Which U.S. volcanoes pose a threat?

·  In 2009, more than 300 flights were canceled, and Ted Stevens Anchorage International Airport was shut down when Redoubt Volcano erupted clouds of volcanic rock and ash. Redoubt ranks in the very high threat category.

The effects of eruptions are usually greatest near the volcano but can also extend far downstream, downwind, and can even persist over time. Airborne ash clouds have caused hundreds of millions of dollars of damage to aircraft and nearly brought down passenger jets flying in U.S. and international airspace. Ash falls have caused agricultural losses and disrupted the lives and businesses of hundreds of thousands of people in Washington and Alaska. In California, noxious gas emissions have resulted in direct loss of life, and in Hawai’i, have given rise to widespread human respiratory ailments.

What makes a volcano dangerous?
Twenty-four factors are used to evaluate U.S. volcanoes and people’s exposure to volcanic hazards.

The geographic footprint of U.S. volcanoes is large, extending from arctic Alaska in the north to tropical American Samoa south of the Equator; from Colorado to the Commonwealth of the Northern Mariana Islands in the western Pacific. The tall stratocones and submarine volcanoes erupt in diverse ways and produce different hazards, so the risks to humans are not equivalent from one volcano to another. A periodic assessment of hazards and the populations likely to be affected in a future eruption helps prioritize efforts where they are needed most.

The updated Volcanic Threat Assessment combines 24 criteria (15 hazard factors and 9 exposure factors) that describe an individual volcano’s hazard potential and the exposure of people and property to those hazards. The hazard factors include volcano type, eruptive history, explosiveness, time between eruptions, types of hazards from past eruptions and effects of the hazards. Also included is an analysis of what the volcano is doing at present, with a focus on seismicity, ground deformation and degassing.

The exposure factors include population within 30 kilometers (18 miles) of the volcano; visitation numbers if the volcano is located in a national park or monument; population beyond 30 kilometers (18 miles) if a far-traveling lahar is a primary hazard; prior eruption fatalities; prior evacuations; aviation impacts, either to the local airport or to regional air transportation routes; impacts on power and transportation infrastructure; and major developments such as parks.

USGS scored the hazard and exposure factors for each volcano. Based upon the scores, the volcanoes were grouped into one of five threat categories: very low, low, moderate, high and very high.

What’s new since 2005?
A total of 161 volcanoes are judged to be potential threats, eight fewer than in 2005.

In 2018, as in 2005, Kīlauea ranked as the U.S. volcano with the highest threat score. Kīlauea is the most active U.S. volcano; it erupts fluid lava flows but is also capable of explosive eruptions. The eruptive activity in 2018 and the destruction of residential subdivisions on its flanks are clear examples of why Kīlauea is a very high threat volcano.

Eleven of the eighteen very high threat volcanoes are in Washington, Oregon or California, where explosive and often snow- and ice-covered volcanoes can project hazards long distances to densely populated and highly developed areas. Five of the eighteen are in Alaska near important population centers, economic infrastructure or below busy air traffic corridors.

Several volcanoes were added to the 2018 assessment, notably these: Salton Buttes, California (high threat); Soda Lakes, Nevada (moderate threat); and Tana, Alaska (low threat). Six additional volcanoes from the Commonwealth of the Northern Mariana Islands, along with three volcanoes from American Samoa, were also added to the assessment. Volcanoes were also removed from the list after more sophisticated age-dating techniques found a lack of eruptive activity over the past 11,000 years.

The updated rankings help prioritize resources
The focus is on reducing risks at those volcanoes with the greatest potential to create disruptions in local and global communities.

USGS says that the new assessment supports USGS efforts in risk research and applications – information that can directly support decision makers in their efforts to better understand societal risks from hazards and manage risk in their communities.

The threats to communities, property and infrastructure drive the need to decipher past eruptive behavior, monitor the current activity and mitigate the damaging effects of volcanic eruptions. “The threat assessment helps guide the decision-making process about where to build or how to strengthen volcano monitoring networks and where more work is needed on emergency preparedness and response,” said Ewert.

The USGS Volcano Hazards Program monitors and studies active and potentially active volcanoes, assesses their hazards and conducts research on how volcanoes work to issue timely warnings of potential volcanic hazards to emergency-management professionals and the public. Subscribe to the Volcano Notification Service for customized emails about volcanic activity at U.S.-monitored volcanoes.

— Read more in John W. Ewert et al., 2018 update to the U.S. Geological Survey national volcanic threat assessment (USGS, 2018)