Hidden magma pools pose eruption risks we cannot yet detect – sciencedaily

The ability of volcanologists to estimate the risk of eruption largely depends on knowledge of the places where magma puddles are stored, deep in the earth’s crust. But what if the magma cannot be spotted?

Shane Rooyakkers, postdoctoral researcher at GNS Science in New Zealand, grew up in the shadow of Mount Taranaki on the country’s north island, hiking the island’s many volcanoes. Today, his research reveals hidden dangers that may have been under his feet from the start.

A new study, published yesterday in Geology, explores a threat recently discovered by volcanologists: surprisingly shallow pools of magma that are too small to be detected with common volcanic monitoring equipment. Such a body of magma was discovered in Iceland in 2009, when scientists from the Iceland Deep Drilling project accidentally drilled directly into molten rock two kilometers shallower than the depths where magma had been detected before. Magma began to move up the borehole, reaching several meters before being stopped with cold drilling fluids. The study adds critical information to the puzzle by connecting the hidden magma to a centuries-old eruption.

Rooyakkers, who is the lead author of the study and completed the work at McGill University, compared the composition of the extinct magma, which had formed smooth volcanic glass, with rocks from an eruption of the same volcano. , Krafla, in 1724. Prior to its study, scientists believed that the shallow magma they had drilled into had been placed after a series of eruptions in the 1980s. No one expected the hidden magma was linked to the 1724 eruption, so what Rooyakkers found was a surprise.

“When we looked at the 1724 compositions, we found an almost perfect match with what was sampled while drilling,” says Rooyakkers. “This suggests that in fact this magma body has been there since 1724 and has already been involved in an eruption at Krafla. So it begs the question, ‘Why didn’t geophysics detect it?'”

The answer is size. Most of the magma detection relies on seismic imagery, as oil companies use it to detect reserves deep below the seabed. In the event of an earthquake, the instruments detect the time it takes for sound waves to pass through the crust. Depending on the density of the rocks, the sound waves return at different times. So if there is water, oil, or magma stored underground, sound waves should reflect it. But these hidden magma chambers are too small for these instruments, along with other detection tools, to find.

“In traditional approaches to monitoring volcanoes, the emphasis is on knowing the location of magma and active magma bodies,” says Rooyakkers. “Krafla is one of the most monitored and instrumented volcanoes in the world. They threw everything except the kitchen sink in terms of geophysics. And yet we still didn’t know there was this body of rhyolitic magma sitting just two kilometers deep capable of producing a dangerous eruption. “

Studies like those by Rooyakkers suggest that smaller, more widely distributed magma bodies may be more common than previously thought, challenging the conventional idea that most eruptions are fueled by larger magma chambers. and deeper that can be reliably detected.

Beyond the inability to monitor magmatic activity, planning eruptions and estimating risks become more difficult if scientists suspect the presence of hidden magma bodies. For example, the Krafla volcano is generally dominated by basalt, a type of magma that tends to passively erupt (like the recent eruption at Fagradallsfjall in Iceland) rather than in an explosion. But the magma body hidden in Krafla is made of rhyolite, a type of magma that often creates violent explosions when it erupts.

“So the problem in this case would be that you have a shallow rhyolitic magma that you don’t know about, so it wasn’t factored into the risk planning,” Rooyakkers explains. “If he is hit by new rising magma, you could have a much more explosive eruption than you expected.”

As volcanologists become aware of the dangers associated with these shallow, distributed magma systems, they can work to improve surveillance, trying to capture these hidden magma pools. Covering a volcanic area in more detectors can be expensive, but by improving the resolution of magmatic imagery, scientists can save a community or business far more than the cost of the study. Risks vary from volcano to volcano, but in general, as we learn more about these magmatic systems, scientists concerned with estimating dangers may be aware of the possibility of hidden magma.

Despite the risks he discovers, will the Rooyakkers still live around volcanoes?

“Oh yeah, sure,” he laughed. “I mean, there’s a risk with anything, isn’t there?”

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