U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS

Since about 1760, when written records began 

to be kept for Alaska, more than 260 eruptions 

have been reported from 41 volcanoes there. 

Forty of these are found on a volcanic chain that 

stretches more than 1,500 miles along the Aleutian Islands, Alaska Peninsula, and Cook Inlet.

In 1988, the Alaska Volcano Observatory (AVO)—a cooperative effort of the U.S. 

Geological Survey (USGS), the University of 

Alaska Fairbanks Geophysical Institute, and the 

Alaska Division of Geological and Geophysical 

Surveys—was founded to monitor and minimize 

the effects of volcanic eruptions in Alaska. 

Of the four dominant Cook Inlet volcanoes, 

only Augustine Volcano was monitored in 1988 

with an adequate real-time seismic network necessary for detecting the small earthquakes that 

typically occur before eruptions. Located 180 

miles southwest of populous Anchorage, Augustine is the most active and youngest of the Cook 

Inlet volcanoes. By the end of 1989, however, 

AVO scientists had installed new seismic stations 

and upgraded monitoring equipment on Augustine, Redoubt, and Mount Spurr Volcanoes.

These volcano-monitoring networks proved 

essential for AVO to forecast eruptions of 

Redoubt in 1989-90 and Mount Spurr in 1992. A 

seismic network installed at Iliamna in 1992 allowed scientists to detect the rise of molten rock 

(magma) into the volcano 4 years later, an event 

that could have been a precursor to an eruption.

The explosive onset of the eruption of 

Redoubt Volcano on December 14, 1989, was 

preceded by only about 23 hours of intense 

swarms of small earthquakes. Because the 

volcano was monitored, this was suffi cient time 

for AVO to warn offi cials and the public of the 

volcano’s restlessness and for AVO and Federal, 

State, and local government agencies to activate 

emergency plans before the eruption began.

A larger explosive event on December 15 sent 

a column of volcanic ash (rock fragments smaller 

than 1/12 inch) more than 40,000 feet above sea 

level. The ash was blown northward by strong 

winds, and the resulting eruption cloud nearly 

brought down a 747 jetliner carrying 244 people.

En route from Amsterdam to Anchorage, 

the plane unknowingly descended into the ash 

cloud and quickly lost power in all four engines 

as gritty ash and sulfurous gas fi lled the aircraft. 

Gliding powerless for more than four frightening minutes, the plane fell nearly 12,000 feet to 

within a few thousand feet of the ground. DisasIn 1999, Shishaldin Volcano on Unimak Island erupted explosively, releasing small columns of volcanic ash and 

gas (photo at left). This activity was punctuated by larger explosive events producing ash clouds that could be 

identifi ed on satellite images (right). These ash clouds drifted for hundreds of miles, crossing heavily traveled 

international air routes. Completion of a seismic network on Unimak in 1998, as well as an ongoing satellite 

monitoring program, enabled Alaska Volcano Observatory scientists to issue timely notifi cations of the unrest 

and explosive activity at the volcano. (USGS photo by Robert McGimsey, April 1999.) 

volcanoes have signifi cantly affected 

air traffi c over the North Pacifi c, as 

well as Alaska’s oil, power, and fi shing industries and local communities. 

Since its founding in the late 1980s, the 

Alaska Volcano Observatory (AVO) has 

installed new monitoring networks and 

used satellite data to track activity at 

Alaska’s volcanoes, providing timely 

warnings and monitoring of frequent 

eruptions to the aviation industry and 

the general public. To minimize impacts 

from future eruptions, scientists at AVO 

continue to assess volcano hazards 

and to expand monitoring networks.

The Alaska Volcano Observatory—

Expanded Monitoring of Volcanoes Yields Results

Recent explosive eruptions at some

 of Alaska’s 41 historically active 

This photo shows Kanaga Volcano, located 1,215 miles southwest of Anchorage in the western Aleutian Islands, 

during its 1993-95 eruption. At that time, the nearest volcano monitored with a real-time seismic network 

adequate to detect the small earthquakes that typically precede an eruption was Katmai, about 900 miles 

away. Since 1996, in order to detect volcanic unrest and eruptions along the Aleutian volcanic chain, scientists 

of the Alaska Volcano Observatory have signifi cantly increased the number of volcanoes they monitor and 

study. A primary goal of the expansion is to provide accurate information about eruptions that could generate 

clouds of volcanic ash, a major threat to fl ying jet aircraft. By early 2004, Kanaga was among 27 volcanoes in 

Alaska monitored with ground-based seismic networks. All Alaskan volcanoes are monitored by satellite, using 

remote-sensing methods. (Copyrighted photo courtesy of Joe Meehan.)

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2004–3084 

2004
ter was averted when the engines were restarted 

and the jetliner landed safely in Anchorage. The 

747 encountered the eruption cloud about 150 

miles downwind from Redoubt, 90 minutes after 

the strong explosive event.

To help prevent such incidents, AVO scientists have worked closely with Federal agencies—including the Federal Aviation Administration (FAA), National Weather Service (NWS), 

National Oceanic and Atmospheric Administration (NOAA) Anchorage Volcanic Ash Advisory 

Center, and Department of Defense—as well as 

the Kamchatkan Volcanic Eruption Response 

Team (a joint Russian-U.S. effort) and the aviation industry, to improve the fl ow of information about volcanic activity and eruption clouds 

in the North Pacifi c to airline dispatchers and 

pilots. This information is crucial for preventing 

other aircraft from accidentally encountering 

eruption clouds.

Expanded Monitoring Effort

The 1989-90 eruption of Redoubt Volcano 

and its adverse effects on aviation ushered in 

a new era of hazard assessment and volcano 

monitoring in Alaska. With an average of one 

to two eruptions a year in Alaska and cargo 

and passenger fl ights steadily increasing 

throughout the 1990s across the North Pacifi c, 

the need for improved monitoring capabilities 

farther out along the Aleutian Island chain 

became paramount.

In 1996, with Congressional funding provided through the FAA, AVO scientists began 

an aggressive effort to monitor and assess 

hazards at volcanoes on the Alaska Peninsula 

and along the distant Aleutian Islands, as well 

as east of Anchorage at Mount Wrangell. By 

the fall of 2003, AVO was operating more than 

150 seismic stations in 15 networks capable of 

detecting earthquake activity at 27 volcanoes. 

AVO has also increased the corresponding data acquisition, storage, and analysis 

capability needed to track volcanic activity at 

Alaska’s volcanoes.

Seismic Networks

A network of six to eight seismometers 

around a volcano is required to detect and 

accurately locate earthquake activity typically 

associated with magma rising into a volcano. 

Many eruptions are preceded by very small 

earthquakes (magnitude less than 1) or low-level 

tremor caused by the rising magma. Two or 

three seismometers are insuffi cient for accurately locating these events. Also, some redundancy 

is needed, particularly in Alaska, in case a 

few stations fail due to extreme weather or are 

destroyed by bears or eruptions.

Satellite Monitoring

AVO also uses remote-sensing data from 

several meteorological satellites to conduct daily 

monitoring and analysis of volcanic activity in 

the North Pacifi c region. Satellite images are routinely analyzed twice a day to identify signs of 

eruptions, such as volcanic ash clouds or thermal 

anomalies or “hot spots,” which may indicate 

erupting lava or hot volcanic-gas emissions. During a period of unrest at one or more volcanoes, 

satellite data are analyzed more frequently, 

within minutes of when they are received.

Such regular analysis of satellite images has 

proven effective in detecting potentially hazardous activity at volcanoes in Alaska and Russia 

that are not seismically monitored. For example, 

AVO scientists detected an explosive eruption 

from Cleveland Volcano on February 19, 2001, 

during a routine daily analysis of satellite-generated images. The volcano was not monitored 

with a seismic network, but around-the-clock 

analysis of satellite data allowed AVO scientists 

to quickly provide accurate information of subsequent ash clouds from explosive eruptions.

Assessing hazards

Just as important as knowing when a volcano 

is likely to erupt based on monitoring studies is 

knowing the type and size of eruptions that can 

be expected from a volcano in the future, including the areas likely to be directly affected. Such 

information is determined in part by studying 

what has happened in the past by mapping the 

geology around volcanoes. These data are then 

incorporated into volcano-hazard assessment 

reports. The expanded monitoring program at 

AVO since 1996 has included an aggressive 

effort to map and understand the eruptive history 

of all active Alaskan volcanoes.

AVO’s volcano-monitoring capability 

has signifi cantly expanded since its inception in 1988. With each new seismic network, 

volcano-hazard assessment, eruption, or period 

of volcano unrest, scientists are increasing their 

ability to provide timely, accurate information 

about Alaska’s active volcanoes.

Geologists examine rock layers in Aniakchak 

Caldera to decipher the volcano’s eruptive history 

and determine its future potential hazards. The 

caldera, a large volcanic depression, formed during 

a catastrophic explosive eruption 3,500 years ago. 

These deposits within the caldera were formed by 

subsequent smaller eruptions.

Scientists of the Alaska Volcano Observatory have increased the number of volcanoes monitored with 

seismic networks from 4 in 1995 to 27 in early 2004. Griggs and Snowy Volcanoes near Katmai, although 

not historically active and therefore not shown, are monitored and included in the 27. New networks will 

be installed in the next few years at volcanoes in the far western Aleutian Islands. Data and images from 

meteorological satellites are also used daily to identify hot areas on volcanoes in Alaska and the Russian Far 

East and eruption clouds resulting from explosive activity.

For more information contact:

Alaska Volcano Observatory

Tel. (907) 786-7497, Fax (907) 786-7425

4200 University Drive

Anchorage, AK 99508

http://www.avo.alaska.edu/

or

USGS Volcano Hazards Program

http://volcanoes.usgs.gov/

See also other USGS Alaskan volcano Fact Sheets

http://volcanoes.usgs.gov/Products/sproducts.html

This Fact Sheet and any updates to it are available online at

http://pubs.usgs.gov/fs/2004/3084/

COOPERATING ORGANIZATIONS

Alaska Division of Geological and Geophysical Surveys

Geophysical Institute University of Alaska Fairbanks

Department of Defense

Federal Aviation Administration

Kamchatka Volcanic Eruption Response Team

National Oceanic and Atmospheric Administration

National Weather Service/Anchorage Volcanic Ash 

Advisory Center

U.S. Coast Guard

Steven R. Brantley, Robert G. McGimsey, and 

Christina A. Neal

Edited by James W. Hendley II and Peter Stauffer

Graphic design by Susan Mayfi eld and Sara Boore

Banner design by Bobbie Myers

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