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October 7, 2019

Let's ditch the overused, misrepresentative, and misapplied "supervolcano" term. Instead, let's call them "caldera systems."

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

Photo of Huckleberry Tuff at Golden Gate, Yellowstone National Park...
Only a few kilometers from Mammoth Hot Springs, this is one of the most accessible and widely seen exposures of the Huckleberry Ridge Tuff, which erupted during the first volcanic cycle of Yellowstone about 2.1 million years ago. This exposure of the Huckleberry Tuff is located about 20 km north from the rim of the caldera that formed as a consequence of its eruption. The Huckleberry Ridge Tuff is also found on top of Mount Everts, the broad peak 6 km in the distance. Based on the distribution of the tuff, scientists infer a broad river valley probably existed between here and Mount Everts and the source of the eruption. The tuff filled or partly buried the valley.

I have a confession to make. I really don't like the term "supervolcano." And I'd like to use this week's edition of Yellowstone Caldera Chronicles to rant about the topic. I'd also like to propose that we use a different term.

The first known use of "supervolcano" is actually from the mid-1900s. In 1925, geologist Edwin Hodge proposed that the Three Sisters volcanic region of central Oregon was actually the site of one very large volcano, which he called "Mt. Multnomah." This idea was later disproven by geologist Howell Williams (the same scientist who deciphered the geologic history of Crater Lake in Oregon), and a review of Williams's 1948 book on Oregon volcanoes referred to Hodge's Three Sisters hypothesis as a "supervolcano."

The term lay dormant (pun intended) for decades, and was mostly absent from the scientific literature until the 2000s. The term "super eruption" had been used, however, to describe some of the largest known eruptions on Earth, like that of Toba, Indonesia, 74,000 years ago.

By the early 2000s, "supervolcano" started creeping into scientific articles. More general and widespread use of the term exploded (so to speak) following the 2005 release of the British-Canadian docudrama "Supervolcano," a disaster television film that centered around a hypothetical large eruption of Yellowstone.

Ever since, use of "supervolcano" has, ahem, blown up. (Okay, okay, I'll stop with the volcano puns now.)

It seems innocent—is there really a downside to the term? After all, it does conjure an eruption of incredible size, which is something that modern humans have (fortunately) never witnessed. But it also oversimplifies the process and causes misunderstanding.

I have three main reasons for disliking the term.

Map of the known ash-fall boundaries for several U.S. eruptions
Eruptions of the Yellowstone volcanic system have included the two largest volcanic eruptions in North America in the past few million years; the third largest was at Long Valley in California and produced the Bishop ash bed. The biggest of the Yellowstone eruptions occurred 2.1 million years ago, depositing the Huckleberry Ridge ash bed. These eruptions left behind huge volcanic depressions called "calderas" and spread volcanic ash over large parts of North America (see map). If another large caldera-forming eruption were to occur at Yellowstone, its effects would be worldwide. Thick ash deposits would bury vast areas of the United States, and injection of huge volumes of volcanic gases into the atmosphere could drastically affect global climate. Fortunately, the Yellowstone volcanic system shows no signs that it is headed toward such an eruption in the near future. In fact, the probability of any such event occurring at Yellowstone within the next few thousand years is exceedingly low.

First, it's trite. Remember back in the 2000s when people used "uber" in front of a word to mean "very"? The pizza wasn't just delicious, it was uber-delicious. The summer wasn't hot, it was uber-hot. It was so uber-annoying! Fortunately, the fad faded. The same can be said for "supervolcano." Adding "super" boils a complex and important aspect of volcanology down into something that sounds like a catch phrase. Ay carumba!

Second, it's misleading. Calling something a "supervolcano" makes it sound like a volcano that only has massive eruptions. Of course, this is not true. Most Yellowstone eruptions that involve magma reaching the surface are lava flows. In fact, there have been about 80 lava flows of varying compositions in and around Yellowstone since the last time the system experienced a catastrophic explosion. Yellowstone is a lot more than just explosions, and calling it a "supervolcano" is a gross oversimplification.

Third, it's misapplied. Volcanologists have come to refer to super eruptions as those that have generated 1000 km3 of ash and other volcanic products. This is equivalent to an "8" on the "Volcano Explosivity Index" scale, which is sort of like a Richter scale for volcanic eruptions. That means a VEI=8 eruption generated 10 times more material than a VEI=7 eruption, and 100 times more than a VEI=6 eruption. For reference, the 1980 Mount St. Helens eruption was VEI=5. The 1991 eruption of Pinatubo, Philippines, was VEI=6. So a VEI=8 is truly epic. Why is it, then, that volcanoes that have never had VEI=8 eruptions are called "supervolcanoes"? For example, the largest eruption of Campi Flegrei, Italy, occurred about 39,000 years ago and was VEI=7. Yet Campi Flegrei is also often called a "supervolcano." Is it a supervolcano if it has never had a super eruption?

My wise colleague, Dr. Jamie Farrell, assistant research professor at the University of Utah and chief seismologist of the Yellowstone Volcano Observatory, likes to say that "there are no supervolcanoes, only volcanoes that have super eruptions." I couldn't agree more.

So I have a suggestion. Let's ditch the overused, misrepresentative, and misapplied "supervolcano" term. Instead, let's call them "caldera systems." This would refer to any volcano that has experienced an explosion massive enough that the surface has collapsed into a partially emptied magma chamber. Campi Flegrei, Crater Lake (Oregon), and Yellowstone would all qualify. And if you must use "super," use it when referencing specific eruptions—like the massive explosion from Yellowstone 631,000 years ago. That was a VEI=8 super eruption that occurred from a caldera system. See? Doesn't that sound better?

Okay, rant over. Have a super day!

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