平特五不中

Scientists鈥� ability to estimate eruption risks is largely reliant on knowing where pools of magma are stored, deep in the Earth鈥檚 crust. But what happens if the magma can鈥檛 be spotted?

, a former PhD student at 平特五不中鈥檚 Department of Earth and Planetary Sciences and now a postdoctoral scholar at GNS Science in New Zealand, grew up in the shadow of Mount Taranaki on the country鈥檚 North Island, hiking on the island鈥檚 many volcanoes. Now, his research is revealing hidden dangers that may have been beneath his feet all along.

A new study, published in , explores a threat scientists discovered only recently: surprisingly shallow magma pools that are too small to be detected with common volcano monitoring equipment. Such a magma body was discovered in Iceland in 2009, when scientists with the Iceland Deep Drilling Project accidentally drilled directly into the molten rock two kilometers shallower than the depths where magma had been detected before. Magma began to creep up the drill hole, reaching several meters before it was stopped with cold drilling fluids.

Hidden magma lurking for several hundred years

鈥淭he new study discovered that this same magma pool belched out a small explosive eruption in 1724. So the magma pool has been lurking for at least several hundred years,鈥� says John Stix, a 平特五不中 professor in the Department of Earth and Planetary Sciences and study co-author.

Rooyakkers, who is the lead author on the study and completed the work while at 平特五不中, compared the composition of the quenched magma, which had formed smooth volcanic glass, with rocks from an eruption from that same volcano, Krafla, in 1724. Before his study, scientists thought the shallow magma they drilled into had been created after a series of eruptions in the 1980s. No one expected the hidden magma to be related to the 1724 eruption, so what Rooyakkers found was a surprise.

鈥淲hen we looked at the compositions from 1724, we found an almost perfect match for what was sampled during the drilling,鈥� Rooyakkers says. 鈥淭hat suggests that, this magma body has been there since 1724 and has previously been involved in an eruption at Krafla. So that raises the question of, 鈥榃hy did geophysics not pick it up?鈥欌��

Too small to detect with seismic imaging

The answer is size. Most magma detection relies on seismic imaging, often used by oil companies to detect reserves deep under the seafloor. When there鈥檚 an earthquake, the instruments detect how long it takes for sound waves to travel through the crust. Depending on the density of the rocks, the soundwaves return at different times. If there鈥檚 water, oil, or magma stored underground, the soundwaves should reflect it. But these hidden magma chambers are too small for these instruments, as well as other detection tools, to find.

鈥淚n traditional approaches to volcano monitoring, a lot of emphasis is placed on knowing where magma is and which magma bodies are active,鈥� says Rooyakkers. 鈥淜rafla is one of the most intensely-monitored and instrumented volcanoes in the world. They鈥檝e thrown everything but the kitchen sink at it in terms of geophysics. And yet we still didn鈥檛 know there was this rhyolitic magma body sitting at just two kilometers鈥� depth that鈥檚 capable of producing a hazardous eruption.鈥�

Studies like Rooyakkers鈥� suggest that smaller, more widely-distributed magma bodies might be more common than previously thought, challenging the conventional view that most eruptions are fed from larger and deeper magma chambers that can be reliably detected.

Estimating risk more difficult

Beyond not being able to monitor magmatic activity, planning for eruptions and estimating risks becomes more difficult if scientists suspect that hidden magma bodies could be present. For example, the Krafla volcano is usually dominated by basalt, a type of magma that tends to erupt passively (like the recent eruption at Fagradallsfjall in Iceland) rather than in an explosion. But the hidden magma body at Krafla is made of rhyolite, a magma type that often creates violent explosions when it erupts.

鈥淪o the concern in this case would be that you have a shallow rhyolitic magma that you don鈥檛 know about, so it hasn鈥檛 been considered in hazards planning,鈥� Rooyakkers explains. 鈥淚f it鈥檚 hit by new magma moving up, you might have a much more explosive eruption than you were anticipating.鈥�

As scientists become aware of the hazards associated with these shallow, distributed magma systems, they can work on improving monitoring, trying to capture these hidden magma pools. Covering a volcanic area in more detectors may be costly, but by improving the resolution of magma imaging, scientists may save a community or company far more than the cost of the study. The risks vary from volcano to volcano, but in general, as we learn more about these magma systems, scientists concerned with estimating hazards can be aware of the possibility of hidden magma.

Despite the risks he鈥檚 uncovering, will Rooyakkers still live around volcanoes? 鈥淥h yeah, for sure,鈥� he says with a laugh. 鈥淚 mean, there鈥檚 risk with anything, isn鈥檛 there?鈥�

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