The USGS volcano threat assessment -- how was it determined, and what does it mean for Yellowstone

First, let's review what the assessment is and what it isn't. The assessment is NOT a list of which volcanoes are most likely to erupt, nor is it a ranking of the most "active" volcanoes. Instead, the assessment is a quantification of the relative threat posed by the volcanoes in the United States. Threat is defined as the combination of a volcano's hazard potential, and the exposure of people and property to those hazards. In other words, a volcano that only erupts lava flows but doesn't have anyone living on it has very low threat, since even though there is a hazard (lava), there are no people or property at risk from that hazard. A volcano that might experience only small explosions but that is surrounded by towns and near an airport has a higher threat, since lots of people and property are exposed to the hazard (even if the hazard might be a relatively small one).

Another important definition is "active volcano." What does it mean to be "active"? Both the original 2005 report and the 2018 update define "active" to mean that the volcano has experienced an eruption in the past 11,650 years—the Holocene geological epoch. How then, could Yellowstone even be considered for a ranking, given that the last time magma reached the surface at Yellowstone was 70,000 years ago?

It turns out that, in addition to volcanoes that have erupted in the Holocene, the report also considers caldera systems that show unrest—for example, earthquake activity, ground deformation, and/or gas discharge—even if they have not erupted recently. There are three such caldera systems in the USA: Valles caldera, New Mexico (which last erupted over 50,000 years ago), Long Valley caldera, California (last eruption was over 15,000 years ago), and Yellowstone.

Now that we have established which volcanoes are considered, we need to address how "scores" are tabulated. In the report, 24 factors that describe a volcano's hazard potential and the exposure of people and property to those hazards are considered. The hazard factors include such categories as the size of the largest explosion to have occurred at the volcano, the average recurrence of eruptions, what types of eruptions have taken place, and whether or not the volcano shows signs of unrest. Exposure factors include nearby population, nearby aviation activity, and nearby infrastructure (like power and transportation resources).

The overall threat score is determined by multiplying the sum of the hazard factors by the sum of the exposure factors. The top three volcanoes, in order, are Kilauea (Hawaii), Mount St. Helens (Washington), and Mount Rainier (Washington). A general categorization was also introduced – "very high threat," "high threat," "moderate threat," "low threat," and "very low threat."

In 2005, Yellowstone was ranked #21 in the threat assessment. Now for the big reveal…in 2018…Yellowstone was ranked…#21! This qualifies Yellowstone as a "high threat" volcanic system. This is not a change or any sort of upgrade in the threat posed by Yellowstone, and carries no implications for future eruptive activity. It is merely a way to provide a sense of Yellowstone's threat level compared to other volcanoes in the USA.

Relative to the other US calderas, Yellowstone lies in the middle. Long Valley caldera is #18 ("very high threat") and Valles caldera is #68 ("moderate threat").

Despite the fact that Yellowstone has not experienced any magmatic eruptions in 70,000 years, the system reached its lofty ranking (compared to other volcanoes in the country) because of the long-past history of very large explosions, more recent history of steam explosions, observed seismic, deformation, and degassing activity, and the presence of a population (over 4 million people visit Yellowstone National Park each year).

The threat ranking is intended as a guide in terms of which volcanoes should be prioritized for upgrades in monitoring capabilities. Yellowstone is already among the best-monitored volcanoes in the world, but we expect that the upgraded threat assessment will be helpful in refining the monitoring plan, which is due for revision.

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If you have any questions about the threat assessment or how Yellowstone fits in to the 2018 report, feel free to contact us any time at yvowebteam@usgs.gov.