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Lava Flows, Domes and Dome Collapses

Molten rock moving across the ground as a lava flow seldom threatens human life directly because its slow rate of advance allows people to move out of the way. Once lava erupts from a vent, its potential pathway is predictable because lava moves down hill according to the local topography.

Future lava flows in the Long Valley area will be either relatively fluid (basalt lava) or viscous (dacite or rhyolite lava). Basalt lava typically forms thin flows that may reach distances of more than 50 km (30 mi) from their vent. Dacite and rhyolite lavas typically produce short, thick flows that seldom move as far as 5 km (3 mi) from the vent. These short flows often build mound-shaped features called lava domes.

The principal hazard associated with formation of a dome results from pyroclastic flows, and from rock fragments thrown out by explosions, which may reach 5 to 10 km from the dome). The sides of a growing lava dome are steep and unstable, and sometimes collapse to form pyroclastic flows that can move outward at least 5 km from the dome. Pyroclastic flows may travel progressively farther as a dome grows higher or if the dome is located well above surrounding areas (for example, on Mammoth Mountain, which was built by a series of overlapping lava domes).