Yellowstone's supervolcano may have become less intense, suggests discovery of two ancient eruptions

Scientists found two super-eruptions, 8.99 and 8.72M years ago, which show the volcano on average erupted once every 500,000 years but over the past 3M years, the hotspot has seen just two supereruptions

                            Yellowstone's supervolcano may have become less intense, suggests discovery of two ancient eruptions
Fountain Paint Pot (National Park Service)

In May this year, over 280 earthquakes were centered in Yellowstone National Park, according to the US Geological Survey. A new study, however, suggests that the Yellowstone hotspot's activity may finally be declining and estimates that there may have been a three-fold decrease in its capacity to produce super-eruption events.

Yellowstone is among the most seismically active places in the US and sees around 700 to 3,000 earthquakes every year, but most are not felt. The Yellowstone National Park sits above three calderas (volcanic craters), which were the result of a series of supermassive volcanic eruptions. Yellowstone is known as a supervolcano because it has had at least one eruption in which 1,000 km3 or more of material erupted, which is also the threshold for the highest magnitude 8 eruption on the Volcano Explosivity Index (VEI). A research team has now discovered two volcanic super-eruptions associated with the Yellowstone hotspot track, which occurred millions of years ago, including what they believe was the volcanic province's "largest and most cataclysmic event". The results indicate the hotspot, which today fuels the famous geysers, mud pots and fumaroles in Yellowstone National Park, maybe waning in intensity, and that massive eruptions are now occurring way less frequently than they once did.

The two events have been named the McMullen Creek eruption, occurring about 8.99M years ago, and the Grey's Landing eruption, occurring about 8.72M years ago. The team used a combination of techniques, including bulk chemistry, magnetic data and radio-isotopic dates to correlate volcanic deposits scattered across tens of thousands of square kilometers. "We discovered that deposits previously believed to belong to multiple, smaller eruptions were, in fact, colossal sheets of volcanic material from two previously unknown super-eruptions at about 9.0 and 8.7 million years ago," says lead author of the study Thomas Knott, a volcanologist at the University of Leicester, in the analysis published in Geology.

The newly-identified super-eruptions occurred during the Miocene, a period spanning 23 to 5.3M years ago. These two new eruptions bring the total number of recorded Miocene super-eruptions at the Yellowstone-Snake River volcanic province to six. According to the experts, this implies that the recurrence rate of Yellowstone hotspot super-eruptions during the Miocene was, on average, once every 500,000 years. In comparison, over the past three million years, the Yellowstone hotspot has experienced just two super-eruptions, an average of one every 1.5M years or so. "It, therefore, seems that the Yellowstone hotspot has experienced a three-fold decrease in its capacity to produce super-eruption events. This is a very significant decline," says Knott. 

Morning Glory Pool, Yellowstone National Park (Jim Peaco for the National Park Service)

The younger of the two, the Grey's Landing super-eruption, is now the largest recorded event of the entire Snake-River-Yellowstone volcanic province. Based on the most recent collations of super-eruption sizes, the scientists say that it is one of the top five eruptions of all time. The team, which also includes researchers from the British Geological Survey and the University of California, Santa Cruz, estimates the Grey's Landing super-eruption was 30% larger than the previous record-holder (Huckleberry Ridge Tuff) and had devastating local and global effects. According to the analysis, the Grey's Landing eruption covered an area the size of New Jersey in "searing-hot volcanic glass that instantly sterilized" the land surface — somewhere in the region of 23,000 square kilometers. The study describes that anything located within this region would have been buried and most likely vaporized during the eruption.

Super-eruptions are among the most extreme events to affect Earth’s surface, but too few examples are known to assess their global role in crustal processes and environmental impact. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization, says the research team. "This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning," they add.

The study grew out of a larger project investigating the productivity of major continental volcanic provinces. The findings, however, do not have much bearing on analyzing the risk of another supereruption occurring currently in Yellowstone. According to the researchers, the last super-eruption there was 630,000 years ago, suggesting we may have up to 900,000 years before another eruption of this scale occurs. However, the team cautions this estimate is far from exact and emphasizes that continuous monitoring in the region, which is being conducted by the US Geological Survey, "is a must". 

During May 2020, the University of Utah Seismograph Stations, responsible for the operation and analysis of the Yellowstone Seismic Network, located 288 earthquakes in the Yellowstone National Park region. The largest event was a minor earthquake of magnitude 3.1 located 5 miles west of Norris Junction in Yellowstone National Park on May 29. "Earthquake sequences like these are common and account for roughly 50% of the total seismicity in the Yellowstone region. Yellowstone earthquake activity remains at background levels," says the US Geological Survey.

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