Obsidian Dome is a large volcanic formation in Inyo National Forest, about 11 miles north of Mammoth Lakes in eastern California. It belongs to the Mono–Inyo volcanic chain and rises from a base elevation of about 8,200 feet to a summit of approximately 8,556 feet. The dome is roughly one mile long, slightly less than one mile wide, and about 400 feet high. Tree-ring evidence indicates that Obsidian Dome, Glass Creek Dome, and Deadman Creek Dome erupted during the late summer of 1350 CE, making this one of the world’s most precisely dated prehistoric eruptions (U.S. Geological Survey [USGS], 2023).
Travelers can reach the area from Mammoth Lakes by taking U.S. Highway 395 north and turning onto Glass Flow Road, also called Obsidian Dome Road. The final approach includes approximately 2.7 miles of unpaved road, and vehicle access may be limited by snow. The observation site is open 24 hours, charges no entrance fee, and has no potable water, so visitors should bring water and check current road and weather conditions before leaving. Short, rough paths lead onto the dome, but the loose volcanic blocks can be sharp and unstable; sturdy shoes, sun protection, and careful footing are important (U.S. Forest Service [USFS], 2025).
Geologically, Obsidian Dome formed when rising magma heated groundwater and caused steam-driven explosions, followed by explosive releases of ash and pumice. As the eruption lost gas, thick, silica-rich rhyolite lava slowly pushed onto the surface and cooled into obsidian, a natural volcanic glass. Visitors can see flow banding, small mineral crystals, gas bubbles called vesicles, and a dark “squeeze-up” where bubbly lava was forced through the cooling surface. Recent regional research used earthquake vibrations recorded along a 100-kilometer fiber-optic cable to create detailed images beneath Long Valley and the Mono–Inyo chain. The results revealed shallow hydrothermal structures but no clear conduit connecting them to the deeper magma chamber (Biondi et al., 2023). In 2026, USGS scientists also conducted the most extensive geodetic survey of Long Valley since the late 1990s to study recent ground subsidence and underground fluid movement (Crozier, 2026).
Although the black and gray lava appears barren, the surrounding land supports an open forest and shrub community adapted to cold winters, dry summers, volcanic soil, and high elevation. Jeffrey pines grow around the parking and trail areas, while manzanita, antelope bitterbrush, grasses, and hardy wildflowers occupy places where enough soil has collected. Jeffrey-pine habitat provides food or shelter for yellow-pine chipmunks, golden-mantled ground squirrels, mule deer, Steller’s jays, mountain chickadees, nuthatches, and other birds and small mammals; black bears also live in the broader Eastern Sierra region (USFS, n.d.). Visitors should observe wildlife from a distance, keep dogs leashed, carry out all trash, and leave volcanic rocks in place so the site remains intact for science and future travelers.
References:
Biondi, E., Zhu, W., Li, J., Williams, E. F., & Zhan, Z. (2023). An upper-crust lid over the Long Valley magma chamber. Science Advances, 9(42), eadi9878.
Crozier, J. A. (2026, August 4). Peering beneath the surface of Long Valley Caldera with geodetic surveys. U.S. Geological Survey.
U.S. Forest Service. (2025). Obsidian Dome Observation Site.
U.S. Forest Service. (n.d.). Pinus jeffreyi, Jeffrey pine.
U.S. Geological Survey. (2023). Long Valley Caldera field guide: Obsidian Dome.
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