the barren domes in the middle distance (see the Obisidian Dome field guide page for more information on the eruption sequence).

To the south of the domes is Deer Mountain (115,000 years old). Immediately south of Deer Mountain are the Inyo Craters, which were created by steam explosions in 1350 CE (see the Inyo Craters field guide page for more information on the Inyo Craters). To the south of Deer Mountain is the east side of Mammoth Mountain. In front of Mammoth Mountain is Earthquake Dome, which was part of the Mammoth Mountain eruption sequence.

Panorama looking west from the summit of Lookout Mountain
Panorama looking west from the summit of Lookout Mountain. (Credit: Roy Bailey. Public domain.)
panorama from summit of Lookout Mountain
Second half of panorama from summit of Lookout Mountain. The left edge of this diagram connects to the right edge of the diagram at left. (Credit: Roy Bailey. Public domain.)


Hildreth, Wes, 2004, Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: several contiguous but discrete systems, Journal of Volcanology and Geothermal Research, Vol. 136, p. 169–198.C.I. Millar, King, John C., Westfall, Robert D., Alden, Harry A., Delany, Diane L., 2006, Late Holocene forest dynamics, volcanism, and climate change at Whitewing Mountain and San Joaquin Ridge, Mono County, Sierra Nevada, CA, USA, Quaternary Research 66, 273–287.

Field Stop Location: Lookout Mountain

Quadrangle: Old Mammoth, California 7.5 minute topographic quadrangle
Coordinates: 37°43.724' N, 118°56.848' W 
Approximate Elevation: 8,327 ft (2,538 m)

Directions to Lookout Mountain:

The summit of Lookout Mountain is accessible by a gravel road (directly opposite the signed Mammoth Scenic Loop Road, turnoff at 37°42.555' N, 118°57.143' W). The road is not plowed in the winter.

Directions from Mammoth Lakes exit U.S. 395 and CA-203 Go this distance
1. Zero your odometer at the intersection of Highway 395 and Highway 203. Head north on U.S 395 towards Lee Vining and Mono Lake. Go 5.2 miles
2. Turn right onto the gravel road for Lookout Mountain which is the opposite direction from the Mammoth Scenic Loop Road. Go 0.9 miles
3. Continue straight past the large road to the left. Go 0.1 miles
4. Take the next left at sign "Lookout Mountain" onto road "2S02" (37°43.026'N 118°56.917' W) and follow the climbing road up to the summit. Go 1.5 miles
5. Bear left uphill. Go 0.6 miles
6. Park at the crest of the mountain.