U.S. Geological Survey Identifier

Title: Long Valley Observatory

Volcano-Hazard Zones for Small- to Moderate-sized Eruptions
in the Long Valley -- Mono Lake Area, California

Each episode of volcanic activity in the past 5,000 years from along the Mono-Inyo Craters volcanic chain has erupted less than 1 km3 of magma. Based on the known aerial extent of the rock deposits formed by these small- to moderate-sized eruptions and experience gained from historical eruptions of similar magnitude, scientists have identified areas that are likely to be affected by similar activity in the future. The outer boundaries of these hazard zones (see anchors below) are generalized and enclose minimum areas that would be endangered by small- to moderate-sized explosive eruptions.

Small burst of incandescent lava fragments erupted by cinder-and-spatter cone within Veniaminoff caldera, Alaska
Small burst of lava fragments, Veniaminoff, Alaska

The size of the next eruption in the Long Valley area will most likely be similar to the small to moderate sized eruptions that have occurred in the past 5, 000 years. The hazard zones below were prepared based on these recent eruptions (explosive and non-explosive activity) that ejected less than 1 km3 of material from one or more vents along the Mono-Inyo Craters volcanic chain. Infrequent but much larger eruptions have occurred from other vents in the caldera in the past few hundred thousand years. The most recent activity in the area, however, suggests that an eruption of up to 1 km3 of magma from a vent or vents along the Mono-Inyo chain is the most likely eruption size to use for emergency-response planning.

Reducing the risk to life and property from future volcanic activity is best achieved when the most dangerous areas are avoided during eruptions. When emergency plans are made before the next eruption then people can determine what steps to take, including the evacuation of some areas, if and when scientists of the U.S. Geological Survey consider that an eruption is imminent.
For key elements of a volcano emergency plan, see The Development of Volcanic Emergency Plans prepared by an international group of scientists and emergency-management professionals. USGS scientists monitoring the Long Valley and Mono Lake area use a color-coded alert scheme to notify people of the volcano status and likelihood of an eruption.

Volcano-Hazard Zones

Tephra -- volcanic ash and pumice fallout

Ashfall over former Clark Airbase, Philippines
Ashfall over former Clark
Airbase, Philippines

During a typical explosive eruption of a Mono-Inyo vent, tephra (volcanic ash and larger rock fragments) may accumulate near the vent to a thickness of 10 m. Areas downwind of an explosive eruption could be covered with a layer of volcanic ash and pumice more than 20 cm (8 in) thick at a distance of 35 km (22 mi) and 5 cm (2 in.) at 85 km (53 mi).

It is not possible now to predict wind direction or directions that will exist at the time of a future explosive eruption. However, records suggest that on an annual basis winds in the area blow toward an east or northeasterly sector more than 50 percent of the time, and toward some easterly direction more than 80 percent of the time. In general, the grain size and thickness of ash accumulations gradually decrease with increasing distance from a vent.

Tephra hazard-zone maps and illustrations


Pyroclastic flows and surges

Small pyroclastic flow from Soufriere Hills volcano, Montserrat
Pyroclastic flow, Soufriere
Hills volcano, Montserrat

Areas as far as 15 km (10 mi) from an explosive eruption could be swept by hot, fast-moving pyroclastic flows and surges. This zone is based on the distance that pyroclastic flows are known to have traveled in the past few thousand years. The actual areas covered by pyroclastic flows or surges during a future explosive eruption depends primarily on the location of the vent(s), the surrounding topography, and volume of magma erupted.

Pyroclastic flows generated from vents on Mammoth Mountain could travel farther than 15 km (10 mi) because of the extra momentum that would be gained as the flows descend hundreds of meters down Mammoth's steep flanks and move across low-lying areas.

Hazard-zone maps and illustrations


Lava flows, domes, and dome collapses

Channelized `a`a lava flow, Mauna Loa Volcano
Fluid basalt lava flow
Mauna Loa, Hawai`i
1984 eruption

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).

Wilson Butte lava flow, California
Viscous rhyolite lava flow
Wilson Butte dome,
Long Valley, California

Hazard-zone maps and illustrations


Volcanic eruption clouds

Eruption column from the eruption of Galunggung Volcano, Indonesia
Eruption column, Galunggung
Volcano, Indonesia

Explosive eruptions inject large amounts of volcanic ash and corrosive gases high into the atmosphere, especially into cruising altitudes used by commercial jet airplanes. Volcanic eruption clouds may drift for hundreds to thousands of kilometers from an erupting volcano, and can contaminate heavily used aeronautical routes over wide areas. Volcanic ash can damage aircraft flying surfaces and electronics, and cause engine failure. Eruption clouds also may cause flights to be diverted, delayed, or canceled.

Since winds above the Long Valley area blow toward an easterly direction more than 80 percent of the time, air routes east of the volcano are likely to be contaminated with volcanic ash during a future eruption. For example, air routes within a few hundred miles east of Long Valley that accommodate jets either approaching and departing San Francisco or flying in a north-south direction toward or away from Las Vegas, Nevada.

Maps and illustrations


Case Example: The Inyo eruptions about 600 years ago

Back to volcano hazard introduction



Miller, C.D., Mullineaux, D.R., Crandell, D.R., and Bailey, R.A., 1982, Potential hazards from future volcanic eruptions in the Long Valley-Mono Lake area, East-Central California and Southwest Nevada -- a preliminary assessment: U.S. Geological Survey Circular 877, 10 p.

Miller, C.D., 1989, Potential hazards from future volcanic eruptions in California: U.S. Geological Survey Bulletin 1847, 17 p.


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U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA
URL http://lvo.wr.usgs.gov/zones/index.html
Contact: Long Valley Web Team
Last modification: 20 August 1999 (SRB)