When it comes to ranking the threat levels of volcanoes, one size does not fit all. All volcanoes pose a risk in some way or another, but those risks depend on a variety of factors including (but not limited to) distance to human settlements, pyroclastic flows, and infrastructure downstream and downwind.
For the first time since 2005, the US Geological Survey has updated its assessment of the most threatening volcanoes based on 24 hazard factors. The 2018 update uses new resources to determine which volcanic systems should be added or removed. Of a total of 161 volcanoes, 12 volcanoes have moved up in threat level and 20 others have dropped.
All of the 18 volcanoes designated as a “very high threat” are found on the West Coast, 11 of which are in Washington, Oregon, or California where "explosive and often snow- and ice-covered edifices can project hazards long distances to densely populated and highly developed areas," according to the report.
Five of the 18 most dangerous volcanoes are in Alaska near important population centers, economic infrastructure, or below busy air traffic corridors. The remaining two volcanoes are on the Hawaiian Islands, where population and developed areas are built along the flanks of active volcanoes.
Active volcanoes are those that have erupted during the Holocene (the last 11,700 years) or those in long-lived Caldera systems like Yellowstone. Home to 10 percent of the world’s known volcanoes, the US is one of Earth’s most volcanically active countries. From Arctic Alaska to tropical American Samoa, the nation has seen some of the most dramatic eruptions in modern history, including the lava flows expelled earlier this year by Hawaii’s Kilauea and the engine-halting ash from the Redoubt Volcano in Alaska to the devastating 1980 eruption of Mount Saint Helens.
The threat assessment places volcanoes in the US into five threat categories: very low, low, moderate, high, and very high.
“Within all five threat categories there are changes in relative rankings of volcanoes, and in a few cases, volcanoes moved between categories owing to changes in our understanding of their hazard, unrest, and exposure factors,” reads the report.
It's important to note that this report doesn't mention which volcanoes will erupt next or how well they are monitored, but instead ranks the potential of these volcanoes to cause damage.
Now, without further ado, the “very high threat” volcanoes are: