Editor’s note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution comes from Mark Stelten, U.S. Geological Survey Research Geologist and Deputy Scientist-in-Charge of the Yellowstone Volcano Observatory.

Geologists at the Yellowstone Volcano Observatory are often asked to predict how likely future eruptions are in Yellowstone, but that’s no easy task in the national park.

People visit Yellowstone National Park every year to observe wildlife and its wide variety of hydrothermal features. The question lurking in the minds of many visitors as they pass through one of the world’s largest active volcanoes is: When will Yellowstone’s next eruption occur? When a volcano is dormant, this question can be resolved by examining trends in monitoring data such as seismicity, ground deformation, and gas emissions. But what about extinct volcanoes like Yellowstone that show no signs of activity anytime soon?

For volcanoes that are not currently active, we generally do not predict dates of future eruptions, but instead estimate the probability of an eruption occurring within a certain time period (for example, within the next year or the next 10 years). This is kind of like long-term weather forecasts; for example, predicting the likelihood of more hurricanes in the upcoming hurricane season than the average year.

Predictions of volcanic eruptions are based, to some extent, on knowledge of the frequency of occurrence of eruptions at a particular volcano. As an analogy, let’s say you live next to a baseball field and want to get a feel for the next time a baseball is thrown into your yard. One way to estimate this is to calculate the average recurrence rate by dividing the number of baseballs in your yard by the duration of your observation period (e.g. 1 year) to obtain the annual number of baseballs in your yard. This average recurrence rate is then calculated for the next day, week, month, etc. can be translated into the probability of a baseball hitting your yard. Similarly, predicting volcanic eruptions requires knowing the number of eruptions that occur over time. Geologists do this by geological mapping. geochronology To determine the eruption history of a volcano.

Map of the Yellowstone caldera showing the locations and ages of the most recent rhyolite eruptions in Yellowstone, Central Plateau Member rhyolites. Unit boundaries are taken from Christiansen (2001). The West Thumb area of ​​Yellowstone Lake is noted because it is thought to be the site of an explosive eruption and the source vent of the Bluff Point Tuff. Central Plateau Member rhyolites have been divided into five unofficial groups based on recent 40Ar/39Ar eruption ages. Each group of unofficial bursts is shown in the same color. The numbers and descriptions on the map have been added to indicate the location of different lava flows. Group mean ages and their 95% confidence intervals are located next to the unit list.

Knowing the average rate of volcanic eruptions is just the beginning. Geologists also need to understand whether volcanic eruptions are one-off events that occur independently of other eruptions, or whether they occur in groups as part of a larger volcanic event. Going back to the baseball analogy, since baseball is only played during certain times of the year, baseballs are much more likely to be hit in your yard during the baseball season rather than the off-season. Recent research has shown that many volcanic systems, including Yellowstone, operate in a similar way. multiple explosions occurring in rapid successionseparated by long periods of little or no eruption. To accurately predict volcanic eruptions, this “grouping” of eruptions needs to be well characterized.

Schematic summary of rhyolite eruptions in the Yellowstone Plateau volcanic field over the last 1.3 million years. Smaller rhyolite eruptions are known as intra-caldera eruptions; this means that they occurred within existing caldera structures. Additional rhyolite eruptions occurring outside the caldera are not included in this figure.

Determining the speed and pattern of volcanic eruptions is only part of the job. Once the history of volcanic eruptions is known over time, the next task is to try to understand where the volcano currently stands in terms of its life cycle. Going back to the baseball analogy one last time, it’s like trying to figure out whether it’s currently baseball season or off-season. The challenge with places like Yellowstone is that they produce large but infrequent eruptions; Eruption periods are thousands to hundreds of thousands of years apart (where an episode may contain one or more eruptions). This means that there are very few observations on which to base our prediction, and (fortunately) there are not many opportunities to test these predictions. For example, there have been no eruptions in Yellowstone National Park for the last 70,000 years. From 160,000 years ago to 70,000 years ago, rhyolite lava flows (or groups of lava flows) erupted on average every 20,000 years. Does this mean we are currently out of volcanic season? Or does this mean that we have “the right” to have an explosion (By the way, that’s never really true)? The truth is that we can’t say for sure based on statistical estimation methods alone. Instead, we must combine such predictions with real-time monitoring of the volcano to assess the state of the volcanic system.

Based on our current knowledge of Yellowstone’s eruption history, the annual probability of a volcanic eruption is on the order of 0.001%, but even this low number is likely an overestimate in the short term. According to monitoring data, there is no sign of an impending volcanic eruption and we know that The magmatic system beneath Yellowstone is mostly solid. But one day, perhaps thousands or tens of thousands of years from now, Yellowstone’s volcanic offseason may end, and volcanologists will watch for signs of incoming baseballs.

Panoramic view of the West Yellowstone rhyolite lava flow taken along Highway 20 (between the West entrance of Yellowstone National Park and Madison Junction). The stream is approximately 111,000 years old and has a volume of approximately 41 km3 (10 mi3).