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Long Valley Caldera Field Guide - Obsidian Dome

At the top of an obsidian dome, view rocks that look different but have the same composition. 

Eruption Sequence

Obsidian Dome viewed from Glass Creek Dome, Long Valley Caldera....
Obsidian Dome viewed from Glass Creek Dome, Long Valley Caldera.(Public domain.)

Obsidian, Glass Creek, and Deadman Creek domes all erupted in 1350 CE (Millar, et al. 2006). The vents from which the domes erupted are all aligned to the north-south. Based on the north-south alignment of the vents, the magma that fed these eruptions was most likely brought to surface along a dike (Miller, 1985). A dike is a long crack that is filled with magma - think of it as a curtain made of magma.

The volcanic activity at the three vents began with initial phreatic (steam) explosions. Phreatic explosions are caused by the heating of ground water and the intruding magma dike was most likely the heat source. The heated water expanded and exploded, blowing steam and rocks into the air. The best examples of explosion craters in the area are the Inyo Craters on the summit and south flank of Deer Mountain. Explosion craters from other phreatic eruptions can be seen next to Obsidian Dome and between Obsidian Dome and Glass Creek Dome. The phreaticexplosions were followed by pyroclastic eruptions (explosive eruptions of magma). Pyroclastic eruptions require a significant gas content to propel the magma into the air where it cools forming ash, lapilli, and pumice (<2mm to 64 mm). Once there is no longer enough gas to propel the magma into the air, the eruptions often conclude with passive extrusion of lava (dome growth). After extrusion of the Obsidian, Glass Creek, and Deadman Creek domes, the Inyo Craters were formed by another series of phreatic (steam) explosions (see the Inyo Craters field guide page for more information).

The locations of the domes and craters show the dike crossing the topographic caldera boundary. ObsidianDome erupted outside the caldera, Glass Creek Dome erupted just north of the caldera wall and flowed down the wall into the caldera, and Deadman Creek Dome erupted within the topographic margin of the caldera. In addition to crossing the topographic caldera boundary, Deadman Creek Dome and Glass Creek Dome contain some of the residual Long Valley Caldera magma, whereas Obsidian Dome does not.

Geologist examining highly vesciculated obsidian in the Obsidian Do...
Geologist examining highly vesciculated obsidian in the Obsidian Dome squeeze-up.(Public domain.)

Did You Know?

The eruption date for the Deadman Creek, Glass Creek, and Obsidian domes is one of the best constrained prehistoric eruption dates in the world. Using tree rings, Millar and others, found that the eruptions occurred in late summer 1350 CE.

Similar Composition Rocks

Follow the trail up to the summit of Obsidian Domealong the south side to view two kinds of rock that have similar compositions but look different. Theobsidian blocks along the southern edge of Obsidian Dome were erupted from a vent 1 km (0.6 mi) to the north. Views from the first trail switchback include Glass Creek Dome to the left and an explosion crater down to the left. Near the summit is a section of obsidian that stands out because it is much darker in color. The dark colored rock is the same chemical composition as the surrounding obsidian but contains many more vesicles (bubbles). This type of feature is commonly referred to as a squeeze-up because gas-rich magma from the interior of the flow is squeezed up to the surface. As the material reaches the surface the gases expand forming bubbles in the cooling rocks. Obsidian from Obsidian Dome is not a good source for making arrowheads due to the vesicles (bubbles) and minerals, but there are good examples of flow banding (look for stripes in the obsidian).


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: Obsidian Dome

Quadrangle: Mammoth Mountain, California 7.5 minute topographic quadrangle
Coordinates: about 37°44.810' N, 119°01.305' W
Approximate Elevation at Base of Dome: 8,200 ft (2500 m)

Directions to Obsidian Dome:

Obsidian Dome is accessible via a 2.7 mile dirt road. This stop also provides access to Glass Creek Dome (a mixture of obsidian and pumice), Obsidian Dome (large flow with different obsidian textures), and explosion craters (large holes in the ground formed by a blast of steam, generally generated by gases from rising magma). Paths lead to the tops of the domes providing access to unique views of the domes. If you do not want to drive the full 2.7 miles, you can stop and view a part of Obsidian flow from a parking area on the left side of the road. At this stop you can see large chunks of obsidian with vesicles (bubbles) in them from the escaping gas and small minerals.

If you continue to drive to the parking area between Obsidian Flow and Glass Creek Flow you can gain an appreciation for the size of the flow since the length is about one mile, the width is a little less than a mile, and the height is about 400 feet at the summit.

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 US-395 towards Lee Vining and Mono Lake. Go 11.0 miles
2. Turn left onto Obsidian Dome road Go 0.3 miles
3. Continue onto Glass Flow Road. 0.6 miles
4. Continue left onto Obsidian Dome Road at Obsidian Flow and Hartley Springs Campground turnoff. From here you will drive around the dome to its south side. 0.6 miles
5. Slight left at 2S10 0.7 miles
6. Continue onto Glass Creek Rd 0.3 miles
7. At the fork in the road, stay to the left to park near Obsidian Flow among the large Jeffrey pines (about 37°44.990' N, 119°01.292' W). Walk up Obsidian Flow bulldozed trail.