Frequently Asked Questions About Volcanic Hazards

A restless volcano endangers any nearby residents with clouds of ash, falling blocks of rock, pyroclastic flows or ash hurricanes, lava flows, and floods of debris or lahars. These hazards are typical of snow- and ice-covered stratovolcanoes like those in the Pacific Northwest and Alaska. Since 1980, volcanic activity has killed more than 29,000 people worldwide. Most of the deaths were caused by lahars and pyroclastic flows; a few hundred people were killed by ash falls, which collapsed the roofs of buildings.

Q: What kinds of hazards are associated with volcanic eruptions?

A: Debris flows, or lahars, are slurries of muddy debris and water caused by mixing of solid debris with water, melted snow, or ice. Lahars destroyed houses, bridges, and logging trucks during the May 1980 eruption of Mount St. Helens, and have inundated other valleys around Cascade volcanoes during prehistoric eruptions. Lahars at Nevado del Ruiz volcano, Colombia, in 1985, killed more than 23,000 people. At Mount Rainier, lahars have also been produced by major landslides that apparently were neither triggered nor accompanied by eruptive activity. Lahars can travel many tens of miles in a period of hours, destroying everything in their paths.

Tephra (ash and coarser debris) is composed of fragments of magma or rock blown apart by gas expansion. Tephra can cause roofs to collapse, endanger people with respiratory problems, and damage machinery. Tephra can clog machinery, severely damage aircraft, cause respiratory problems, and short out power lines up to hundreds of miles downwind of eruptions. Explosions may also throw large rocks up to a few miles. Falling blocks killed people at Galeras Volcano in Colombia in 1992, and at Mount Etna, Italy, in 1979.

Pyroclastic surges and flows are hot, turbulent clouds of tephra (known as surges), or dense, turbulent mixtures of tephra and gas (known as flows). Pyroclastic flows and surges can travel more than a hundred miles per hour and incinerate or crush most objects in their path. Though most extend only a few miles, a pyroclastic surge at Mount St. Helens in 1980 extended 18 miles (28 km) and killed 57 people. Pyroclastic surges at El Chichón volcano in Mexico in 1982 killed 2000 people, and pyroclastic flows at Mount Unzen, Japan, in June, 1991, killed 43 people. Speeding vehicles cannot outrun a pyroclastic flow or surge.

Lava flows erupted at explosive stratovolcanoes like those in the Pacific Northwest and Alaska are typically slow-moving, thick, viscous flows. Kilauea volcano on the Island of Hawaii has produced thin, fluid lava flows throughout its history, and almost continuously since 1983. Lava flows destroyed a visitor center at Kilauea in 1989 and overran the village of Kalapana on the volcano's southeast flank in 1991.

Q: Can volcanoes be dangerous even when they don't erupt?

A: Definitely. Many stratovolcanoes have a plumbing system of hot acid water that progressively breaks down hard rock to soft, clay-rich material. The volcano is gradually weakened, and large parts may suddenly fail. Resulting water-rich landslides are especially dangerous because they can occur without any volcanic or seismic warning.

The risk of mudflows formed this way is especially high along rivers downstream from Mount Rainier, because of the large population on floodplains, the huge weakened edifice of the volcano, and a long history of large flows that occurred when the volcano was otherwise dormant.

Q: How can residents who live near volcanoes prepare for future eruptions?

A: Residents can obtain copies of USGS volcano-hazard reports to determine whether they live or work in areas at risk from volcanic activity. Everyone should plan how they and their family will respond to a natural disaster, including unrest or eruptive activity at nearby volcanoes. Preparation might include knowing where to go when family members are separated, where to go for emergency housing, what emergency supplies to keep on hand, and how to be self sufficient for several days, as recommended by local emergency management agencies. Residents who live within 100 miles of a volcano should also find out what their local officials are doing to prepare their community for the possibility of renewed volcanic activity. Lastly, enjoy the scenic, recreational, and inspirational benefits of living near an active volcano!

Q: How does the USGS provide eruption warnings?

A: The USGS volcano observatories post updates about volcanic activity on our web site. Information about our alert system is available online.

If activity at a volcano increases, we provide hazards-zone maps and other information about the frequency of eruptions and extent of specific hazards to public officials, land-use planners, and emergency-management agencies. The assessments we've already completed are available online in our hazard assessments section. The USGS works with the Federal Aviation Administration and National Weather Service to provide airline pilots with timely information about hazardous volcanic ash clouds.

When communities are at risk, scientists give hazards information directly to public officials to help them make decisions about land-use or evacuations. Unlike what is often portrayed in movies, warnings are delivered only after a thorough analysis of all existing information and careful consultation among members of the USGS response team. Our goal is always to keep natural processes from becoming natural disasters.

Q: How many active volcanoes are there in the United States?

A: There are about 169 volcanoes in the United States that scientists consider active. Most of these are located in Alaska, where eruptions occur virtually every year. Others are located throughout the west and in Hawaii (see our Volcano Activity Map for their locations). Kilauea volcano in Hawaii is one of the most active volcanoes on Earth. It has been erupting almost continuously since 1983!

Q: How many active volcanoes are there on Earth?

A: There are about 1500 potentially active volcanoes worldwide, aside from the continuous belt of volcanoes on the ocean floor. About 500 of these have erupted in historical time. Many of these are located along the Pacific Rim in what is known as the "Ring of Fire." In the U.S., volcanoes in the Cascade Range and Alaska (Aleutian volcanic chain) are part of the Ring, while Hawaiian volcanoes form over a "hot spot" near the center of the Ring.