Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Wendy Stovall, volcanologist with the U.S. Geological Survey and Deputy Scientist-in-Charge of the Yellowstone Volcano Observatory.

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Here at YVO we receive a lot of questions related to Yellowstone supereruption "what ifs" and "whens", even though that is the least possible scenario for future volcanic activity. News articles, websites, and videos often exaggerate the rarest events, while ignoring hazards that may actually happen during a person's life. So right here, right now, we're going to flip this around and discuss Yellowstone's geologic hazard scenarios in order from most to least likely.

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1. Hydrothermal explosions are the most likely of Yellowstone's volcano hazards. With little-to-no warning, thermal features can erupt as rock-hurtling geysers that excavate shallow craters. Smaller explosions occur more often and can impact an area tens of feet in diameter, whereas less-frequent, larger eruptions could send hot water and rocks a few thousand feet.

   On average, one hydrothermal explosion occurs every two years at Yellowstone. Because these events are typically small and can occur any time, including when few visitors are present, the likelihood of harm to any individual is reduced. A famous example is the 1989 explosion of Porkchop geyser in the Norris Geyser Basin.

   Did you know? Hot springs have injured or killed more people in Yellowstone than encounters with wildlife. Boardwalks and trails are there to protect visitors, and off-boardwalk hiking is very dangerous - scalding water underlies most of the thin, breakable crust around hot springs.

2. 

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   Damage from large earthquakes (in the magnitude 7 range) ranks as the second most likely Yellowstone hazard. As discussed last week, there are many faults within the Yellowstone Plateau. A couple thousand earthquakes occur every year, but 99% of those are magnitude 2 or below and are undetectable by humans. Earthquakes within the caldera max out around magnitude 6 owing to the nature of the faults and rocks. However, outside the caldera, regional faults associated with Basin and Range extension are capable of producing large earthquakes, like the M7.3 1959 Hebgen Lake event.

   Such large earthquakes can occur once or twice per century, but like all other geologic hazards, they do not operate on a schedule.

3. It's not until third place that magma is the culprit for creating a hazard – lava flows. Almost all of the past few dozen or so magma-generated Yellowstone eruptions were lava flows interspersed with a few moderate volcanic-ash eruptions. The individual rhyolite lava flows are some of the most massive on Earth - they reach 30 km (19 mi) from their source vents and can have thicknesses greater than 400 m (1300 ft, or the height of the Empire State Building)! The eruptions occurred as groups, clustered in time, separated by non-eruptive periods. Yellowstone's most recent volcanic eruption was a lava flow that formed the Pitchstone Plateau 70,000 years ago. Since the last "supereruption", rhyolite lava has filled in much of the Yellowstone caldera—the total flow volume is around 600 km³ (140 mi³), which would fill the Dallas Cowboy's AT&T stadium 200,000 times!!

   If future volcanic activity at Yellowstone led to an eruption, it would probably result in a lava flow or, less-likely, moderate ashfall. However, we're currently in an eruption hiatus, and history tells us these types of eruptions are clustered events. Plus, current monitoring data shows that volcanic activity at Yellowstone is not increasing.

4. Of all the possible volcano hazard scenarios for Yellowstone, by far the least likely includes another major explosive caldera-forming eruption. This is certainly the worst-case scenario for Yellowstone, but the chances of it happening in our lifetime are, literally, one-in-a-million. At some point in the future, a supereruption might occur somewhere else on earth. But we may be waiting tens of thousands of years before another such event (worldwide), and it's possible that a supereruption may never happen again at Yellowstone. We'll leave it at that…

At YVO, we hear from people who are literally losing sleep with worry over a possible super eruption. But, this limited perspective misses the bigger picture of Yellowstone hazards: The one event that most people are aware of—a massive explosion—is also the least likely to occur, by far. Even lava flows or moderate ash eruptions are unlikely. On the scale of human lives, the most likely hazards are small hydrothermal explosions, like the September 2018 eruption of Ear Spring, or a strong earthquake, like the M7.3 1959 Hebgen Lake event. The spectrum of Yellowstone activity is much more complex than just rare catastrophic eruption!