Alaska’s Active Volcanoes: Federal Role in Research, Monitoring, and Warning
Congressional research reportMay 8, 2025
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Alaska’s Active Volcanoes: Federal Role in
Research, Monitoring, and Warning
May 8, 2025
Congressional Research Service
https://crsreports.congress.gov
R48529
SUMMARY
Alaska’s Active Volcanoes: Federal Role in
Research, Monitoring, and Warning
Alaska has about 140 active volcanoes, and the U.S. Geological Survey (USGS) considers 86 to
be threatening to people and property. Most of Alaska’s active volcanoes are stratovolcanoes,
capable of explosive and effusive eruptions potentially impacting local, regional, and global
communities and aviation. A primary risk is ash, which may harm human health and the
environment, damage property, damage aircraft, and alter flight operations.
R48529
May 8, 2025
Linda R. Rowan
Analyst in Natural
Resources and Earth
Sciences
On October 16, 2024, increasing volcanic activity at Mount Spurr in Alaska caused the USGS to
raise Mount Spurr’s volcano and aviation alert levels, warning that an eruption may be possible and increasing concern about
risks to people, property, and aviation. Mount Spurr is about 81 miles from Anchorage, and past eruptions have caused
damage. Warning about hazardous volcanic activity relies on various federal, state, and local research, monitoring,
forecasting, and alerting collaborations. Alaska’s volcanoes are remote and hard to access, and they exist in harsh
environments, making research and monitoring difficult. The USGS considers Mount Spurr to be one of 18 very-high-threat
to high-threat Alaska volcanoes that are not monitored at a level commensurate with their threats; under-monitoring can
hamper timely alerts that would help reduce volcanic risks.
Congress in 2019 and 2022 passed legislation authorizing various federal roles to monitor, warn, and protect people from
harm caused by volcanic activity. Issues for the 119th Congress include whether this earlier legislation has been fully
implemented and what (if any) additional investments are warranted to enhance the efficacy and efficiency of the
international, federal, state, and local collaborations to reduce the threats to lives, property, and aviation in Alaska and to
aviation beyond Alaska (i.e., local, regional, national, and international air travel).
Legislation in 2019 and 2022
In 2019, Congress passed legislation that authorized a National Volcano Early Warning and Monitoring System (NVEWS;
§5001 of P.L. 116-9; 43 U.S.C. 31k) and authorized appropriations of $55 million, which expired in FY2023. The law
directed the USGS to establish NVEWS to monitor volcanoes, modernize and unify the monitoring systems of volcano
observatories in the United States, warn U.S. citizens of volcanic activity, and protect citizens from “undue and avoidable
harm.” In 2022, Congress passed legislation (§10501 of P.L. 117-263) that amended NVEWS to direct cooperation and
coordination between the USGS’s NVEWS and the National Oceanic and Atmospheric Administration’s (NOAA’s) Volcanic
Ash Advisory Centers (VAACs) to strengthen monitoring and warning of volcano hazards in the atmosphere.
Monitoring and Warning
The Alaska Volcano Observatory (AVO)—a partnership between the USGS, the Geophysical Institute of the University of
Alaska Fairbanks and the State of Alaska Division of Geological and Geophysical Surveys—studies, monitors, forecasts,
warns, and seeks to reduce risks from Alaska’s volcano hazards. As of May 2025, the USGS, with AVO serving a leading
role, is prioritizing additional monitoring of 18 very-high-threat to high-threat Alaska volcanoes, including Mount Spurr.
NOAA’s Anchorage Volcanic Ash Advisory Center (AVAAC), with the AVO, studies, monitors, forecasts, and warns about
Alaska’s volcanic ash hazards in the atmosphere. As of May 2025, NOAA is preparing an implementation plan for its
VAACs to cooperate with NVEWS and the USGS is collaborating with NOAA on multiple aspects of volcano hazards and
warnings.
Considerations for Congress
The 119th Congress may consider the future of the authorizations and appropriations for the activities that various federal
agencies, and in particular the USGS, perform to support the monitoring and warning of volcano hazards affecting the United
States, including Alaska’s volcano hazards. S. 1052, introduced on March 13, 2025, would reauthorize NVEWS, authorizing
appropriations for USGS of $75 million until FY2033, extending the period of authorization of sums necessary for NOAA to
carry out its NVEWS activities to FY2034 and amending NVEWS by adding “infrasound arrays, visible and infrared cameras
and advanced digital telemetry networks” to the emerging technologies the USGS should apply to modernize NVEWS.
Another measure, H.R. 3176, was introduced on May 5, 2025, to reauthorize NVEWS. Congress may assess its options by
evaluating the efficiency and effectiveness of NVEWS activities as part of the nation’s preparedness and efforts to reduce
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Alaska’s Active Volcanoes: Federal Role in Research, Monitoring, and Warning
volcano hazards’ impacts on lives, property, aviation, and economic activity. For example, Congress may evaluate the level
of monitoring and warning of very-high-threat to high-threat Alaska volcanoes and how AVAAC/AVO volcanic ash warning
products are used by the Federal Aviation Administration, the Department of Defense, and international aviation systems.
Congress also may wish to learn more about the USGS’s plans for meeting the objectives of NVEWS and about how AVO
and AVAAC are cooperating to advance NVEWS.
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Alaska’s Active Volcanoes: Federal Role in Research, Monitoring, and Warning
Contents
Introduction ..................................................................................................................................... 1
Primer on Alaska’s Active Volcanoes and Hazards ......................................................................... 2
Ashfall Risks and Largest Eruption in 20th Century.................................................................. 7
Ash Risks for Aviation and Alaska’s Large Aviation System ................................................... 11
The 1989-1990 Redoubt Eruption: Lessons Learned About Hazards, Monitoring, and
Warnings..................................................................................................................................... 12
Seismic Monitoring ................................................................................................................. 14
Pyroclastic Flows and Lahars.................................................................................................. 14
Volcanic Ash and Aviation ...................................................................................................... 14
Eruption Uncertainties ............................................................................................................ 15
National Volcano Early Warning and Monitoring System ............................................................ 16
Role of the Alaska Volcano Observatory ....................................................................................... 17
Research .................................................................................................................................. 18
Monitoring .............................................................................................................................. 18
Warnings ................................................................................................................................. 21
Role of Anchorage Volcanic Ash Advisory Center........................................................................ 26
Research .................................................................................................................................. 28
Models .............................................................................................................................. 28
Satellite Observations ....................................................................................................... 30
Ash Concentration and Aircraft Damage .......................................................................... 30
Monitoring .............................................................................................................................. 32
Warning ................................................................................................................................... 32
Other Volcano Warning Products in Use in Alaska ....................................................................... 33
Dissemination of Volcano Warning Products in Alaska ................................................................ 36
Congressional Considerations ....................................................................................................... 36
Figures
Figure 1. Threat Assessment of Some Active Volcanoes in Alaska................................................. 3
Figure 2. Detailed Maps of Some of the Highest Threat Volcanoes in Alaska ................................ 4
Figure 3. U.S. Subduction Zones Map and Subduction Zone Model .............................................. 5
Figure 4. Shishaldin Volcano Ash Cloud, Pyroclastic Flows, and Lahars ....................................... 6
Figure 5. Mount Cleveland Ash Plume ........................................................................................... 7
Figure 6. Extent of Ashfall From Some Major Alaskan Eruptions.................................................. 9
Figure 7. Volume of Major Alaskan and Other Eruptions ............................................................. 10
Figure 8. Satellite Image of Novarupta and Other Volcanoes in Katmai National Park ................ 11
Figure 9. Examples of Hazards from the 1989-1990 Redoubt Eruption ....................................... 13
Figure 10. Volcano Monitoring ..................................................................................................... 20
Figure 11. Alert-Level Terms for Volcanic Activity for Informing Ground Populations .............. 21
Figure 12. Aviation Color Codes for Volcanic Activity ................................................................. 22
Figure 13. Alaska Volcano Observatory Volcano Status Map on February 20, 2025 .................... 23
Figure 14. Great Sitkin Volcano: Alert Status Map on February 20, 2025 .................................... 23
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Figure 15. Great Sitkin Volcano Activity Notice on February 19, 2025 ....................................... 25
Figure 16. Volcanic Ash Advisory Centers (VAACs): Areas of Responsibility ............................ 27
Figure 17. Models of Mount Spurr Ashfall and Ash Cloud Dispersion ........................................ 29
Figure 18. Example of a Volcanic Ash Graphic (VAG) ................................................................. 33
Figure 19. Some Volcano Warning Products in Use in Alaska ...................................................... 35
Tables
Table A-1. Acronyms Used in This Report.................................................................................... 39
Appendixes
Appendix A. Acronyms Used in This Report ................................................................................ 39
Appendix B. Glossary of Volcano Terms ...................................................................................... 40
Appendix C. Examples of Problems for Aircraft from Volcanic Ash and Gas Hazards ................ 43
Contacts
Author Information........................................................................................................................ 43
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Introduction
The United States has 170 potentially active volcanoes, and the U.S. Geological Survey (USGS)
considers about 161 of these volcanoes threatening.1 In 2019, Congress passed the John D.
Dingell Jr. Conservation, Management, and Recreation Act (P.L. 116-9), in which Title V, Section
5001 (43 U.S.C. 31k), authorized the establishment of the National Volcano Early Warning and
Monitoring System (NVEWS).2 The objectives of NVEWS are to organize, modernize, and unify
the monitoring systems of the five U.S. volcano observatories—Alaska, California, Cascades,
Hawaiian, and Yellowstone—and to monitor all volcanoes at a level commensurate with their
threats.3 The USGS Volcano Hazards Program established the Volcano Science Center to
administer and unify the volcano observatories to research and monitor active volcanoes and
warn about volcano hazards.4
Alaska has about 140 active volcanoes, and the USGS considers 86 to be threatening. Since
October 2024, Mount Spurr—about 81 miles from Anchorage, AK—has been in a heightened
state of unrest. The USGS has warned of a possible eruption while noting that, at times, bad
weather and snow have limited monitoring, and limits to monitoring may inhibit timely volcano
warnings to reduce the risks to people and property. Most of Alaska’s volcanoes are remote and
inaccessible in the winter and bad weather. They also exist in harsh environments, making
monitoring challenging. Congress may consider whether NVEWS enables the Alaska Volcano
Observatory (AVO) to monitor all of Alaska’s active volcanoes at levels commensurate with their
threats.
In 2022, Congress passed the James M. Inhofe National Defense Authorization Act for Fiscal
Year 2023 (P.L. 117-263) and Title CV, Volcanic Ash and Fumes (§10501), amended NVEWS to
strengthen the monitoring and warning systems by integrating relevant capacities of the National
Oceanic and Atmospheric Administration (NOAA), including the Volcanic Ash Advisory Centers
(VAACs) located in Anchorage, AK, and Washington, DC. A primary risk from Alaska’s
volcanoes is from ash and ashfall, which may harm human health and the environment and may
damage property and aviation. Past eruptions of Redoubt Volcano, Mount Spurr, and Augustine
Volcano have disrupted aviation in Alaska and elsewhere. Congress may consider how the USGS
and NOAA volcanic ash models, forecasts, and warnings, which are generated using different
data and algorithms, may be integrated or improved and how these models and volcanic ash
warning products are impacted by modernizing and unifying monitoring systems. Congress also
may consider how other federal agencies, such as the Federal Aviation Administration (FAA), and
other governments or international organizations use the volcanic ash models and volcanic ash
warning products. In addition, Congress may consider how flight operators (e.g., American
Airlines, United Airlines) use volcanic ash warning products, because the 2010 Eyjafjallajökull
1 The U.S. Geological Survey (USGS) assessment of threatening volcanoes considers volcanoes that have been active
within the past about 11,000 years plus “three notably large and long-lived caldera systems (Yellowstone, Wyoming;
Valles, New Mexico; and Long Valley, California).” John W. Ewert et al., 2018 Update to the U.S. Geological Survey
National Volcanic Threat Assessment, USGS, Scientific Investigations Report 2018-5140, 2018, https://doi.org/
10.3133/sir20185140 (hereinafter USGS, Volcanic Threat Assessment).
2 CRS In Focus IF11987, The National Volcano Early Warning System, by Linda R. Rowan.
3 The USGS volcano observatories are the Alaska Volcano Observatory, the California Volcano Observatory, the
Cascades Volcano Observatory, the Hawaiian Volcano Observatory, and the Yellowstone Volcano Observatory.
USGS, “USGS Operates Five U.S. Volcano Observatories,” https://www.usgs.gov/programs/VHP/usgs-operates-fiveus-volcano-observatories.
4 USGS, “Volcano Hazards Program,” https://www.usgs.gov/vhp; and USGS, “Volcano Science Center,”
https://www.usgs.gov/centers/volcano-science-center.
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eruption in Iceland led to changes in volcanic ash warning products and flight operation
procedures during a volcanic ash event in some European and North Atlantic airspaces.
Primer on Alaska’s Active Volcanoes and Hazards
Alaska has about 140 active volcanoes and the U.S. Geological Survey (USGS) considers 86 to
be threatening to people and property (Figure 1 and Figure 2).5 Most of Alaska’s active
volcanoes originate along the Alaska-Aleutian Arc Subduction Zone and are stratovolcanoes
capable of explosive and effusive eruptions impacting local, regional, and global communities
and aviation (Figure 3 and Appendix B). Alaska’s volcanoes may have variable amounts of
perennial to seasonal snow and ice, which may contribute to more catastrophic eruptions.
Eruptions may incorporate the snow and ice into ash plumes or lava flows, increasing their size
and extent. Eruptions may melt the snow and ice, triggering steam- and water-enriched volcano
hazards, such as phreatic eruptions, pyroclastic flows, and lahars (mudflows) (Figure 4, Figure 5,
and Appendix B). On average, about two volcanoes erupt every year in Alaska. Because many
eruptions continue for weeks to months, it is not uncommon for three or four Alaska volcanoes to
have experienced eruptive activity in any single year.6 Volcano hazards include ash, gases, debris
avalanches, ejecta, lahars, lava flows, phreatic eruptions, and pyroclastic flows (Appendix B).7 In
addition, earthquakes, landslides, and tsunamis may be associated with tectonics near a volcano,
volcanic growth, and/or volcanic activity. According to AVO, the most significant and common
hazards from Alaska’s volcanoes that impact people, property, and aviation are ash clouds and
ashfall.8 Some of the past eruptions in Alaska have been explosive, generating ash clouds and
ashfall. Some of Alaska’s active volcanoes have eruptions that may range from a moderate to
moderate-large volcanic explosivity index (VEI of 2 or 3), with some instances of eruptions with
large to very large VEIs ranging from 4 to 6.9 The largest eruption in the 20th century was the
1912 Novarupta-Katmai eruption in Alaska with a VEI of 6.10
5 According to the Alaska Volcano Observatory (AVO), Alaska has about 140 volcanoes that have been active within
the past 2.6 million years. About 90 of these volcanoes have been active within the past 11,000 years. AVO, “Alaska
Volcano Observatory,” https://avo.alaska.edu/volcano/. The USGS assessment of threatening volcanoes considers
volcanoes that have been active within the past 11,000 years plus “three notably large and long-lived caldera systems
(Yellowstone, Wyoming; Valles, New Mexico; and Long Valley, California)”; therefore, the USGS’s 2018 assessment
considers 90 volcanoes in Alaska. USGS, Volcanic Threat Assessment.
6 AVO, “Frequently Asked Questions,” https://avo.alaska.edu/about/faq.
7 AVO, “Alaska Volcano Observatory,” https://avo.alaska.edu/volcano/.
8 AVO, “Alaska Volcano Observatory,” https://avo.alaska.edu/volcano/.
9 The volcanic explosivity index (VEI) provides a relative measure of the amount of erupted ash and may be useful for
assessing the relative risks of ash and ashfall to people, property, and aviation. VEI does not measure the amount of
erupted lava and is not useful for assessing the risks of lava flows or other ground-based hazards. USGS, “The
Volcanic Explosivity Index: A Tool for Comparing the Sizes of Explosive Volcanic Eruptions,” https://www.usgs.gov/
observatories/yvo/news/volcanic-explosivity-index-a-tool-comparing-sizes-explosive-volcanic. Large to very large
explosive eruptions (volcanic explosivity index of 4 to 8) may have global impacts on climate; modern technologies,
such as satellites, telecommunications, power infrastructure, and aviation; and other aspects of modern society with
global interdependencies. Such impacts are not discussed in this report. See Chris Newhall et al., Anticipating Future
Volcanic Explosivity Index (VEI) 7 Eruptions and Their Chilling Impacts. Geosphere vol. 14, no. 2 (2018), pp. 572603, https://doi.org/10.1130/GES01513.1 (hereinafter Newhall, Anticipating Future Eruptions) for an overview.
10 The 1991 Pinatubo eruption in the Philippines was the only other 20 th century eruption with a VEI of 6. Ben
Anderson, “Alaska’s Biggest Volcanic Eruptions,” Anchorage Daily News, February 28, 2012, https://www.adn.com/
science/article/alaskas-biggest-volcanic-eruptions/2012/02/28/.
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Figure 1. Threat Assessment of Some Active Volcanoes in Alaska
Sources: John W. Ewert et al., 2018 Update to the U.S. Geological Survey National Volcanic Threat Assessment, U.S.
Geological Survey (USGS), Scientific Investigations Report 2018-5140, 2018, https://doi.org/10.3133/sir20185140.
Map from the USGS and ESRI.
Notes: NVEWS = National Volcano Early Warning and Monitoring System. Triangles with different colors and
sizes denote the location and threat level of some volcanoes in Alaska (see legend). The USGS uses 24 volcano
hazard factors and the exposure of people and property to those factors to determine each volcano’s threat
level.
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Figure 2. Detailed Maps of Some of the Highest Threat Volcanoes in Alaska
Sources: John W. Ewert et al., 2018 Update to the U.S. Geological Survey National Volcanic Threat Assessment, U.S.
Geological Survey (USGS), Scientific Investigations Report 2018-5140, 2018, https://doi.org/10.3133/sir20185140.
Maps from the USGS and Esri. Modified by CRS.
Notes: NVEWS = National Volcano Early Warning and Monitoring System. Triangles with different colors and
sizes denote the location and threat level of some volcanoes in Alaska. Each volcano name is listed by each
triangle. Population centers near volcanoes, such as Dutch Harbor (top), Homer (left), and Anchorage (right),
are noted with black squares and labels. The USGS uses 24 volcano hazard factors and the exposure of people
and property to those factors to determine each volcano’s threat level.
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Figure 3. U.S. Subduction Zones Map and Subduction Zone Model
Sources: Top: U.S. Geological Survey (USGS), “Pacific Ocean Subduction Zones,” https://www.usgs.gov/media/
images/pacific-ocean-subduction-zones. Bottom: USGS, “Subduction Zone Graphic,” https://www.usgs.gov/media/
images/subduction-zone-graphic.
Notes: Top: Map focused on the Pacific Ocean basin and surrounding “Ring of Fire” subduction zones, where
tectonic plates collide, generating volcanoes, earthquakes, tsunamis, and mountainous landslide-prone terrain.
Some tectonic plates are labeled. Subduction zones are shown by high densities of earthquakes (circles) and
volcanoes (triangles). Subduction zones within the United States and its territories are shaded red. There is one
U.S. subduction zone in the Atlantic Ocean—the Caribbean Subduction Zone, which impacts Puerto Rico and
the U.S. Virgin Islands. Most of Alaska’s active volcanoes are generated at the Alaska-Aleutian Arc Subduction
Zone, where the Pacific plate collides and is subducted under the North American plate. Other types of plate
boundaries are shown with thin red lines. Colors reflect topography and bathymetry; darker blues and purple
offshore are deeper water, while brown and orange are higher land features, such as mountain ranges. Bottom:
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Schematic cross section of a subduction zone model showing the four major hazards—earthquakes, landslides,
tsunamis, and volcanic eruptions. When tectonic plates converge (movement indicated by the black arrows), one
plate slides beneath (subducts) the upper plate. At shallow depths, the interface between the plates—a fault—
may become stuck (locked), causing stresses to build until they exceed the fault’s strength and break free,
resulting in an earthquake. Volcanoes may form along the surface above a subduction zone because friction
between the moving plates produces heat and the heat produces magma (bright yellow features on the
schematic) that rises to the surface and erupts. Landslides may be triggered by earthquakes or volcanic activity.
Tsunamis may be triggered by earthquakes, landslides, or volcanic activity. For the Alaska-Aleutian Arc
Subduction Zone, the Pacific Plate is being subducted beneath the North American Plate along the converging
plate boundary shown in the top panel. Joan S. Gomberg and Kristin A. Ludwig, “Reducing Risk Where Tectonic
Plates Collide,” USGS, fact sheet 2017-3024, 2017, https://doi.org/10.3133/fs20173024.
Figure 4. Shishaldin Volcano Ash Cloud, Pyroclastic Flows, and Lahars
(October 3, 2023)
Source: Photograph by Chris Barnes, available at Alaska Volcano Observatory, “Shishaldin 2023/07,”
https://avo.alaska.edu/image/view/194974.
Notes: The October 3, 2023, eruption of Shishaldin viewed from the northwest. The ash cloud from the
eruption is extending away from the volcano to the southeast while steam emissions continue from the crater.
Steam and dark deposits on the flanks are from hot pyroclastic flows and lahars that flowed down the sides of
the volcano. Shishaldin’s flanks are covered in snow and ice, which may make the volcano hazards more
threatening. Roundtop Mountain Volcano is visible in the far left. Blankets of low-lying clouds surround the
volcanoes. See Figure 2, top panel, for a more detailed map of these volcano locations.
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Figure 5. Mount Cleveland Ash Plume
(May 23, 2006)
Source: Courtesy of Jeffrey N. Williams, Flight Engineer and National Aeronautics and Space Administration
(NASA) Science Officer, International Space Station Expedition 13 Crew, NASA Earth Observatory.
Notes: Eruption of Mount Cleveland on May 23, 2006, as photographed from the International Space Station at
an orbital altitude of approximately 400 kilometers. The photograph (north at the top; Carlisle Island to the
northwest) shows the ash plume moving southwest from the summit. Banks of fog (arcuate clouds at upper
right) are common features around the Aleutian Islands. The plume reached a height above sea level of about 8.2
kilometers (about 5 miles). A mix of snow and lava flows are also visible on the flanks of the volcano. See Figure
2, top panel, for a more detailed map showing the location of Cleveland and Carlisle Island volcanoes.
Ashfall Risks and Largest Eruption in 20th Century
A primary risk from Alaska’s volcanoes is from ash and ashfall, which may harm human health
and the environment and may damage property and aviation. For example, communities in
southcentral Alaska have been affected by ashfalls from relatively moderate eruptions of
Redoubt, Spurr, and Augustine (Figure 6 and Figure 7). For the most part, Alaska’s active
volcanoes are too far from people and property for lava flows or lahars to present a risk.11
Studying previous volcanic eruptions provides a greater understanding of volcanic activity,
volcano hazards, and the exposure of people and property to volcano hazards. The largest
eruption on Earth in the 20th century was the 1912 Novarupta eruption (also known as the 1912
11 Other risks exist from other volcanic hazards. The level of the risks varies depending on the size and extent of the
eruption and the magnitude of the exposure (i.e., the number of people and extent of the built environment in the path
of the volcanic hazard).
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Novarupta-Katmai eruption), which created the Katmai Caldera and the Valley of Ten Thousand
Smokes (Figure 8). The eruption launched an ash cloud about 20 miles into the air and caused
massive ashfall in Alaska and extensive ashfall as far away as Africa.12 In Kodiak, the ash and
sulfur dioxide gas caused injuries and environmental damage, including making water
undrinkable; damaged buildings and infrastructure; and disrupted radio communications and
shipping. Animal and plant life were decimated, especially birds and fish. The salmon fishing
industry was devastated, especially from 1915 to 1919.13 According to the USGS, if a similar
sized eruption occurred today, the state’s economy would be brought to a standstill and hundreds
of people would be injured or killed.14 For comparison, the second-largest eruption in the 20th
century, the 1991 Pinatubo eruption in the Philippines, killed 850 people and the much smaller
1980 Mount Saint Helens, WA, eruption killed 57 people (Figure 7).15
The enhanced understanding afforded by examining these 20th century eruptions can inform
research, monitoring, and warning for volcanoes in Alaska and elsewhere. At the same time, each
eruption is unique, and even with research, monitoring, and warning, authorities must make
decisions about risk reduction—that is, avoidance or protective actions for possible volcano
hazards—with some uncertainties about the volcano hazards.16
12 Wes Hildreth and Judy Fierstein, The Novarupta-Katmai Eruption of 1912—Largest Eruption of the Twentieth
Century: Centennial Perspectives, USGS, USGS Professional Paper 1791, 2012, https://pubs.usgs.gov/pp/1791/
pp1791.pdf (hereinafter Hildreth and Fierstein, Novarupta-Katmai Eruption); USGS, “The Impact of the 1912
Novarupta/Katmai Eruption on the Pacific Northwest,” https://www.usgs.gov/news/impact-1912-novaruptakatmaieruption-pacific-northwest (hereinafter USGS, “Impact of the 1912 Novarupta/Katmai Eruption”); and Judy Fierstein et
al., “Can Another Great Volcanic Eruption Happen in Alaska?,” USGS, USGS Fact Sheet 075-98, 1998,
https://pubs.usgs.gov/fs/fs075-98/ (hereinafter Fierstein et al., “Another Great Volcanic Eruption”).
13 Hildreth and Fierstein, Novarupta-Katmai Eruption; USGS, “Impact of the 1912 Novarupta/Katmai Eruption.”
14 Fierstein et al., “Another Great Volcanic Eruption.
15 According to the USGS, the 1991 Pinatubo eruption could have been even worse without the collaborative work of
the USGS and Philippine Institute of Volcanology and Seismology to monitor, forecast, and warn the public. The
USGS estimates this work saved more than 5,000 lives and $250 million (in 1991 dollars) in property. USGS,
“Remembering Mount Pinatubo 25 Years Ago,” https://www.usgs.gov/news/featured-story/remembering-mountpinatubo-25-years-ago-mitigating-a-crisis. According to the USGS, the Mount Saint Helens catastrophe could have
caused fewer fatalities if the restricted zone around the volcano had been larger and had considered the potential for a
lateral blast. Of the 57 fatalities, 51 occurred outside the restricted zone set up for a forecasted eruption. Lower
Columbia Currents, “Did 57 People Have to Die at Mount St. Helens?,” https://andrestepankowsky.substack.com/p/
did-57-people-have-to-die-at-mount; USGS, “1980 Cataclysmic Eruption,” https://www.usgs.gov/volcanoes/mount-st.helens/science/1980-cataclysmic-eruption.
16 Avoidance and protective actions may include evacuations; shelter in place; land, air, or water restrictions; closures;
shutting down or changing operations; moving property; and, where possible, removing or cleaning up ashfall deposits
to prevent further damage to people, property, or infrastructure. For example, see this study of aircraft avoidance
actions for volcanic ash: Juliette Delbrel et al., “An Investigation of Changes to Commercial Aircraft Flight Paths
During Volcanic Eruptions,” Journal of Applied Volcanology, vol. 14, no. 2 2025, https://doi.org/10.1186/s13617-02500150-7 (hereinafter Delbrel et al., “Aircraft Flight Paths in Eruptions”) and Newhall, Anticipating Future Eruptions.
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Figure 6. Extent of Ashfall From Some Major Alaskan Eruptions
Sources: U.S. Geological Survey (USGS), “The Impact of the 1912 Navarupta/Katmai Eruption on the Pacific
Northwest,” https://www.usgs.gov/news/impact-1912-novaruptakatmai-eruption-pacific-northwest; and Judy
Fierstein et al., “Can Another Great Volcanic Eruption Happen in Alaska?,” USGS, USGS Fact Sheet 075-98,
1998, https://pubs.usgs.gov/fs/fs075-98/.
Notes: Figure shows extent of ashfall from the 1912 Novarupta eruption (gray shaded area; also known as the
1912 Novarupta-Katmai eruption), from a 1976 Augustine eruption (blue shaded area), a 1990 Redoubt eruption
(orange shaded area), and a 1992 Spurr eruption (yellow shaded area). The Novarupta-Katmai eruption was the
largest eruption in the 20th century. Red triangles denote the locations of volcanoes within 500 miles of
Anchorage with notable ashfalls in the past 4,000 years. Black squares denote the location of labeled cities.
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Figure 7.Volume of Major Alaskan and Other Eruptions
Sources: U.S. Geological Survey (USGS), “The Impact of the 1912 Navarupta/Katmai Eruption on the Pacific
Northwest,” https://www.usgs.gov/news/impact-1912-novaruptakatmai-eruption-pacific-northwest; and Judy
Fierstein et al., “Can Another Great Volcanic Eruption Happen in Alaska?,” USGS, USGS Fact Sheet 075-98,
1998, https://pubs.usgs.gov/fs/fs075-98/.
Notes: The 1912 Novarupta eruption (also known as the 1912 Novarupta-Katmai eruption) was the largest on
Earth in the 20th century. The research shown in this figure calculated the amount of erupted volcanic materials,
as expressed in the original volume of molten rock (i.e., magma), shown by schematic ash plumes and labeled
with the estimated volume in cubic miles (mi3). The volume of Novarupta’s erupted material was more than
double that of the second-largest eruption of that century. There were no fatalities from the Novarupta
eruption, primarily because only a small, distant population was exposed to some ashfall hazards.
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Figure 8. Satellite Image of Novarupta and Other Volcanoes in Katmai National Park
Sources: Image produced by Steve J. Smith, Volcanology/Remote Sensing Graduate Student from the
Geophysical Institute University of Alaska Fairbanks in 2000; Alaska Volcano Observatory, “Alaska Volcano
Observatory,” https://avo.alaska.edu/image/view/2152.
Notes: True color composite satellite image of Katmai National Park region of Alaska on August 16, 2000.
Composite produced with a red-green-blue channel combination of band 3 – band 2 – band 1, respectively. This
combination produces an image with colors similar to human eyesight. The eruption of Novarupta and Mount
Katmai on June 6, 1912, was the largest volcanic eruption anywhere in the world for the entire 20th century. The
60-hour eruption began at Novarupta, then Mount Katmai collapsed to form a 2-mile-wide caldera. The last part
of the eruption was the formation of a lava dome at Novarupta. Most of the pyroclastic deposits from the
eruption were deposited in what is now known as the Valley of Ten Thousand Smokes. The valley received this
name because the deposits remained so hot for several years after the eruption that steam was released
continuously from fumaroles, so it looked as if the ground was always smoking. Some of these deposits may be
resuspended depending on weather conditions, and the resuspended ash may become a volcano hazard that
leads to a volcanic ash warning. ETM+ is the Enhanced Thematic Mapper Plus instrument on Landsat 7.
Ash Risks for Aviation and Alaska’s Large Aviation System
Volcanic ash and sulfur dioxide gas may pose a serious risk for aircraft in flight or on the ground
(Appendix B and Appendix C).17 Ash clouds can damage aircraft in flight, such as engine and
17 Federal Aviation Administration (FAA), Chapter 7-6-10, “Flight Operations in Volcanic Ash,” in Aeronautical
Information Manual (AIM), September 2024, https://www.faa.gov/air_traffic/publications/atpubs/aim_html/
(continued...)
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avionic system failures.18 Ash can make takeoff and landing an aircraft treacherous and can
damage aircraft on the ground, incurring costs for cleaning, repair, or even replacement. Alaska’s
aviation system, which is an international cargo hub and serves more than 80% of Alaskan
communities that are only accessible by air, faces significant exposure to volcanic ash and gases
from numerous volcanoes (Figure 1).19 In addition, Alaska’s largest commercial airports—
Anchorage, Fairbanks, and Juneau and the Department of Defense’s (DOD’s) Joint Base
Elmendorf-Richardson—are near very-high-threat to high-threat volcanoes. A review of 94
confirmed aircraft encounters with volcanic ash clouds from 38 volcanoes between 1953 and
2009 found that Augustine and Redoubt volcanic eruptions have each caused five or more, mostly
damaging, aircraft encounters.20
The 1989-1990 Redoubt Eruption: Lessons Learned
About Hazards, Monitoring, and Warnings
The 1989-1990 Redoubt eruption led to a greater understanding of Alaska’s volcano hazards: the
extent and cost of damage caused by these hazards, the extent to which people and property were
exposed to Alaska’s volcano hazards (i.e., volcanic risks), and the strengths and weaknesses of
research, monitoring, and warnings. From December 14, 1989 to August 30, 1990, a series of
explosive and effusive eruptions of Redoubt, about 110 miles southwest of Anchorage (Figure 2),
generated ash plumes, ash clouds, ashfalls, pyroclastic flows, lava domes and lava flows, and
lahars (Figure 6, Figure 9, and Appendix B).21 Redoubt’s eruption damaged property and
contributed to other costs associated with evacuations of facilities and closures of airspace,
airports, roads, oil production facilities, oil pipelines, schools, businesses, other facilities, and
chap7_section_6.html (hereinafter FAA, “Flight Operations in Volcanic Ash,”). International Civil Aviation
Organization (ICAO), Manual on Volcanic Ash, Radioactive Material and Toxic Chemical Clouds, Doc 9691, AN954,
3rd edition, 2015 (hereinafter ICAO, Manual on Volcanic Ash).
18 Marianne Guffanti et al., Encounters of Aircraft with Volcanic Ash Clouds: A Compilation of Known Incidents, 19532009, USGS, Data Series 545, 2010 (hereinafter Guffanti et al., Encounters of Aircraft, 2010). Uta Reichardt et al.,
“Volcanic Ash and Aviation: Recommendations to Improve Preparedness for Extreme Events,” Transportation
Research Part A, vol. 113 (2018), https://doi.org/10.1016/j.tra.2018.03.024 (hereinafter Reichardt et al., “Volcanic Ash
and Aviation”).
19 Alaska Department of Transportation and Public Facilities, Airports and Aviation Annual Report 2021, 2021,
https://dot.alaska.gov/documents/aviation/2021-Annual-Report.pdf. See also Alaska Department of Transportation and
Public Facilities, “DOT&PF Aviation and Airports,” https://dot.alaska.gov/airport-portal.shtml and USGS, Volcanic
Threat Assessment.
20 Six other volcanoes caused five or more aircraft encounters, besides Augustine and Redoubt. The other volcanoes
include Chaiten in Chile, Mount Saint Helens in the United States, Pacaya in Guatemala, Pinatubo in the Philippines,
Sakura-jima in Japan, and Soufriere Hills in Montserrat, Lesser Antilles, United Kingdom. Guffanti et al., Encounters
of Aircraft, 2010. For comparison, 92 aircraft reported encountering the 2010 Eyjafjallajökull eruption. Delbrel et al.,
“Aircraft Flight Paths in Eruptions.”
21 The 1989-1990 eruption was similar in terms of explosivity and effusivity to past eruptions at Redoubt and was not
as violent or voluminous as some other eruptions elsewhere in the 20th century, such as the 1980 eruption of Mount
Saint Helens (Figure 7). Steven R. Brantley, The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31,
1990, USGS Circular 1061, 1990, https://dggs.alaska.gov/webpubs/usgs/c/text/c-1061.pdf (hereinafter, USGS,
Eruption of Redoubt).
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other infrastructure (Figure 9).22 In 1990, the USGS described the Redoubt eruption as the
second-most costly U.S. eruption, behind only the 1980 eruption of Mount Saint Helens.23
Figure 9. Examples of Hazards from the 1989-1990 Redoubt Eruption
Source: Photos from Alaska Volcano Observatory (AVO) as described below.
Notes: Upper left: Photograph by R. Clucas, April 21, 1990. AVO activity archive. Ascending eruption cloud
from Redoubt volcano as viewed to the west from the Kenai Peninsula. The mushroom-shaped plume rose from
avalanches of hot debris (pyroclastic flows) that cascaded down the north flank of the volcano. A smaller, white
steam plume rose from the summit crater. Upper right: Photograph by R. McGimsey, U.S. Geological Survey
(USGS), AVO activity archive. Gray ash from the February 21, 1990, eruption of Redoubt Volcano blankets the
snow in Indian, AK, a community about 32 kilometers (20 miles) southeast of Anchorage and 200 kilometers
(124 miles) northeast of Redoubt Volcano. Lower left: Photograph by C. Gardner, USGS, AVO activity archive.
Aerial view, looking southwest, of the Drift River valley following the 1989-1990 eruption of Redoubt volcano.
Two bedrock islands (informally called the Dumbbell Hills) are visible at bottom center. Lahar deposits cover the
valley floor. Lower right: Photograph by C. Neal, USGS, AVO activity archive. Pyroclastic-flow deposits from the
April 15 (lower two-thirds of section) and April 21 (upper one-third of section), 1990, eruptions of Redoubt
volcano exposed in a gully along the western margin of the piedmont lobe of drift glacier. Shovel (about 1.5 feet
tall) at base of section shows scale to understand the thickness of these deposits. See Figure 2, bottom right,
for a more detailed map of the location of Redoubt, near Anchorage.
22 USGS, Eruption of Redoubt; Thomas J. Casadevall, “The 1989-1990 Eruption of Redoubt Volcano, Alaska: Impacts
on Aircraft Operations,” Journal of Volcanology and Geothermal Research, vol. 62, no. 1-4 (1994), https://doi.org/
10.1016/0377-0273(94)90038-8 (hereinafter Casadevall, Redoubt Impact on Aircraft); Zygmunt J. Przedpelski and
Thomas J. Casadevall, “Impact of Volcanic Ash from 15 December 1989 Redoubt Volcano Eruption on GE CF6-80C2
Turbofan Engines,” Proceedings of the first international symposium on volcanic ash and aviation safety, U.S.
Geological Survey Bulletin 2047, 1994, https://avo.alaska.edu/explore/reference/2121 (hereinafter Przedpelski and.
Casadevall, “Impact of Volcanic Ash from 15 December 1989 Redoubt Volcano Eruption”).
23 USGS, Eruption of Redoubt.
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Seismic Monitoring
According to the USGS, seismic monitoring established on Redoubt less than two months before
eruptive activity began allowed for sufficient warning and time for emergency protective actions
by authorities for some eruptive events.24 That is, prior to the eruption, Congress provided
funding for Alaska volcano monitoring in FY1988 appropriations and the Alaska Volcano
Observatory (AVO) was established in 1988.25 AVO installed seismic instruments on Redoubt,
and the seismic network was operational by October 1989, just before the eruption started.
Seismic instruments recorded earthquakes, volcanic tremor attributed to magma movement, and
other ground vibrations attributed to flows and lahars. The seismic instruments provided the only
dedicated and continuous information about the size and extent of eruptive activity at Redoubt.
For example, a series of earthquakes beneath the volcano’s summit began 24 hours before the first
major eruption on December 14, 1989. AVO warned officials that Redoubt might erupt soon, and
officials took protective actions that reduced the risks of the volcano hazards from the first major
eruption.
Pyroclastic Flows and Lahars
The Redoubt eruption’s most damaging volcano hazard for property on the ground was a series of
pyroclastic flows and massive lahars formed when hot lava melted parts of the Drift glacier and
winter snowpack along the volcano’s slope (Appendix B).26 AVO warned officials about some
flows and lahars. Authorities evacuated the Drift River Oil Terminal,27 shut down terminal and oil
pipeline activities, and shut down nearby oil production facilities that were supplying oil to the
pipelines. The flows and lahars damaged some terminal buildings and other infrastructure, but
there were no oil spills (Figure 9).28
There was additional concern about ashfall causing damage to people and property on the ground.
Along the populated Kenai Peninsula, the public was told to stay indoors, and schools and other
facilities were closed to avoid health problems from ashfall. Gas-powered turbines at the Beluga
power plant, a primary power supplier for Anchorage, were partially shut down to avoid ashfall
damage to the turbines.
Volcanic Ash and Aviation
The Redoubt eruption’s most damaging volcano hazard for aviation was ash (Appendix C). The
1989-1990 eruption was a seminal event for understanding ash hazards and ash damage to aircraft
because at least seven aircraft (five in Alaska and two in Texas) flew through ash, causing major
to minor damage and interrupting flight operations.29 Five commercial aircraft near Anchorage
encountered ash over the first three months of the eruption. The most serious encounter occurred
on December 15, the second day of the eruption, when a KLM passenger plane entered an ash
cloud about 150 miles northeast of Redoubt.30 The plane temporarily lost power to all four
24 USGS, Eruption of Redoubt.
25 In FY1988 appropriations, S. Rept. 100-165 provided $750,000 for monitoring of Augustine volcano.
26 USGS, Eruption of Redoubt.
27 Shipnext, “Drift River Oil Terminal (United States),” https://shipnext.com/port/drift-river-marine-terminal-usdrf-usa.
28 USGS, Eruption of Redoubt.
29 USGS, Eruption of Redoubt; Casadevall, Redoubt Impact on Aircraft; Przedpelski and Casadevall, “Impact of
Volcanic Ash from 15 December 1989 Redoubt Volcano Eruption.”
30 According to a former head of the AVO, the pilots of the KLM flight did not see an AVO warning that had been sent
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engines.31 The aircraft was at an altitude of about 5 miles and descended about 2 miles over 12
minutes before two of the engines restarted. The aircraft was able to return to the airport in
Anchorage. The airplane was sandblasted by the ash, leaving pit marks on the windshield, other
windows, and other external surfaces; all four engines and all of the electrical and avionic
equipment had to be replaced; and the plane had to be cleaned of ash.32 Two other commercial
aircraft in west Texas encountered a Redoubt ash cloud, about 55 hours after an eruption and
2,900 nautical miles away from the volcano, highlighting the ash risks for aviation far from an
eruption in space and time (Appendix C). In addition to aircraft damaged by ash, the ash hazards
caused delays and cancellations at Anchorage International Airport, Merrill Field, Elmendorf Air
Force Base, and many smaller airports in Alaska.
There was no dedicated monitoring of the ash hazards before or during the 1989-1990 eruption.
As such, there was little to no information about the size, extent, and direction of the ash clouds
and plumes to be able to forecast or provide specific warnings about ash hazards. The only
information came from the seismic monitoring, reconnaissance observations during volcano
quiescence, and opportunistic observations from some distance away from the volcano, such as
the photograph in Figure 9.33
After the damaging encounters of aircraft with Reboubt ash clouds, the National Weather Service
(NWS) organized an Alaska Aviation Weather Unit (AAWU), which includes a Meteorological
Watch Office (MWO) and the Anchorage Volcanic Ash Advisory Center (AVAAC), to monitor
ash hazards and provide warnings.34 The AVO and AVAAC became contributing members of the
International Civil Aviation Organization’s (ICAO’s) International Airways Volcano Watch.35 The
U.S. volcano observatories submit weekly reports on volcanic activity for U.S. volcanoes, the
Smithsonian Institution Global Volcanism Program submits weekly reports on volcanic activity
for volcanoes outside the United States, and AVO and AVAAC provide information and warnings
for Alaska’s active volcanoes to ICAO.36
Eruption Uncertainties
Not every volcanic eruption in Alaska and elsewhere is preceded by precursory signals, such as
earthquakes, so forecasts and early warnings may not be possible for every volcanic event.
Furthermore, the earthquake and ground vibration activity monitored by the seismic network
during the 1989-1990 Redoubt eruption was not always followed by an eruptive event, so
by fax to officials before taking off. Geophysical Institute, “Redoubt’s Big Impact 30 Years Ago,”
https://www.gi.alaska.edu/alaska-science-forum/redoubts-big-impact-30-years-ago.
31 Casadevall, Redoubt Impact on Aircraft.
32 Casadevall, Redoubt Impact on Aircraft.
33 When it was considered safe and feasible during periods of Redoubt’s quiescence, reconnaissance flights and field
work combined with other observations from aircraft and satellites helped to measure the size and extent of some of
Redoubt’s hazards after the first series of eruptions. USGS, Eruption of Redoubt.
34 National Weather Service (NWS), “Alaska Aviation Weather Unit,” https://storymaps.arcgis.com/stories/
9ec2090f634e48efae91ecb5a2153d20; NWS, “Alaska Aviation Weather Unit (AAWU),” https://www.weather.gov/
aawu/.
35 ICAO, Handbook on the International Airways Volcano Watch, Doc 9766-AN/968, 2024 (hereinafter ICAO,
Handbook). See also Larry Mastin et al., “Progress in Protecting Air Travel from Volcanic Ash Clouds,” Bulletin of
Volcanology, vol. 84, no. 9 (2022), https://doi.org/10.1007/s00445-021-01511-x (hereinafter Mastin et al., Progress in
Protecting Air Travel).
36 Smithsonian Institution, “Global Volcanism Program,” https://volcano.si.edu/. The U.S. volcano observatories are
the Alaska Volcano Observatory, the California Volcano Observatory, the Cascades Volcano Observatory, the
Hawaiian Volcano Observatory, and the Yellowstone Volcano Observatory. USGS, “USGS Operates Five U.S.
Volcano Observatories,” https://www.usgs.gov/programs/VHP/usgs-operates-five-us-volcano-observatories.
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forecasts of eruptive activity were uncertain. In addition, not every volcanic eruption is a singular
event of short duration; some eruptions, such as the 1989-1990 Redoubt eruption, may have
eruptive events over days to months, where forecasts and warnings would benefit most from
preexisting and continuous monitoring.
Studies of the 1989-1990 Redoubt eruption and others concluded that even with volcanic
unpredictability, forecasts and early warnings about changing volcanic activity and warnings after
an eruption starts would benefit from preexisting and continuous monitoring. A 1990 USGS
report on the Redoubt eruption concluded that a suite of monitoring tools could provide forecasts
and early warning for some eruptions and more information about the extent and risk of volcano
hazards for ongoing and not forecasted eruptions.37 Additional assessments of U.S. volcanic risks
recommended advancing volcano research and monitoring to improve forecasts and early
warning about changing volcanic activity and to improve warnings about eruptions.38
National Volcano Early Warning and Monitoring
System
In 2019, Congress passed legislation that authorized a National Volcano Early Warning and
Monitoring System (NVEWS; §5001 of P.L. 116-9; 43 U.S.C. 31k).39 The law directed the USGS
to establish NVEWS to monitor volcanoes, warn U.S. citizens of volcanic activity, and protect
citizens from “undue and avoidable harm.”40 In addition, under the Stafford Act (42 U.S.C.
§5132), the USGS has authority through the President to provide alerts about volcanoes using
federal and other communication services to states and civilian populations in endangered areas.
The USGS organizes NVEWS activities within the Volcano Hazards Program (VHP), and the
Volcano Science Center serves the VHP’s mission. The 2019 law specifies that the system’s
objective is to monitor U.S. volcanoes at a level commensurate with the volcanic threats.
NVEWS is to have two purposes: (1) organize, modernize, standardize, and stabilize the
monitoring systems of the five U.S. volcano observatories and (2) unify the monitoring systems
of these observatories into a single interoperative system.
The USGS has submitted several reports and plans regarding NVEWS. It submitted a five-year
plan for establishing and managing NVEWS to Congress in 2020, annual reports to Congress
charting the progress of NVEWS, a 2024 recommended monitoring plan, and a 2024 USGS
Volcano Science Center Response Plan for Significant Volcanic Events.41 Through these reports
37 USGS, Eruption of Redoubt.
38 John W. Ewert et al., An Assessment of Volcanic Threat and Monitoring Capabilities in the United States:
Framework for a National Volcano Early Warning System, USGS, Open File Report 2005-1164, 2005,
https://pubs.usgs.gov/of/2005/1164/2005-1164.pdf; National Academies of Sciences, Engineering, and Medicine
(NAS), Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing, 2017, doi: https://doi.org/10.17226/
24650 (hereinafter NAS, Volcanic Eruptions).
39 CRS In Focus IF11987, The National Volcano Early Warning System, by Linda R. Rowan.
40 USGS, “National Volcano Early Warning System—Monitoring Volcanoes According to Their Threat,”
https://www.usgs.gov/programs/VHP/national-volcano-early-warning-system-monitoring-volcanoes-according-theirthreat.
41 Peter F. Cervelli et al., Five-Year Management Plan for Establishing and Operating NVEWS: The National Volcano
Early Warning System, USGS Open-File Report 2021–1092, 2021, https://doi.org/10.3133/ofr20211092 (hereinafter
USGS, NVEWS Plan). See also Charlie Mandeville et al., The Volcano Hazards Program—Strategic Science Plan for
2022–2026, USGS Circular 1492, 2022, https://doi.org/10.3133/cir1492; Ashton F. Flinders et al., Recommended
Capabilities and Instrumentation for Volcano Monitoring in the United States, USGS Scientific Investigations Report
2024-5062, 2024, https://doi.org/10.3133/sir20245062 (hereinafter USGS, Recommended Monitoring); Seth C. Moran
(continued...)
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and plans, the USGS has detailed updates to monitoring systems; the establishment of a National
Volcano Information System, which is expected serve the dual purposes of a National Volcano
Data Center and a 24/7 Volcano Watch Office; and efforts to establish advisory committees
(pursuant to §5001 of P.L. 116-9).42
In 2022, Congress passed legislation that amended NVEWS to direct cooperation and
coordination between the USGS volcano monitoring and NOAA’s Volcanic Ash Advisory Centers
(VAACs) (§10501 of P.L. 117-263). The law directs the Secretary of the Interior and the
Secretary of Commerce to develop and execute a memorandum of understanding to establish
cooperative support for NVEWS activities with NOAA.43 The law also directs the Secretary of
Commerce to submit cost estimates for NVEWS activities at NOAA to the Secretary of the
Interior that should be incorporated into the NVEWS management plan. As of May 2025, NOAA
is preparing an implementation plan with cost estimates for its VAACs. The USGS is
collaborating with NOAA on multiple aspects of volcano hazards and warnings, such as volcanoinduced tsunamis and the health impacts of volcanic ash.44 In June 2022, the USGS, in
cooperation with NOAA and other federal agencies, published an updated Alaska Interagency
Plan for Volcanic Ash Episodes.45
The following sections of this report focus on AVO’s and AVAAC’s roles in research, monitoring,
and warning for Alaska’s active volcanoes and how these efforts may meet the objectives of
NVEWS.
Role of the Alaska Volcano Observatory
The AVO researches, monitors, forecasts, warns about, and seeks to reduce risks from volcano
hazards in Alaska.46 AVO is a partnership between the USGS, the Geophysical Institute of the
University of Alaska Fairbanks, and the State of Alaska Division of Geological and Geophysical
Surveys.47 An AVO duty scientist is to be available 24/7 via cell phone. In addition, a duty
seismologist, duty remote sensing scientist, and duty alarm analyst are to be available 24/7 via
cell phone. AVO may establish a 24/7 watch operation for a volcanic event.48 Geologists and
geophysicists at all three entities share research, data processing, analysis, and hazard
communication duties.
et al., The U.S. Geological Survey Volcano Science Center Response Plan for Significant Volcanic Events, USGS
Circular 1518, 2024, https://doi.org/10.3133/cir1518 (hereinafter USGS, Response for Volcanic Event).
42 USGS, “Volcano Watch: A Focus on the National Volcano Information System,” https://www.usgs.gov/
observatories/hvo/news/volcano-watch-a-focus-national-volcano-information-service. Posted by the USGS on April 10,
2025.
43 A memorandum of understanding between the USGS and NOAA already exists to collaborate on activities involving
physical and biological sciences that the USGS can utilize as needed. Correspondence between CRS and USGS, March
31, 2025. See also “MOU GS21000543” in USGS, “List of Memorandums of Understanding (MOUs),” updated
August 23, 2023, https://www.usgs.gov/media/files/list-memorandums-understanding-mous.
44 Correspondence between CRS and USGS, March 31, 2025.
45 USGS, Alaska Interagency Plan for Volcanic Ash Episodes, June 2022, https://www.usgs.gov/programs/VHP/
coordination-plans (hereinafter, USGS, Alaska Interagency Plan).
46 AVO, “Alaska Volcano Observatory,” https://avo.alaska.edu/volcano/. AVO acts as the backup for all USGS
volcano observatories. In the event that the AVO is unable to serve as a backup, the Cascades Volcano Observatory
generally would assume this critical function. USGS, Response for Volcanic Event.
47 USGS, “Volcano Hazards Program,” https://www.usgs.gov/programs/VHP; University of Alaska Fairbanks,
“Geophysical Institute,” https://www.gi.alaska.edu/; State of Alaska, “Geological & Geophysical Surveys,”
https://dggs.alaska.gov/.
48 USGS, Response for Volcanic Event.
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As needed, AVO may involve other USGS Volcano Science Center staff from Volcano
Observatories in the Cascades, Hawaiian, California, and Yellowstone or other USGS offices.49
AVO communicates with international agencies involved in volcanic ash cloud and ashfall
warnings including ICAO, the Canadian Meteorological Centre, and Montreal VAAC. AVO is
available to consult with partner agencies about significant eruptions from Russian volcanoes,
including the Tokyo VAAC and the Petropavlovsk MWO.
Research
AVO describes the scope of its research to include the following50
•
•
•
•
•
Geological mapping to determine eruptive histories of active volcanoes
Investigations of hydrologic hazards such as lahars and floods associated with
eruptions of snow- and ice-clad volcanoes
Characterization and analysis of eruptive processes and their impacts
Geophysical and geochemical exploration of the interiors of volcanoes and
mechanisms of eruption
Development of new instrumentation, analytical tools, and models to aid in
interpretation of volcanic unrest and eruption and hazard detection and
forecasting
Monitoring
In its volcano monitoring, AVO “utilizes a variety of instrumentation and techniques to detect and
interpret signs of volcanic unrest or eruption. Instruments and techniques include seismicity,
infrasound, ground deformation, satellite imagery and data, lightning data, gas emissions,
webcams and direct visual observations, including local and pilot reports”51 (Figure 10).
AVO uses aviation the most of any U.S. volcano observatory for research and monitoring because
there are more than 100 active volcanoes in Alaska, many of which are remote and only
accessible by air. Helicopters and airplanes can take researchers to visit a volcano for field studies
or to install, repair, or maintain monitoring sites (Figure 10). In addition, airplanes, helicopters,
and unoccupied aircraft systems can have monitoring instruments installed on the aircraft to
measure properties of a volcano (Figure 10).52 AVO also uses weather data and weather forecasts
from NOAA and weather and ash data from pilot reports.53 AVO utilizes Earth observations from
49 USGS, Response for Volcanic Event; USGS, Alaska Interagency Plan.
50 Alaska Volcano Observatory, “About AVO–Volcano Hazard Research Program,” https://avo.alaska.edu/about/.
51 Alaska Volcano Observatory, “About AVO–Volcano Monitoring,” https://avo.alaska.edu/about/.
52 FAA, “Drones,” https://www.faa.gov/uas. Unoccupied aircraft systems (UASs) also may be called unmanned aerial
systems or unmanned aircraft systems or referred to as unoccupied aerial vehicles (UAVs) and related terms.
53 A Pilot Weather Report (PIREP) is an inflight weather report submitted by an aircraft pilot or crew member. An
Aircraft Report (AIREP) is also an inflight weather report provided by the pilot or derived from onboard sensors, such
as wind and temperature. PIREPs are U.S.-only reports, whereas AIREPs are worldwide. In instances where volcanic
ash is observed, pilots can report this information in either a PIREP or an AIREP. When volcanic ash details are
communicated in a PIREP, the report is termed an Urgent PIREP; in an AIREP, it is referred to as a Special AIREP.
Pilots typically complete a Volcanic Activity Report (VAR, example of the form available at FAA, AIM,
https://www.faa.gov/air_traffic/publications/atpubs/aim_html/appendix_2.html), which may contain extra details about
the physical characteristics of ash clouds, after concluding flight operations or during flight debriefings. It also can
function as a Special AIREP with an additional section dedicated to describing the ash cloud. Interagency Council for
Advancing Meteorological Services (ICAMS), National Volcanic Ash Operations Plan for Aviation, Second Release,
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satellites for research, monitoring, and warning about Alaska’s volcanoes.54 The satellites may
include the USGS/NASA Landsat Mission,55 NASA’s Earth Observing System,56 NOAA’s
Satellites,57 and the European Space Agency’s Sentinel Mission, among others.58 AVO uses a
web-based tool “VolcView” to examine satellite data and “AshCam” to examine webcam data.59
ICM-P35-2024, December 2024, https://www.icams-portal.gov/resources/icams/related_documents/2024_nvaopa.pdf
(hereinafter ICAMS, National Volcanic Ash Operations).
54 Table 1.2, “Satellite-Borne Suite for Volcano Monitoring,” in NAS, Volcanic Eruptions; Mastin et al., Progress in
Protecting Air Travel.
55 USGS, “Landsat Missions,” https://www.usgs.gov/landsat-missions.
56 NASA, “NASA’s Earth Observing System,” https://eospso.nasa.gov/content/nasas-earth-observing-system-projectscience-office.
57 NOAA, “Satellites,” https://www.noaa.gov/satellites; NWS, “Alaskan Region HQ,” https://www.weather.gov/arh/.
58 European Space Agency, “The Sentinel Missions,” https://www.esa.int/Applications/Observing_the_Earth/
Copernicus/The_Sentinel_missions.
59 USGS, “VolcView,” https://volcview.wr.usgs.gov/; USGS, “AshCam,” https://volcview.wr.usgs.gov/.
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Figure 10. Volcano Monitoring
Sources: U.S. Geological Survey (USGS) Volcano Hazards Program and the Alaska Volcano Observatory.
Notes: Top: USGS Infographic. The USGS monitoring includes measurements of volcanic gases, ground
movement (deformation [i.e., rise, fall, or sideways movement of surface] or vibration [i.e., shaking] related to
magma movement, earthquakes, flows or lahars, or landslides); and remote sensing (via camera, aircraft and
satellite imaging/measurements) of eruptive activities. Bottom left: Photograph by Jimmy Finney, AVO activity
archive. Maintenance of field site RDDF by Malcolm Herstand and Tara Shreve, with Redoubt in the background.
Equipment includes a camera, seismometer, and infrasound plus power supplies and communications. Bottom
right: Photograph by Taryn Lopez, University of Alaska, Fairbanks Geophysical Institute/AVO. USGS Volcano
Emissions Project scientists Laura Clor and Peter Kelly prepare to collect airborne volcanic gas measurement
during the 2017 Cook Inlet Gas Flight. Inset image: map showing the location and abundance of sulfur dioxide gas
released from Redoubt as measured during the flight.
Over the past few years, AVO has replaced all analog-based telemetry with digital-based
telemetry, replaced at least 53 outdated instruments, and installed at least 55 new instruments on
very-high-threat and high-threat Alaska volcanoes and on Mount Edgecumbe.60 Even though
60 Telemetry involves the automatic gathering, processing, and transmission of data from distant or inaccessible points
to a central location for analysis and monitoring. According to the USGS fourth annual report to Congress on NVEWS
covering FY2023, digital telemetry “was achieved in August 2022 and ensures Alaska volcano monitoring networks
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there has been progress in monitoring, the USGS considers Redoubt an under-monitored volcano.
Redoubt does not have the requisite instrumentation recommended by the USGS.61 Among the
other very-high-threat to high-threat volcanoes in Alaska (Figure 2), the USGS considers Akutan
Island, Makushin Volcano, Mount Spurr, Augustine Volcano, Mount Okmok, Iliamna Volcano,
Aniakchak Crater, Mount Katmai, Mount Veniaminof, Korovin Volcano, Hayes Volcano, Mount
Churchill, Kanaga Volcano, Kaguyak Crater, Kasatochi Island, Mount Moffett, and Seguam
Island to be under-monitored compared with their threat levels.62 Warnings of eruptions may not
be possible for under-monitored volcanoes.
Warnings
The AVO provides warnings of volcanic activity using an alert system designed by the USGS
VHP and in coordination with the ICAO and the FAA. The alert system has two parts: (1) ranked
terms (unassigned, normal, advisory, watch, and warning) to inform people on the ground about a
volcano’s status (Figure 11) and (2) ranked colors (unassigned, green, yellow, orange, and red) to
inform the aviation sector about airborne ash hazards (Figure 12).
Figure 11. Alert-Level Terms for Volcanic Activity for Informing Ground Populations
Source: U.S. Geological Survey, Alaska Interagency Plan for Volcanic Ash Episodes, June 2022, https://www.usgs.gov/
programs/VHP/coordination-plans.
now comply with National Telecommunications and Information Administration (NTIA) authorizations for radio
frequency spectrum used by USGS.” Mount Edgecumbe volcano was not considered a threatening volcano in the 2018
USGS assessment; however, volcanic unrest related to a swarm of earthquakes in April 2022 led to a retrospective
analysis of past earthquakes and past ground deformation measured in satellite data showing volcanic activity. The
AVO installed instruments to monitor Edgecumbe and declared it an “officially monitored” volcano in 2023. USGS,
“Newest Volcano Notice Including Edgecumbe, Alaska Volcano Observatory Information Statement,” Edgecumbe,
VNUM #315040, February 9, 2024, https://volcanoes.usgs.gov/hans-public/volcano/ak90.
61 USGS, Volcanic Threat Assessment; USGS, NVEWS Plan; and USGS, Recommended Monitoring.
62 USGS, Volcanic Threat Assessment; USGS, NVEWS Plan; and USGS, Recommended Monitoring.
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Figure 12. Aviation Color Codes for Volcanic Activity
Source: U.S. Geological Survey, Alaska Interagency Plan for Volcanic Ash Episodes, June 2022, https://www.usgs.gov/
programs/VHP/coordination-plans.
Notes: As of November 2024, International Civil Aviation Organization (ICAO) guidance allows a VONA to
include an UNASSIGNED colorless code for volcanoes that have insufficient information available to evaluate the
status of the volcano or volcanic activity. Also as of November 2024, volcano observatories may report
resuspension of ash in a VONA. ICAO, Handbook on the International Airways Volcano Watch, Doc 9766-AN/968,
2024.
AVO notifies the public about the status of each Alaskan volcano via the online AVO Volcano
Map (Figure 13).63 For example, the AVO Volcano Map accessed on February 20, 2025, shows
the Great Sitkin Volcano is at alert level watch and at aviation level orange (Figure 14). In
addition, anyone can sign up to receive emails or text messages about volcanic activity at U.S.monitored volcanoes through the Volcano Notification Service.64
63 AVO, “Alaska Volcano Observatory,” https://avo.alaska.edu/.
64 USGS, “Volcano Notification Service (VNS),” https://volcanoes.usgs.gov/vns2/.
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Figure 13. Alaska Volcano Observatory Volcano Status Map on February 20, 2025
Source: U.S. Geological Survey, “Alaska Volcano Observatory,” https://avo.alaska.edu/, accessed on February
20, 2025.
Notes: Screenshot of the online interactive map. A user can examine a specific volcano for more information
about volcanic activity and monitoring equipment on the interactive map.
Figure 14. Great Sitkin Volcano: Alert Status Map on February 20, 2025
Source: U.S. Geological Survey, “Alaska Volcano Observatory,” https://avo.alaska.edu/, accessed on February
20, 2025.
Notes: Screenshot of the online interactive map. A user can examine a specific volcano for more information
about volcanic activity and monitoring equipment on the interactive map. In many cases, the user can see realtime data as measured by the instrument or camera, by accessing the instrument page from the map.
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AVO may issue a Volcano Activity Notice (VAN) and a Volcano Observatory Notification for
Aviation (VONA).
•
•
A Volcano Activity Notice (VAN) is to be issued when a volcano’s alert level
changes (Figure 11 and Figure 12), there is significant volcanic activity (Figure
15), or there is a significant ash resuspension event. Ash from previous eruptions
may be resuspended in the atmosphere by strong winds and may pose a threat to
people and property.65
Volcano Observatory Notification for Aviation (VONA) is a derivative product of
the VAN that contains information in a format specifically intended for aviation
users (e.g., pilots, dispatchers, air-traffic managers, meteorologists) of volcano
hazard information with emphasis on ash emission. Its purpose is to
communicate volcanic activity details to the aviation sector, specifically focusing
on ash-plume information. The VONA includes an Aviation Color Code and is
transmitted to Air Route Traffic Control Centers, MWOs, and AVAAC.66
65 Resuspension of ash from the 1912 Novarupta-Katmai eruption is among the most common and most impactful for
Alaskan communities and aviation in Alaska (Figure 8). For any alerts, the AVO identifies the volcano that is the
source area for the resuspension and specifies that the volcano is not erupting. AVO identifies the volcano as “Katmai”
for resuspension in the Novarupta-Katmai region. USGS, Alaska Interagency Plan. See also Mastin et al., Progress in
Protecting Air Travel, for more about resuspension ash cloud forecasts and the use of satellite data.
66 As of November 2024, Volcano Observatory Notifications for Aviation (VONAs) include an unassigned colorless
code for volcanoes that have insufficient information available to evaluate the status of the volcano or volcanic activity,
and volcano observatories may report resuspension of ash as a volcanic activity in a VONA. ICAMS, National
Volcanic Ash Operations; ICAO, Handbook.
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Figure 15. Great Sitkin Volcano Activity Notice on February 19, 2025
Source: U.S. Geological Survey, “Alaska Volcano Observatory,” accessed February 20, 2025,
https://avo.alaska.edu/.
The AVO follows the USGS plans for distributing warnings and notices to federal, state, and local
officials as described in the Volcano Science Center Response Plan for Significant Volcanic
Events and the Alaska Interagency Plan for Volcanic Ash Episodes, among other plans.67 AVO
communicates with the NWS; FAA; DOD; and Alaska’s Division of Homeland Security and
Emergency Management, Department of Environmental Conservation, and Department of
Health; among other federal, state, and local agencies, to provide observational data and
consistent interpretations and notifications of volcanic activity and hazards. AVO coordinates as
needed with federal (e.g., U.S. Fish and Wildlife Service, National Park Service), state (e.g.,
Department of Natural Resources), and private land managers that have jurisdiction over the land
encompassing an active or restless volcano.68
67 USGS, Response for Volcanic Event; USGS, Alaska Interagency Plan.
68 USGS, Alaska Interagency Plan.
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Every volcanic event is unique, and a volcanic event may grow in time and space into heightened
volcanic unrest that requires a larger-scale response to mitigate volcanic risks.69 The USGS has
developed plans to scale up or scale down a response, depending on the volcanic event. The AVO
is responsible for an initial response to changes in volcanic activity through release of VANs and
VONAs to the public and federal, state, and local officials.70 If the volcanic event becomes more
significant, the AVO may scale up its response by establishing an Observatory Volcanic Event
Response Team, which typically includes a 24/7 watch. If the volcanic event becomes even more
significant, the Volcano Science Center Director, who is supervised by the USGS Alaska
Regional Director, may establish a Center Volcanic Event Response Team. The plans describe
roles and responsibilities of the team members and considerations for increasing the size of the
team by bringing in staff from other volcano observatories or other USGS programs. The plans
follow the National Incident Management System developed by the Federal Emergency
Management Agency (FEMA) to standardize command, control, and coordination of emergency
responses in the United States.71 In the event of a presidential disaster declaration, AVO typically
coordinates with FEMA. In addition, AVO is part of Alaska’s interagency response to volcanic
ash episodes, national volcanic ash operations plans, and international volcanic ash operations
plans.72
Role of Anchorage Volcanic Ash Advisory Center
The NOAA AVAAC, together with the AVO, studies, monitors, forecasts, and warns about
volcanic ash hazards in the atmosphere to reduce risks to people, property, and aviation.73 The
1989-1990 Redoubt eruption and aircraft encounters with Redoubt ash motivated the NWS to
establish the Alaska Aviation Weather Unit (AAWU), including the AVAAC, soon after the
eruption. AVAAC is part of the AAWU and colocated with the NWS Weather Forecast Office
(WFO) and the Alaska Regional Operations Center in Anchorage. The AAWU is one of three
ICAO Meteorological Watch Offices (MWOs) in the United States and the only MWO of the
three that is also a VAAC.74
69 The USGS defines a volcanic event as any instance of heightened volcanic unrest, which may include a volcanic
eruption but does not require an eruption where magma reaches the surface. USGS, Response for Volcanic Event, p. 5.
70 The USGS’s federal authority to monitor and warn about volcanic activity is codified in a National Volcano Early
Warning and Monitoring System (NVEWS; §5001 of P.L. 116-9; 43 U.S.C. 31k). The law directed the USGS to
establish NVEWS to monitor volcanoes, warn U.S. citizens of volcanic activity, and protect citizens from “undue and
avoidable harm.” Under the Stafford Act (42 U.S.C. §5132), the USGS has authority through the President to provide
alerts about volcanoes using federal and other communication services to states and civilian populations in endangered
areas. The USGS Volcano Science Center is not currently an Integrated Public Alert and Warning System (IPAWS)
alerting authority, required to issue warnings via wireless emergency alerts (WEA), unlike the USGS Earthquake
Science Center in California that has this authority to issue earthquake early warning messages. Federal Emergency
Management Agency (FEMA), “IPAWS Alerting Authorities: Agencies and Organizations,” https://www.fema.gov/
emergency-managers/practitioners/integrated-public-alert-warning-system/public-safety-officials/alerting-authorities/
agencies-organizations. See also CRS Report R48363, The Integrated Public Alert and Warning System (IPAWS):
Primer and Issues for Congress, by Amanda H. Peskin and CRS Report R47121, The ShakeAlert Earthquake Early
Warning System and the Federal Role, by Linda R. Rowan.
71 USGS, Response for Volcanic Event; FEMA, “National Incident Management System,” https://www.fema.gov/
emergency-managers/nims.
72 USGS, Alaska Interagency Plan; ICAMS, National Volcanic Ash Operations; ICAO, Manual on Volcanic Ash;
ICAO, Handbook.
73 NWS, “Anchorage Volcanic Ash Advisory Center,” https://www.weather.gov/vaac/.
74 ICAO, Manual on Volcanic Ash; ICAO, Handbook.
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AVAAC is one of nine VAACs organized by ICAO to monitor volcanic ash around the world and
its potential impacts on aviation (Figure 16).75 A single VAAC generally issues Volcanic Ash
Advisories (VAAs) for a continuous ash cloud at one time, although two VAACs may issue
simultaneous VAAs for different ash clouds from the same volcano. As ash nears and crosses
VAAC boundaries, VAACs transfer, or handover, responsibility of VAA issuance to the
downstream VAAC.76
Figure 16. Volcanic Ash Advisory Centers (VAACs): Areas of Responsibility
Source: International Civil Aviation Organization (ICAO)—global map from ICAO. Modified by CRS.
Notes: The National Oceanic and Atmospheric Administration (NOAA) operates the Anchorage VAAC and
the Washington VAAC. These centers are responsible for observing, measuring, and modeling volcanic ash
plume distribution in the atmosphere and providing volcanic ash advisories about volcano hazards to aviation in
the areas of responsibility shown on the map by thick red lines. ICAO flight information regions for local to
regional areas are shown by blue lines. See also National Weather Service, “Anchorage Volcanic Ash Advisory
Center,” https://www.weather.gov/vaac/, and NOAA, “Washington Volcanic Ash Advisory Center,”
https://www.ospo.noaa.gov/products/atmosphere/vaac/. Air space by country adjacent to the Anchorage and
Washington VAACs shown by color shading, with each country identified in the legend. Volcanoes shown by
yellow circles from NOAA, National Centers for Environmental Information, https://www.ngdc.noaa.gov/hazel/
view/hazards/volcano/loc-data.
75 ICAO, “ICAO,” https://www.icao.int/Pages/default.aspx; ICAO, Manual on Volcanic Ash; and ICAO, Handbook.
76 The Anchorage Volcanic Ash Advisory Center (AVAAC) accepts handovers from the Tokyo Volcanic Ash Advisory
Center (VAAC) and hands over responsibility for issuance of volcanic ash advisories (VAAs) to the Washington
VAAC, whose area of responsibility borders that of AVAAC to the south, and to Montreal VAAC, whose area of
responsibility borders that of AVAAC to the east. AVAAC is generally notified of an eruption on Russia’s Kamchatka
Peninsula or in the Kurile Islands by receipt of a VAA and a volcanic ash graphic (VAG) from the Tokyo VAAC or a
significant meteorological event (SIGMET) from the Petropavlovsk Meteorological Watch Office. AVAAC relies on
the Washington VAAC for its primary backup. ICAMS, National Volcanic Ash Operations.
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Research
Research on volcanic ash hazards and their movement in the atmosphere includes observations
and modeling. In the United States, the USGS, NOAA, NASA, and the National Science
Foundation have supported much of this research at the federal level.77
Models
AVAAC is an operational center that uses research on volcanic ash hazards for monitoring and
warning. AVAAC uses NOAA’s HYSPLIT Volcanic Ash Model to prepare warnings.78 The
HYSPLIT model inputs volcano location and eruption duration, summit height, ash column
height, and forecast meteorology, such as wind direction, and outputs forecasts of the dispersion
and concentration of ash as it moves away from the volcano. In December 2022, the capability to
generate ensemble forecasts became operational; HYSPLIT employs the 31 separate forecasts
members from NOAA’s weather model, the Global Ensemble Forecast System (GEFS).79
The USGS has advanced another model, Ash3d, to study and forecast ash clouds and ashfall
hazards.80 Ash3d inputs a volume of erupted ash from a volcanic source with wind models to
forecast the thickness of ashfall on the ground and the concentration of ash in the dispersing ash
cloud (see Figure 17 for an example). Ash3d uses NOAA’s Global Forecast System at 0.5
degrees of resolution, which does not use the 31 separate forecasts members of GEFS and may
yield different results.81
77 ICAMS, National Volcanic Ash Operations.
78 NOAA, “HYSPLIT,” https://www.arl.noaa.gov/hysplit/; NOAA, “HYSPLIT Volcanic Ash Model,” .
79 ICAMS, National Volcanic Ash Operations; NOAA, “Global Ensemble Forecast System (GEFS),”
https://www.ncei.noaa.gov/products/weather-climate-models/global-ensemble-forecast; National Centers for
Environmental Prediction Central Operations, “NCEP Products Inventory,” https://www.nco.ncep.noaa.gov/pmb/
products/gens/.
80 USGS, “Ash3d,” https://vsc-ash.wr.usgs.gov/ash3d-gui/#!/.
81 NOAA, “Global Forecast System,” https://www.ncei.noaa.gov/products/weather-climate-models/global-forecast;
ICAMS, National Volcanic Ash Operations.
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Figure 17. Models of Mount Spurr Ashfall and Ash Cloud Dispersion
(February 26, 2025, eruption)
Source: U.S. Geological Survey, “Ash3d Public Run Results,” accessed February 27, 2025, https://vscash.wr.usgs.gov/ash3d-gui/#!/publicruns. These map displays of the model results have not been modified by CRS.
Notes: The top panel shows estimated ashfall thickness on the ground. The bottom panel shows the integrated
ash mass load in grams per cubic meter in the atmosphere. Both estimates start with an erupted volume of
0.0171 cubic kilometers dense-rock equivalent (DRE) for a duration of 3 hours for a plume height of 13.7
kilometers (km) above sea level (asl). The estimates use winds from the Global Forecast System (GFS) with a 0.5
degrees resolution for the time listed.
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Satellite Observations
In addition to models, the increased observations by satellites and web cameras have led to better
ash detection and tracking to inform forecasts of ash movement and dispersion over time. Current
geostationary satellites have roughly doubled the spatial resolution (0.5-3 kilometers; about 0.3-2
miles) and image repeat times (< 1 to 15 minutes) compared with satellites in orbit more than 10
years ago. Whereas satellites imaged the 1991 Pinatubo ash cloud once per hour and the 2010
Eyjajallajokull ash cloud once every 15 minutes, 2021 eruptions at Soufriere, St. Vincent, were
imaged once per minute. As of 2022, every VAAC could access high-quality images at least every
15 minutes and several volcanic arcs were routinely imaged every 2.5 or 5 minutes.82
Besides ash dispersion models and forecasts based on NOAA’s GEFS or Global Forecast System
(GFS), AVAAC may use NOAA’s satellite-based ash-tracking products. The Satellite Analysis
Branch of the National Environmental Satellite, Data, and Information Service (NESDIS)
operates the Washington VAAC in conjunction with the NWS’s National Centers for
Environmental Prediction.83 Additionally, the NESDIS Center for Satellite Application and
Research is responsible for developing satellite products that support VAAC operations. Notably,
the NESDIS Volcanic Cloud Analysis Toolkit is a widely used suite of satellite-based products
and services comprising, but not limited to, eruption alerts, automated ash detection and tracking,
automated gas detection and tracking, ash cloud top height, ash loading, and imagery.84
Ash Concentration and Aircraft Damage
The 2010 Eyjafjallajökull eruption in Iceland forced the closure of airspace over large areas of
Europe for an extended period of time based on the London VAAC ash dispersal models and
forecasts.85 More than 100,000 flights were canceled, more than 10 million passengers were
affected, and there was an estimated $5 billion in economic damage.86 During part of the initial
phase of the eruptive event,87 various impacted European airspaces followed the general
protective action of aircraft avoiding any concentration of volcanic ash, which led to airspace
restrictions and closures causing significant impact to air travel. As the eruption continued, the
European Organisation for the Safety of Air Navigation and the European Commission took a
coordinating role and, in agreement with jet engine experts, developed different aircraft
avoidance criteria for the rest of the 2010 eruption, allowing commercial aircraft to fly through
certain low-ash-concentration thresholds.88 The change in procedures required the London VAAC
82 Mastin et al., Progress in Protecting Air Travel.
83 NOAA, “National Environmental Satellite, Data, and Information Service,” https://www.nesdis.noaa.gov/; NOAA,
“Airlines, Observatories, and Others Keep Tabs on Volcanic Activity with VOLCAT,” https://www.nesdis.noaa.gov/
news/airlines-observatories-and-others-keep-tabs-volcanic-activity-volcat.
84 Cooperative Institute for Meteorological Satellite Studies (CIMSS), “Volcanic Cloud Monitoring – NOAA/CIMSS,”
https://volcano.ssec.wisc.edu/; CIMSS, “Cooperative Institute for Meteorological Satellite Studies,”
https://cimss.ssec.wisc.edu/. See also Michael J. Pavlonis et al., “Automated Detection of Explosive Volcanic Eruptions
Using Satellite-Derived Cloud Vertical Growth Rates,” Earth and Space Science, vol. 5 (2018), pp. 843-980,
https://doi.org/10.1029/2018EA000410.
85 Met Office, “Volcanic Ash Advisory Centre (VAAC),” https://www.metoffice.gov.uk/services/transport/aviation/
regulated/international-aviation/vaac/index.
86 Reichardt et al., “Volcanic Ash and Aviation.”
87 The eruption waxed and waned for several weeks, causing multiple VAAs. See Global Volcanism Program, “Report
on Eyjafjallajokull (Iceland),” Bulletin of the Global Volcanism Network, vol. 36, no. 4 (April 2011) https://doi.org/
10.5479/si.GVP.BGVN201104-372020.
88 Tatiana Bolic and Zarko Sikcev, “Eruption of Eyjafjallajökull in Iceland: Experience of European Air Traffic
Management,” Transportation Research Record, vol. 2214, no. 1 (2011), DOI: 10.3141/2214-17.
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to issue supplemental ash dispersal models and forecasts showing low, medium, and high ash
concentrations.
Subsequent studies of this eruption and others have led to advances in volcanic ash
measurements, models, and forecasts.89 Since the 2010 eruption, the ICAO and the European
Union Aviation Safety Agency have developed new procedures for aviation to deal with volcanic
ash hazards in Europe.90 In Western Europe and parts of the North Atlantic regions, the
responsibility for volcanic ash avoidance or the decision to fly or not to fly into an area of known
or forecasted volcanic ash contamination was transferred from air traffic management to
commercial aircraft operators.91 Aircraft operators must have safety risk assessments that include
decisions about flight operations in known or forecasted volcanic ash.92
The ICAO collaborated with the International Coordinating Council of Aerospace Industries
Associations to establish a set of ash concentration thresholds and ranges for commercial
aviation.93 The benefits of advances in ash observations and ash forecasting, however, may be of
limited benefit for flight operators across all nations for developing safety risk assessments,
because there is limited research on the impact of ash on jet engines.94 The ICAO is asking
VAACs to develop a quantitative volcanic ash (QVA) product that provides forecast probabilities
of ash concentration at aircraft altitudes or quantifies the concentration of ash with height,
enabling operators to move away from traditional criteria for visible (to the human eye) or
discernable (by satellite) volcanic ash aloft.95
89 The eruption released ash into the atmosphere for 39 days. Frances M. Beckett et al., “Atmospheric Dispersion
Modelling at the London VAAC: A Review of Developments Since the 2010 Eyjafjallajökull Volcano Ash Cloud,”
Atmosphere vol. 11, no. 4 (2020), pp. 352-378, https://doi.org/10.3390/atmos11040352; Reichardt et al., “Volcanic Ash
and Aviation”; Mastin et al., Progress in Protecting Air Travel.
90 European Union Aviation Safety Agency (EASA), Flight in Airspace with Contamination of Volcanic Ash, Safety
Information Bulletin (SIB) 2023-13, December 19, 2023, https://ad.easa.europa.eu/ad/2023-13 (hereinafter EASA,
Flight in Volcanic Ash); ICAO, Volcanic Ash Contingency Plan, European and North Atlantic Regions, EUR Doc 019,
NAT Doc 006, Part II, Edition 2.2.0, 2024 (hereinafter ICAO, Volcanic Ash European and North Atlantic); Reichardt
et al., “Volcanic Ash and Aviation.”
91 This transfer has implications for U.S. flight operators that may wish to fly in certain European and North Atlantic
flight information regions. Near a volcano with significant volcanic activity, air traffic management makes decisions
about flight restrictions and closures. In addition, any state may in the interest of public safety temporarily impose
flight restrictions or closures over part or all of its territory. The European and North Atlantic flight information regions
in ICAO fall under the responsibility of different VAACs—Montreal, Washington, London, and Toulouse VAACs
serve the North Atlantic region; London, Toulouse, Anchorage, and Tokyo VAACs serve the European region.
EASA’s safety bulletin only applies in the Western European and northern North Atlantic regions, essentially the
regions covered by the London and Toulouse VAACs. Since the 2010 Eyjafjallajökull eruption, the London and
Toulouse VAACs have issued supplementary information about ash concentrations and have worked to improve
forecasts through advances in modeling and observations as well as testing. Other VAACs are collaborating on
research, monitoring, and warning about volcanic ash and improving volcanic ash warning products. EASA, Flight in
Volcanic Ash; ICAO, Volcanic Ash European and North Atlantic. Frances Beckett et al,, “Conducting Volcanic Ash
Cloud Exercises: Practising Forecast Evaluation Procedures and the Pull-Through of Scientific Advice to the London
VAAC,” Bulletin of Volcanology, vol. 86, no. 63 (2024), https://doi.org/10.1007/s00445-024-01717-9 (hereinafter
Beckett, “Conducting Volcanic Ash Cloud Exercises”).
92 EASA, Flight in Volcanic Ash; ICAO, Volcanic Ash European and North Atlantic.
93 ICAMS, National Volcanic Ash Operations.
94 Reichardt et al., “Volcanic Ash and Aviation”; Delbrel et al., “Aircraft Flight Paths in Eruptions.” See also T. V.
Abramchuk et al., “Model for Generation the Liquid Phase Zones for Volcanic Ash Melt in the Combustion Chamber
of a Turbojet: Different Flight Modes for a Mainline Aircraft,” Thermophysics and Aeromechanics, vol. 31 (March 12,
2025), pp. 659-667, https://doi.org/10.1134/S0869864324040048.
95 The ICAO initiated the development of the quantitative volcanic ash (QVA) product so flight operators may use the
QVA with their safety management program to optimize airspace and plan more efficient routes during significant
(continued...)
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Monitoring
AVAAC uses observations, measurements, and forecasts from AVO and NOAA to forecast,
advise, and monitor the movement of volcanic ash in AVAAC’s assigned airspace. AVAAC is
typically staffed 24/7 to provide guidance and support to Alaska’s MWO and the aviation
community.96 Duty meteorologists or satellite analysts continuously monitor remote sensing data,
pilot reports, and AVO VANs. AVAAC runs ash dispersion models, determines the current and
forecast area extent of the ash, and produces ash warnings. AVAAC can request a HYSPLIT run
whenever necessary by contacting the senior duty meteorologist and providing essential details
such as the eruption’s start time, duration, and eruption height.
Warning
AVAAC may issue a volcanic ash advisory with a volcanic ash graphic.97
•
•
A volcanic ash advisory (VAA) provides information about the presence or
anticipated occurrence of volcanic ash that could impact the safety of aircraft
operations. These advisories include details such as the volcano’s identification,
the eruption time, the observed position of the ash cloud, and the forecasted
position of the ash cloud. In cases where the ash cloud is crossing into another
VAAC’s area of responsibility, handoff information also is included in the VAA.
A volcanic ash graphic (VAG) provides information from the VAA in a graphical
format (Figure 18).98
As noted in the “Research” section, above, VAACs are developing QVAs and are expected to
begin providing QVAs as part of VAAs as early as November 2025. In addition, VAAs and VAGs
are anticipated to be digital by 2030.99 The advances in observations and modeling are seen as
improving the timeliness and resolution of the VAAs while reducing uncertainties in the
forecasts.100
volcanic ash cloud events. The London and Toulouse VAACs have been required to issue supplemental QVAs since
the end of 2024 to satisfy updated Europe and North Atlantic guidance. EASA, Flight in Volcanic Ash; ICAO, Volcanic
Ash European and North Atlantic; Beckett, “Conducting Volcanic Ash Cloud Exercises.” The other seven VAACs are
expected to issue QVAs for significant eruptions after 2025, and all VAACs are expected to issue QVAs as a standard
for significant events in Annex 3 by November 2030. ICAO, Annex 3 - Meteorological Service for International Air
Navigation, 20th Edition, 2018. After 2030, VAAs and VAGs are anticipated to be issued only for ash cloud events that
are not significant events. Amendment 82 to ICAO Annex 3 and ICAMS, National Volcanic Ash Operations.
96 ICAMS, National Volcanic Ash Operations.
97 ICAMS, National Volcanic Ash Operations.
98 ICAO specified the format for consistency. ICAMS, National Volcanic Ash Operations.
99 The World Meteorological Organization, sponsored by the ICAO, has developed and currently maintains the
IWXXM data format. This format is designed to report aviation weather information in the widely recognized XML
standard for data exchange over the internet. The product representations of IWXXM are intended for machine-tomachine operational exchanges of meteorological information essential to aviation, which includes volcanic activity.
These IWXXM products are expected to replace legacy Time Activity Curve (TAC)-formatted and text-based products
by 2030. ICAMS, National Volcanic Ash Operations.
100 Mastin et al., Progress in Protecting Air Travel.
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Figure 18. Example of a Volcanic Ash Graphic (VAG)
Source: Interagency Council for Advancing Meteorological Services, National Volcanic Ash Operations Plan for
Aviation, Second Release, ICM-P35-2024, December 2024, https://www.icams-portal.gov/resources/icams/
related_documents/2024_nvaopa.pdf.
Notes: The example is for a 2021 eruption of the Fuego Volcano in Guatemala. The VAG was prepared by the
Washington Volcanic Ash Advisory Center. Each panel shows the forecasted extent of the ash cloud in 6-hour
increments. Top left is the eruption; top right is 6 hours later; bottom left is 12 hours later; and bottom right is
18 hours later.
Other Volcano Warning Products in Use in Alaska
The information and warnings from the AVO and AVAAC, the two primary federal agencies
focused on volcanic activity, are used to develop other volcano warning products for Alaska
(Figure 19). AVAAC is part of the NWS and generally communicates information and warnings
following NWS protocols and coordination. In addition to the VAA warning products from
AVAAC, the NWS may issue other volcano warning products in Alaska (Figure 19). The
Anchorage MWO may issue Significant Meteorological Information—the primary warning
product to the aviation community for volcanic ash. The Anchorage Center Weather Service Unit
(CWSU), which is located in the FAA’s Air Route Traffic Control Center, is staffed by an NWS
meteorologist. The CWSU is the liaison between FAA facilities and other NWS offices.
Forecasters issue a Center Weather Advisory or a Meteorological Impact Statement, as needed, to
provide additional information essential to air traffic managers’ decisionmaking processes.
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The NWS Alaska Region operates three WFOs, located in Anchorage, Fairbanks, and Juneau.
WFOs participate in the volcanic eruption response by issuing ashfall statements, advisories, and
warnings for the public and marine communities (Figure 19). Ashfall is included in Terminal
Aerodrome Forecasts as appropriate. The WFO also issue flash-flood products as deemed
necessary for impactful lahars or volcanic debris flows after coordination with AVO and the
Alaska Pacific River Forecast Center. In addition, the WFOs assist in the coordination of
information during volcanic events by soliciting ashfall reports and briefing local community
members and leadership about potential hazards.
In the United States, the FAA advises pilots not to fly through volcanic ash and provides advice
about flight operations if a pilot encounters ash while flying, landing, or taking off.101 The FAA
may restrict air space based on the presence of volcanic ash.102 The FAA may issue a Notice to
Air Missions about volcanic ash hazards, an Urgent Pilot Report, or a Temporary Flight
Restriction (Figure 19).103 DOD and Alaska state agencies issue other volcano warning products
(Figure 19).
101 FAA, “Flight Operations in Volcanic Ash.”
102 FAA, Aeronautical Information Manual (AIM), “3-5-3 Temporary Flight Restrictions,” https://www.faa.gov/
air_traffic/publications/atpubs/aim_html/chap3_section_5.html.
103 A Notice to Air Missions prescribes direction used to format and distribute information regarding unanticipated or
temporary changes to services, components of, or hazards in the National Airspace System. FAA, “Notice to Air
Missions (NOTAM),” https://www.faa.gov/documentLibrary/media/Order/7930.2S_Chg_2_dtd_12-2-21.pdf. FAA also
changed the acronym from Notice to Air Men to Notice to Air Missions. ICAO and ICAMS may use the term Notice to
Air Men, Notice to Airman, or Notice to Airmen.
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Figure 19. Some Volcano Warning Products in Use in Alaska
Source: U.S. Geological Survey, Appendix J in Alaska Interagency Plan for Volcanic Ash Episodes, June 2022,
https://www.usgs.gov/programs/VHP/coordination-plans.
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Dissemination of Volcano Warning Products in
Alaska
If a volcanic event may be an immediate threat to life, the State Emergency Operation Center
(SEOC), located on Joint Base Elmendorf-Richardson near Anchorage, can coordinate
dissemination of volcano warnings through the national Emergency Alert System (EAS), Wireless
Emergency Alert (WEA), and National Warning System/Alaska Warning System (AKWAS).104
Each agency distributes alert and safety information (i.e., volcano warning products) through
various communication portals. AVO issues notification of volcanic activity via telephone calldown to partner agencies as well as via email, website, social media postings, and the Volcano
Notification Service. NWS uses NOAA Weather Wire, marine high-frequency and very-highfrequency radio, NOAA Weather Radio, the statewide Alaska television weathercast, and EAS in
addition to distribution of text and graphics by its own telecommunications Gateway and through
radio facsimile, satellite-based SafetyNet Service, and the internet. The United States Coast
Guard broadcasts NWS marine forecasts over high-frequency radio. FAA distributes aviation
weather forecasts and warnings from the NWS, flight information, pilot reports, and terminal
information via the Aeronautical Fixed Telecommunications Network, and radio, data, and voice
communications for transmitting flight plans, clearances, and other information.105 SEOC can
redistribute critical information via email systems, Alaska Land Mobile Radio, Amateur Radio,
commercial radio, television, and cable and internet services in addition to EAS, WEA, and
AKWAS.
Congressional Considerations
The number of active volcanoes in Alaska, and the difficulty of monitoring those volcanoes due
to their remoteness and harsh environment, present unique challenges for the USGS and its
partners. Congress may wish to assess the USGS’s plans for addressing these challenges in
Alaska and meeting the objectives of NVEWS (§5001 of P.L. 116-9; 43 U.S.C. 31k) and its
progress on these plans.106
Congress may want information on the USGS’s ability to efficiently and effectively employ
remote sensing (e.g., satellite-based observations) to provide additional volcanic activity
information that could enhance warning capability relative to the cost of installing and
maintaining additional monitoring equipment at volcanoes in Alaska. Congress also could
evaluate current USGS efforts to organize, modernize, standardize, and stabilize the monitoring
104 USGS, Alaska Interagency Plan; Federal Communications Commission, “The Emergency Alert System,”
https://www.fcc.gov/emergency-alert-system; Federal Communications Commission, “Wireless Emergency Alerts
(WEA),” https://www.fcc.gov/consumers/guides/wireless-emergency-alerts-wea; State of Alaska, Emergency Alert
System Plan, 2025, https://ready.alaska.gov/Documents/Operations/EAS/EASPlan/
Alaska%20EAS%20Plan%202025.pdf. See also CRS Report R48363, The Integrated Public Alert and Warning System
(IPAWS): Primer and Issues for Congress, by Amanda H. Peskin.
105 SKYbrary, “Aeronautical Fixed Telecommunications Network,” https://skybrary.aero/articles/aeronautical-fixedtelecommunication-network-aftn. See also FAA, “Section 4. International Operations and Messages,”
https://www.faa.gov/air_traffic/publications/atpubs/fs_html/chap6_section_4.html.and ICAO Store, “Annex 10 –
Aeronautical Telecommunications,” https://store.icao.int/en/annexes/annex-10.
106 The USGS submitted a five-year plan for establishing and managing NVEWS to Congress in 2020 (USGS, NVEWS
Plan), annual reports to Congress charting the progress of NVEWS, a 2024 report with recommendations for volcano
monitoring (USGS, Recommended Monitoring), and a 2024 response plan for volcanic events (USGS, Response for
Volcanic Event).
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systems of the five U.S. volcano observatories and consider whether the USGS NVEWS plans
may ensure Alaska’s active volcanoes are monitored at a level commensurate with each volcano’s
threat. 107 The USGS is developing a National Volcano Information System that is expected to
include a 24/7 Watch Office and a National Volcano Data Center (pursuant to §5001 of P.L. 1169), but neither had been fully established as of the end of the five-year authorization of
appropriations for NVEWS (which ended in FY2023).108 The 2024 USGS Volcano Science
Center Response Plan for Significant Volcanic Events calls for temporary 24/7 watch capabilities
for significant volcanic events as described in the plan.109
AVAAC is responsible for VAAs (including VAG) and for developing QVAs in Alaskan air space
and other flight regions within its area of responsibility (Figure 16). It remains unclear how
AVAAC’s VAA products are coordinated with the USGS Ash3d products, the Washington VAAC,
and the NESDIS Volcanic Cloud Analysis Toolkit. The ICAO initiated the development of the
QVA, which quantifies ash concentrations in ash cloud forecasts, so that flight operators may use
the QVA with their safety risk assessment to optimize airspace and plan more efficient routes
during significant volcanic ash cloud events. ICAO expects all VAACs to issue QVAs as a
standard product by November 2030 and to retire VAAs sometime after 2030. Although the FAA
uses VAAs among other volcano warning products (Figure 19), it has not provided guidance
about using QVAs in the future.
In 2022, Congress passed legislation that amended NVEWS to direct cooperation and
coordination between the USGS’s volcano monitoring and NOAA’s VAACs (P.L. 117-263). As of
May 2025, the Secretary of Commerce and the Secretary of the Interior had not completed
implementation plans and cost estimates for cooperative support for integrating VAAC research,
modeling, and warning with NVEWS. Congress may consider oversight of NOAA’s VAACs to
understand how AVAAC products may evolve and how AVAAC/AVO volcanic ash products may
be used by the FAA, DOD, and others. In addition, flight operators may benefit from
AVAAC/AVO research, monitoring, and warning in preparing safety risk assessments about
volcanic ash that are necessary for flight operations in parts of Europe and the North Atlantic.
Congress may consider oversight of NVEWS, particularly plans, progress, and priorities;
amending NVEWS further; or reauthorizing appropriations for NVEWS (authorized
appropriations expired in FY2023) to meet the objectives of NVEWS while ensuring research,
monitoring, and warning about Alaska’s active volcanoes at levels commensurate with their
threats.
In the 119th Congress, S. 1052, introduced on March 13, 2025, would reauthorize NVEWS,
authorizing appropriations for USGS of $75 million until FY2033, extending the period of
authorization of sums necessary for NOAA to carry out its NVEWS-related activities to FY2034,
and amending NVEWS (41 U.S.C. § 31k(b)(2)(B)) by adding “infrasound arrays, visible and
infrared cameras and advanced digital telemetry networks” to the emerging technologies the
USGS should apply to modernize NVEWS. Another measure, H.R. 3176, was introduced on May
107 For example, the USGS indicates that some very-high to high-threat Alaska volcanoes may remain under-monitored
in its five-year plan for NVEWS. USGS, NVEWS Plan; USGS, “National Volcano Warning System—Monitoring
Volcanoes According to Their Threat,” https://www.usgs.gov/programs/VHP/national-volcano-early-warning-systemmonitoring-volcanoes-according-their-threat. See also CRS In Focus IF11987, The National Volcano Early Warning
System, by Linda R. Rowan.
108 43 U.S.C. 31k; USGS, NVEWS Plan; CRS In Focus IF11987, The National Volcano Early Warning System, by
Linda R. Rowan; USGS, “Volcano Watch: A Focus on the National Volcano Information System,” posted by the
USGS on April 10, 2025, https://www.usgs.gov/observatories/hvo/news/volcano-watch-a-focus-national-volcanoinformation-service.
109 USGS, Response for Volcanic Event.
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5, 2025, to reauthorize NVEWS. Over the past few years, AVO has replaced all analog-based
telemetry with digital-based telemetry for Alaska’s volcanoes, replaced at least 53 outdated
instruments, and installed at least 55 new instruments, including some infrasound and cameras, on
very-high-threat and high-threat Alaska volcanoes and on Mount Edgecumbe.110
110 The emerging technologies included in 41 U.S.C. § 31k(b)(2)(B) are “digital broadband seismometers, real-time
continuous Global Positioning System receivers, satellite and airborne radar interferometry, acoustic pressure sensors,
spectrometry to measure gas emissions, and unoccupied aerial vehicles.”
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Appendix A. Acronyms Used in This Report
Table A-1. Acronyms Used in This Report
AAWU
Alaska Aviation Weather Unit
AKWAS
Alaska Weather Alert Service
AVAAC
Anchorage Volcano Ash Advisory Center
AVO
Alaska Volcano Observatory
CWSU
Central Weather Service Unit
DOD
Department of Defense
EAS
Emergency Alert System
FAA
Federal Aviation Administration
FEMA
Federal Emergency Management Agency
GEFS
Global Ensemble Forecast System
GFS
Global Forecast System
ICAO
International Civil Aviation Organization
MWO
Meteorological Weather Office
NASA
National Aeronautics and Space
Administration
NESDIS
National Environmental Satellite, Data,
and Information Service
NOAA
National Oceanic and Atmospheric
Administration
NVEWS
National Volcano Early Warning and
Monitoring System
NWS
National Weather Service
QVA
Quantitative Volcano Ash
SEOC
State Emergency Operation Center
USGS
U.S. Geological Survey
VAA
Volcanic Ash Advisory
VAAC
Volcanic Ash Advisory Center
VAG
Volcanic Ash Graphic
VAN
Volcano Activity Notice
VEI
Volcanic Explosivity Index
VHP
Volcano Hazards Program
VONA
Volcano Observatory Notice for Aviation
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Appendix B. Glossary of Volcano Terms
Terms describing volcanic features, volcanic processes, and volcano hazards may vary based on
volcano type (stratovolcano or shield volcano), from place to place, and over time, as volcano
science advances. The following volcano terms are commonly applied to volcanoes in the United
States. Alaska’s active volcanoes are mostly stratovolcanoes and are different from Hawaii’s
active volcanoes, which are shield volcanoes. Nonetheless, Hawaiian terms, such as pahoehoe
may be applied to some of Alaska’s volcanoes. Volcano hazards that may harm people and the
environment and damage property include lava flows, lahars, pyroclastic flows, volcanic gases,
boiling water/fluid, steam, volcanic ejecta, and volcanic ash. Earthquakes and landslides are
related hazards as the volcano grows or as magma moves.
Glossary
Aa
Rubbly surface composed of broken lava blocks. Walking on such a flow is difficult
and may be painful.
Ash
Fine fragments (typically less than 2 to 4 millimeters or 0.08 to 0.16 inches) of
volcanic rock formed by a volcanic explosion or ejection from a volcanic vent.
Ashfall (Fallout or Tephra
Fall)
A rain of ash and volcanic debris that falls to the ground from an eruption.
Basalt
Volcanic rock or lava that is typically black to gray with few to no visible crystals
(i.e., minerals); rich in iron and magnesium; and lower in silica (silicon dioxide) than
most other volcanic rocks, such as andesites, dacites, or rhyolites. Basalt is
generally more fluid and darker in color than other volcanic rocks because it
contains less silica. Basalt forms from direct melting in the Earth’s subsurface and
the lava cools quickly on the surface, leaving little to no time for larger crystals that
are visible with an unaided eye to form in the rock. Basalt is the most common
rock type in the Earth’s crust, and most of the ocean floor is made of basalt. Flood
basalts are large volumes of lava that erupted over a few million years, such as the
Columbia River basalts covering parts of Washington, Oregon, and Idaho. The
Hawaiian Islands are composed mostly of basalt.
Caldera
A large, basin-shaped depression with a diameter many times larger than its depth
and included vents; may range from 2 to 50 kilometers (1 to 30 miles) across.
Commonly formed when magma is withdrawn or erupted from a shallow
underground magma reservoir. When large volumes of magma are removed, the
overlying rock collapses to form these large depressions. Kilauea and other
Hawaiian volcanoes have classic calderas.
Caldera Complex
A type of volcano consisting of a caldera with a volcanic field filled with eruptive
vents over a large and complex magma reservoir. The volcano does not have a
shield shape or cone shape but typically is recognized by its extensive caldera and
volcanic activity. The Yellowstone Caldera Complex covers about 43 miles by 28
miles on the surface. Other caldera complexes include Valles in New Mexico, Long
Valley in California, and Aniakchak Caldera, Fisher Caldera, and the Atka Volcanic
Complex in Alaska.
Crater
Small to large circular depression (typically a few hundred meters to tens of
kilometers or less than a mile to several miles across). Created primarily by
explosive excavation of rock during eruptions. A crater may become larger and
more irregular in shape due to multiple eruptions and volcano growth. Craters are
different from calderas in size, shape, and origin, but the terms may be applied
interchangeably.
Debris Avalanche
Moving masses of rock, soil, snow, and/or ice that occur when the flank of a
mountain or volcano collapses and slides downslope. Debris avalanches may
transform into lahars and travel tens of kilometers further away from the volcano.
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Effusive Eruption
An eruption dominated by the outpouring of lava flows onto the surface.
Ejecta (Tephra)
Material explosively ejected from a volcano. Any type and size of rock fragment
that is forcibly ejected and travels an airborne path. May include ash, bombs, and
scoria.
Explosive Eruption
An energetic eruption that produces mainly ash, pumice, and fragmental ballistic
debris.
Fire Fountain (Lava
Fountain)
Molten hot lava with yellow to orange colors that look like fire erupting in a shape
that resembles water squirting from a fountain.
Fumarole
Vents from which volcanic gas escapes into the atmosphere.
Geyser
A hot spring characterized by intermittent discharge of water ejected turbulently
and accompanied by vapor. Geysers are rare; there are a few thousand worldwide.
About half of Earth’s geysers are in Yellowstone National Park and are associated
with the Yellowstone Caldera Complex. Geysers require water, heat, magmatism,
rhyolite flows to supply heat and silica, and fractures/cavities that provide a conduit
to the surface.
Lahar (Volcanic Mudflow
or Debris Flow)
A mixture of water and volcanic debris that moves rapidly down a slope or
entrained in a stream. The consistency of the mixture can range from muddy
dishwater to wet cement. Lahars form by the rapid melting of snow and ice due to
pyroclastic flows, intense rainfall on loose volcanic rocks, breakout of a lake
dammed by volcanic deposits, or as a consequence of debris avalanches.
Lava Dome
A steep-sided mass of viscous and often blocky lava extruded from a vent; typically
has a rounded top and covers a roughly circular area. May be isolated or,
alternatively, associated with lobes or flows of lava from the same vent. Typically
silicic (rhyolite or dacite) in composition. Novarupta is a lava dome.
Lava Flow
Masses of molten rock that pour onto the Earth’s surface during effusive eruption.
Molten and solidified rock are referred to as lava flows. Lava flows vary in shape,
thickness, length, and width depending on the type of lava, the volume of the
eruption, the duration of the eruption, and the shape of the surface over which the
lava flows. The type of lava varies from the most fluid and least silica-rich to the
least fluid and most silica-rich—that is, from basalt to andesite to dacite and finally
to rhyolite.
Lava Tube
Conduits through which lava travels beneath the surface of a lava flow. Lava tubes
may be partially filled to empty and may erode, creating gaps or uneven surfaces.
Laze Plume
A white plume consisting of hydrochloric acid, steam, and fine volcanic glass
particles. Laze forms when hot lava hits the ocean.
Magma Reservoir
(Magma Chamber)
The zone beneath the surface of a volcano where molten rock is
concentrated/stored; the subsurface source of molten rock that produces lava
flows.
Pahoehoe
Hawaiian term for basaltic lava with a smooth, hummocky, or ropy surface. A
pahoehoe typically advances as a series of small lobes and toes that continually
break out from a cooled crust.
Pele’s Hair
Thin glass fibers (about a micrometer thick and can be as long as a few feet)
formed when bubbles of gas near the surface of a lava flow burst, stretching the
skin of the molten lava into long threads. The threads are so light, they can be
carried away from the volcano by the wind. Pele’s hair can accumulate in low-lying
areas and form dense mats. These tiny pieces of glass can cause harm if embedded
in skin, eyes, or elsewhere.
Phreatic Eruption
Steam-driven explosions of steam, water, ash, and volcanic blocks. Such explosions
may happen when water on or below the surface is heated by magma, lava, or hot
rocks, causing the water to boil and flash to steam
Pumice
Highly vesicular (i.e., full of holes) and silicic volcanic ejecta. Pumice essentially is
magma that has been frothed up by escaping gases and then cools and solidifies
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during eruption. Rhyolitic pumice is typically of low enough density that it floats on
water.
Pyroclastic Flow
A hot (greater than 800 degrees Celsius or 1472 degrees Fahrenheit), chaotic
mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per
second or tens of miles per hour) away from a volcanic eruption.
Rhyolite
Volcanic rock or lava that is typically white to light gray with few to some visible
crystals (minerals); rich in silica, sodium, and potassium; and higher in silica than
most other volcanic rocks, such as dacites, andesites, or basalts. Rhyolitic lavas are
viscous (i.e., not fluid but resistant to flow) and tend to form thick blocky lava
flows or steep-sided piles of lava called lava domes. Rhyolite magmas tend to erupt
explosively, commonly producing ash and pumice.
Shield Volcano
A broad, shield-shaped volcano that is built by successive, mostly effusive eruptions
of primarily basalt. Aa and pahoehoe lava flows are common. Some shield
volcanoes are topped with a caldera. The Hawaiian Islands are composed of shield
volcanoes.
Stratovolcano
A steep, cone-shaped volcano that is built by successive explosive and effusive
eruptions of basalt, andesite, dacite, and/or rhyolite. A stratovolcano may consist
of multiple vents, cones, domes and/or craters and is sometimes called a composite
volcano. Lava flows, pyroclastic flows, lahars, ash, pumice, phreatic explosions, and
other volcanic activity may occur. Most of Alaska’s volcanoes are stratovolcanoes.
The Cascade Range in California, Oregon, and Washington consists of mostly
stratovolcanoes.
Tuff
A general term for consolidated (hardened and/or compacted) pyroclastic rocks.
Vog
Volcanic smog consisting of gas, aerosol of tiny particles, and acidic droplets
formed when sulfur dioxide and other gases emitted from a volcano chemically
interact with sunlight, atmospheric oxygen, moisture, and dust. Vog and other
volcanic gas emissions can pose environmental and health risks. Vog is a hazard
that is most often associated with Hawaiian volcanoes, and there is a Hawaii
Interagency Vog Information Dashboard for more information and current
conditions.
Volcanic Gases
Gases emitted by a volcano may include (1) carbon dioxide, which may become
concentrated in low-lying areas and can be lethal to people and animals in high
concentrations; (2) sulfur dioxide, which can irritate eyes, skin, and the respiratory
system and can cause acid rain, air pollution, and vog; (3) hydrogen sulfide, which
can irritate the upper respiratory tract and with long exposure can cause
pulmonary edema or, with high concentration, death; and (4) hydrofluoric acid,
hydrochloric acid, or hydrobromic acid, which are toxic acids that may cause acid
rain or may be present in ashfall leading to poisoning of surface waters and lands
that may impact drinking water supplies, agriculture, grazing, and fishing.
Sources: U.S. Geological Survey (USGS), “Volcano Hazards Program Glossary,” https://volcanoes.usgs.gov/vsc/
glossary/; Shaul Hurwitz and Michael Manga, “The Fascinating and Complex Dynamics of Geyser Eruptions,”
Annual Review of Earth and Planetary Sciences, vol. 45 (2017), pp. 31-59, https://www.annualreviews.org/content/
journals/10.1146/annurev-earth-063016-015605; National Park Service, Hawaii Volcanoes National Park, “Pele’s
Hair,” https://www.nps.gov/havo/learn/nature/peles-hair.htm; USGS, “Caldera or Crater ... What’s the
Difference?,” https://www.usgs.gov/observatories/yvo/news/caldera-or-craterwhats-difference; USGS, “What Is
‘Vog’? How Is It Related to Sulfur Dioxide (SO2) Emissions?” https://www.usgs.gov/faqs/what-vog-how-it-relatedsulfur-dioxide-so2-emissions; USGS. “Volcanic Gases Can Be Harmful to Health, Vegetation, and Infrastructure,”
https://www.usgs.gov/programs/VHP/volcanic-gases-can-be-harmful-health-vegetation-and-infrastructure.
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Appendix C. Examples of Problems for Aircraft
from Volcanic Ash and Gas Hazards
During a volcanic eruption, volcanic ash with or without toxic volcanic gases, such as sulfur
dioxide, can reach and exceed the cruising altitude of turbine-powered airplanes within minutes
and may spread over large geographical areas within days. The volcanic risks to aircraft may be
immediate or delayed in time and space to the actual volcanic eruption that produced the volcanic
plume and cloud. Some of the problems an aircraft may encounter include the following:111
•
•
•
•
•
•
•
•
malfunction (or failure) of one or more engines, leading not only to reduction (or
complete loss) of thrust but also to failures of electrical, pneumatic, and hydraulic
systems;
blockage of pitot and static sensors resulting in unreliable airspeed indications
and erroneous warnings;
windscreens rendered partially or completely opaque;
smoke, dust, and/or toxic chemical contamination of cabin air requiring crew use
of oxygen masks, thus impacting communications (electronic systems also may
be affected);
erosion of external and internal aircraft components;
reduced electronic cooling efficiency leading to a wide range of aircraft system
failures;
need for aircraft to be maneuvered in a manner that conflicts with other aircraft;
and
deposits of volcanic ash on a runway degrading braking performance, most
significantly if the ash is wet; in extreme cases, this can lead to runway closure.
Author Information
Linda R. Rowan
Analyst in Natural Resources and Earth Sciences
111International Civil Aviation Organization, Volcanic Ash Contingency Plan, European and North Atlantic Regions,
EUR Doc 019, NAT Doc 006, Part II, January 2024.
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