An inlet of Lake Toba in Indonesia, surrounded by tropical vegetation
Lake Toba in Indonesia is the world’s largest volcanic lake. New research has found that the supereruption that created it 74,000 years ago was followed by smaller eruptions for thousands of years afterward. Credit: Adonara Mucek

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Lake Toba in Sumatra, Indonesia, is the world’s largest volcanic lake, the result of the Youngest Toba Tuff supereruption, which happened 74,000 years ago. And it was a biggie, even for a supereruption.

“The region around Toba for about 100 kilometers in [any] direction was completely inundated with ash,” said Shanaka de Silva, a professor at Oregon State University. “Then the ash went into the oceans on either side, and that generated these big plumes of ash that went up into the atmosphere and then got distributed regionally.”

But it didn’t end there. De Silva was part of new research that found there’s life after a supereruption. By studying resurgence, the poorly understood period of recovery following a supereruption, the researchers found that for thousands of years after the supereruption, the Toba system produced thousands of smaller eruptions, even though the volcano had no hot magma. This means these smaller volcanoes were erupting with semisolid material that had been kept in cold storage, and the volcanic system’s period of resurgence was much longer than previously thought. The study was published in Communications Earth and Environment.

Dating Finesse

A geologist uses two rock hammers to dig into a cliff face.
Shanaka de Silva and other volcanologists conducted fieldwork at Lake Toba between 2012 and 2016. Credit: Adonara Mucek

Previous dating showed that the supereruption and smaller eruptions were of the same age, said de Silva—but the geological evidence said otherwise. The problem was that traditional argon-argon dating techniques weren’t accurate enough to measure (geologically) small time differences, he said.

To get more accurate ages, the researchers used a technique new to volcanology using uranium-thorium-helium thermochronology on samples of feldspars and zircons collected at Toba. They found that helium in samples from the smaller eruptions was younger by up to 13,000 years compared with argon-argon ages. The cold, solid material, not liquid magma, that these smaller eruptions sent out had congealed at the upper part of the magma chamber and had been stored for 5,000–13,000 years at 180°C–500°C. Magma is liquid at around 700°C.

“The really exciting part is the thermochronological approach, which is normally applied to plate tectonics and so on, [applied to] to volcanology,” said de Silva. “It really shows that the modern geochronology techniques can provide insight into a temperature range that is not available to normal investigation techniques in use.”

Cold(er) Storage

Patricia Gregg, an associate professor of geophysics at the University of Illinois at Urbana-Champaign who was not involved with the research, said it’s a compelling and well-done study that could cause a reconsideration of what material is considered eruptible. The most important aspect of the study, she said, is how low it found the volcano’s temperatures to be.

“A lot of material was evacuated from the magma system, and then it just sat there and cooled. And really, what was left behind was this crystal-rich gloop that you just wouldn’t expect to erupt.”

“I think it’s really wild. Because in my mind and a lot of people’s minds, we just assume that there’s going to be this sort of constant flux of magma into the system,” she said. “But now it’s, you’ve had this very large eruption, a lot of material was evacuated from the magma system, and then it just sat there and cooled. And really, what was left behind was this crystal-rich gloop that you just wouldn’t expect to erupt.”

Others are also positing that these systems are much cooler than previously thought, added Gregg. This research is “kind of falling in line with some other really interesting papers that have come out in the last few years,” she said.

Other volcanic sites are supporting the idea that eruptible magma doesn’t need to be hot to move. “At Mount St. Helens and some other places now, we’re finding evidence that solid material can actually be erupted,” said de Silva.

But, said de Silva, these magma systems can’t truly be described as either warm or cold because they exist across a spectrum of temperatures. “You have the cold part of the magma chamber and then you have the warm part of the magma reservoir,” he said.

Gregg said follow-up research should include why this crystal-rich viscous magma erupted. “I think that’s a question for modelers like me: How do you mechanically get this stuff out? Here we have evidence of this stuff coming out, it’s very cold, it’s very viscous, and it’s very crystal rich. How did it erupt?” asked Gregg.

De Silva and his fellow researchers plan to expand their work at Toba to examine more areas and get a better understanding of the supereruption time frame. Samosir, a large island in Lake Toba, is home to about 100,000 people and is a resurgent dome that’s being pushed upward. Mount Sinabung, a nearby volcano that previous research suggested is part of the Toba system, began erupting in 2010 and has remained active ever since. (Its most recent eruption phase began in July.)

“The information that we’re getting is helping us understand how the supereruption cycles actually work,” said de Silva.

—Danielle Beurteaux (@daniellebeurt), Science Writer

Citation: Beurteaux, D. (2021), Life after a supereruption, Eos, 102, Published on 27 October 2021.
Text © 2022. The authors. CC BY-NC-ND 3.0
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