As part of its program of keeping officials and the public apprised of volcanic threats, the USGS periodically reassesses the threat level of U.S. volcanoes and updates volcanic threat assessment documentation.
In 2005, the USGS published its first national volcanic threat assessment in support of establishing a National Volcano Early Warning System. The assessment helps to prioritize U.S. volcanoes for research, monitoring and mitigation efforts based on objective measures of volcano hazards and exposure of people and infrastructure to those hazards. The new publication is an update of that assessment.
“More than ten percent of the world’s known active and potentially active volcanoes are within U.S. territories,” said John Ewert, a USGS geologist and lead author of the updated Volcanic Threat Assessment. “All of these volcanoes pose some degree of risk to people and infrastructure.”
Ewert and his USGS colleagues applied specific criteria to each volcano to derive a score, and grouped volcanoes into five threat categories. “The ranking is not a list of which volcano will erupt next,” said Ewert; “instead, it is a way to help focus attention and resources where they can be most effective.”
The United States is one of Earth’s most volcanically active countries.
Since 1980, there have been 120 eruptions and 52 episodes of notable volcanic unrest at 44 U.S. volcanoes.
The mountainous landscapes of California, Oregon, Washington, Alaska, Hawaiʻi, American Samoa and the Mariana Islands are punctuated by volcanoes. Born along the margins of converging tectonic plates or above intraplate hot spots, these volcanoes are a part of our country’s history and culture. But when erupting, these same volcanoes threaten the health and safety of residents and can damage property and infrastructure.
- In 2018, more than 700 structures were destroyed when swift-flowing lava erupted from fissures in Kīlauea Volcano’s lower East Rift Zone. Lava covered 35.5 square kilometers (13.7 square miles), which included homes, farms, wild spaces, roads, highways and critical infrastructure. Kīlauea is ranked as the U.S. volcano with the highest threat score in the very high threat category.
- In 1980, a powerful explosion at Mount St. Helens devastated huge tracts of forest and killed people tens of miles from the volcanic source, while debris avalanches and mudflows choked major rivers and destroyed bridges. Mount St. Helens is ranked a very high threat volcano.
- 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.
This 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.