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Frequently Asked Questions

Q. Which Cascade volcano will erupt next?

A. No one knows for sure, but seven of them have been active in just the past 300 years: Mount St. Helens (1800-57, 1980-86 and 2004-08), Lassen Peak (1914-17), Mount Baker (1843), Glacier Peak (1700s), Mount Rainier (1894-95), Mount Hood (1780-93), and Mount Shasta (1786). Any of these could be the next to erupt, though the odds are highest at Mount St. Helens.

Q. How often do the Cascade volcanoes erupt?

A. The Cascades are an active volcanic range. During the last 4,000 years, a Cascade Range volcano has erupted a few times per century on average. Eruptions were ongoing at Mount St. Helens and Lassen Peak for about ten-percent of the twentieth century.

Q. Why are the Cascades here?

A. The Cascade chain is 3000 km (1600 mi) long, stretching from Mount Meager in British Columbia to Lassen Peak in northern California. The roughly linear string of volcanoes in the Cascade Range is not an accident of nature. Volcanoes stand directly above a subducting oceanic plate (80-120 km or 50-75 mi deep) where the conditions are right to form magma.

Q. How does magma form?

A. Magma forms in the mantle, where the subducted oceanic plate has sunk to great depths. Temperatures and pressures at such depths are sufficiently high to cause water within the oceanic plate to sweat into the mantle rock. The addition of water to hot mantle rocks causes rock to melt and form magma. This magma begins to rise because it is less dense than surrounding solid mantle rock.

Q. What kinds of unusual activity might be noticed before an eruption?

A. Common symptoms of volcanic unrest include an increase in the frequency or intensity of earthquakes beneath a volcano; the occurrence of volcanic tremor; swelling, subsidence, or cracking of the ground; increased steam emission or small steam explosions; melting snow or ice; changes in existing fumaroles or hot springs, or the appearance of new ones; and increased discharge of magmatic gases. Volcanologists assess the significance of volcanic unrest partly by monitoring the pace and intensity of such activity.

Q. What starts an eruption?

A. Gases, such as water vapor, CO2, SO2 and other rarer gases, are the driving forces that power explosive volcanic eruptions. However, gases are not the only players in a volcanic eruption. The size and explosiveness of an eruption are also controlled by the amount of magma in the magma chamber, the magma's chemical composition, and the pressure change in the narrow conduit that leads to Earth's surface.

Q. What hazards are associated with volcanic eruptions?

A. Before an eruption begins, rising magma opens cracks (fractures) in rocks beneath the volcano, commonly causing earthquakes, deformation (rising or falling) of the land surface and release of gases.
 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 at least once 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 28 km (18 mi) and caused 57 fatalities. Pyroclastic surges at El Chichón volcano in Mexico in 1982 caused 2000 fatalities, and as recently as 2010, more than 380 people lost their lives to pyroclastic flows and surges near Merapi volcano in Indonesia. 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 that typically threaten only people and things near the flow front. 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. On a small scale, rock fall and debris flows threaten valley floors at or near the volcano. On a larger scale, 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 Cascade stratovolcanoes because of the large populations on floodplains downstream. Such landslides have occurred in the Cascades at Mount Baker, Mount Rainer, and Mount Adams. In 2010, a large (about 50 million cubic meters) landslide at Mount Meager, the northern most Cascade volcano (in Canada), traveled 10 km (6mi) down valley in a matter of minutes. Mount Meager is in a remote area, but a landslide of this magnitude from a US Cascade volcano could present significant risks to nearby populace.

Q. How does the CVO monitor Cascade Range volcanoes?

A. In cooperation with universities and state agencies, the USGS-CVO monitors seismic activity, ground deformation, volcanic gases, thermal emissions, and changes in water levels and chemistry. When unusual activity is detected, a response team may do more ground surveys and install more instruments, if possible, to better determine if an eruption is likely.

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. When activity at a volcano increases, we provide more specific information about the eruption potential and possible effects 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 can I get updates and notifications?

A. The Volcano Notification Service (VNS) is a free service that sends you notification emails about volcanic activity happening at US monitored volcanoes. You can customize the VNS to only deliver notifications for certain volcanoes, or range of volcanoes, as well as choose the separate notification types you want to receive. Notifications are issued by the five U.S. Volcano Observatories: Alaska (AVO), Cascades (CVO), Hawaiian (HVO), California (CalVO), and Yellowstone (YVO). You can also follow USGS Volcano Hazards Program activities on the USGS Facebook page and register for updates, notification of new publications, and information about natural events. Information is also available on the CVO home page HOT STUFF current events.

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. See additional details on the CVO Preparedness webpage. Lastly, enjoy the scenic, recreational, and inspirational benefits of living near an active volcano!

Q. How many volcanoes exist in the United States and its territories?

A. The U.S. and its territories have about 170 volcanoes that have been active during the past 10,000 years, and most could erupt again in the future. In the past 500 years, 80 U.S. volcanoes have erupted one or more times.

Q. How many potentially 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.

Q. How many people live close to volcanoes?

A. According to 2009 World Bank population data, it is estimated that at least 600 million people live in areas potentially affected by volcanic hazards. [Source: Auker, M.R., Stephen, R., Sparks, J., Siebert, L., Crosweller, H.S., Ewert, J., 2013, A statistical analysis of the global historical volcanic fatalities record: Journal of Applied Volcanology 2013, 2:2]

Q. How many people have died as a result of volcanic eruptions?

A. Between 1600 to 2010 AD, 278,880 fatalities are recorded from 533 fatal incidents at 198 volcanoes across 38 countries. The fatality count is dominated by a handful of disasters, at Tambora (1815), Krakatau (1883), Pelée (1902), Nevado del Ruiz (1985), Unzen (1792), Grímsvötn (1783), Santa Maria (1902, 1929), Kilauea (1790), Kelut (1919), Tungurahua (1640), Galunggung (1822), and Vesuvius (1631). The number and probability of fatalities are strongly influenced by volcano explosivity and population density around volcanoes. The occurrence and extent of lahars (mudflows) and pyroclastic density currents have caused 50% of fatalities. With the largest disasters removed, over 90% of fatalities occurred between 5 km and 30 km from volcanoes, though the most devastating eruptions impacted far beyond these distances. [Source: Auker, M.R., Stephen, R., Sparks, J., Siebert, L., Crosweller, H.S., Ewert, J., 2013, A statistical analysis of the global historical volcanic fatalities record: Journal of Applied Volcanology 2013, 2:2]