When engineers began drilling into an Icelandic volcano named Krafla, things took a turn for the weird. The team’s objective was to approach the boundary of a magma reservoir 2.5 miles below the surface, tapping into superheated fluids that could produce geothermal energy. But when the drill was just over a mile down, molten rock began creeping up the drill.
On that brisk spring day in 2009, the engineers had accidentally hit a pocket of magma sitting right below the surface that no one knew was there.
Krafla is “one of the best studied volcanoes on the planet,” says Hugh Tuffen, a volcanologist at Lancaster University in the United Kingdom who wasn’t involved with the research. It has been repeatedly surveyed using a range of techniques, so scientists thought they had a decent grasp of its underground workings. “It’s remarkable that this magma was able to hide.”
The incident at Krafla is one of three recent encounters with surprise magma pockets in Earth’s uppermost crust; similar drilling projects found unexpected reservoirs at Kīlauea volcano in Hawai‘i and Menengai volcano in Kenya. Now, researchers writing in the journal Geology are making the case that hidden magma pockets may exist around active volcanic centers the world over. Partly thanks to their comparatively small size—each one about a quarter of a cubic mile—the techniques that scientists normally deploy to locate magma bodies cannot see them.
The new study also demonstrates that these covert magma pockets can remain silent for an extremely long time. Based on chemical analysis, the drilled magma is a match for lava that erupted at Krafla way back in 1724. That means the pocket has remained undetected for three centuries, right through the advent of modern geophysical science.
But if a pocket like this gets infiltrated by a sheet of hot magma or a burst of volcanic gases from below, it can be reawakened. That may trigger an eruption—and because some covert magma bodies, like the one at Krafla, are made of a goopier gas-trapping magma, this could result in a more explosive event.
The research aims to be “a bit of a wake-up call,” says study leader Shane Rooyakkers, a postdoctoral researcher at GNS Science in New Zealand. Covert magma bodies add a wildcard aspect to a volcano’s hazard potential, so they need to be found.
“If these little pockets of potentially eruptible melt are the norm rather than the exception, then they’re little ticking time bombs just beneath volcanoes,” says Emma Liu, a volcanologist at University College London who wasn’t involved with the work.
Magma reservoirs can be found in a variety of ways. Seismic waves are a good choice; they change speed and trajectory as they pass through different materials. Molten rock, being a liquid, shows up as a different entity to the solid walls keeping it confined.
But magma is not completely molten. It’s a mixture of solids (crystals) and liquids (melt). If a magma reservoir has significantly cooled, it will have far more crystals than melt and will appear more like crust to seismic surveys, says Liu.
Another issue is that seismic waves designed to spy magma reservoirs have large wavelengths. Any features smaller than these wavelengths, including small magma pockets, cannot be properly imaged. Similarly, surveys that detect underground features by searching for electrical conductivity—a technique that reveals the presence of fluids, including magma—also can’t see small magma pockets; no one is sure why not.
“It’s almost like you have a net,” says Liu. The bigger, more molten magma reservoirs get caught, but “anything smaller can just squeak through.”
Krafla is a six-mile-wide cauldron scored by a 60-mile-long collection of volcanic fissures. Survey work there had pinned its magma reservoir as being two to four miles underground, well below drilling depths. But the National Power Company of Iceland encountered magmatic fragments above that depth when they drilled a well in 2008. And several times back in 2009, the very first well of the experimental Iceland Deep Drilling Project, hoping to reach the boundary of a deep magma reservoir, instead punctured 1,650-degree Fahrenheit magma just 1.3 miles down.
“I’d say it was more than a surprise. They were pretty shocked,” says study co-author John Stix, a geoscientist at McGill University.
Giving a bomb its fuse
Stealthy magma has long been acknowledged to exist, but scientists only have a vague understanding of it precisely because of its incognito nature. Hoping to better understand the properties of covert magma, Rooyakkers took samples of the magma serendipitously drilled in 2009—a goopy type known as a rhyolite—and forensically compared them with rhyolitic debris from the volcano’s past eruptions. He found a geochemical match to an early stage of the Mývatn Fires, a prolific lava-spewing eruption sequence that ran from 1724 to 1729.
An explosion in 1724 carved out a 1,000-foot crater named Víti. Based on the debris it left behind, and the composition of the magma involved, Rooyakkers and his colleagues put together a picture of what happened. A sheet of runny magma called basalt rose to the surface and intersected what may have been a covert pocket of dormant rhyolite in the uppermost crust. That gave the rhyolite the extra heat, gas, and momentum it needed to ascend, whereupon it encountered a zone of trapped steam, causing a violent blast.
Here, the presence of the steam played a pivotal role in creating an explosion. But as geologists know from more standard eruptions, the existence of rhyolite alone ups the odds of a blast, because its viscous nature easily stops trapped gas from escaping to the surface.
The 2005 eruption at Ethiopia’s Dabbahu volcano, for example, featured a fairly major blast that led to the evacuation of 6,000 people from nearby villages. That explosive episode was triggered by an injection of basalt from below into a covert rhyolitic magma pocket. In this instance, that was like giving an explosive charge a lit fuse.
Reactivating rhyolite doesn’t guarantee a new bout of volcanic violence, though. Take Iceland’s 2010 Eyjafjallajökull eruption, which led to the most expansive shutdown of European airspace since World War II. Basalt once again intersected rhyolite, bringing it to the surface—but that didn’t appear to alter the already vigorous style of the eruption.
Smaller is better
The good news is that if these covert magma bodies are truly quite small, “then the hazard is going to be small,” says Dave McGarvie, a volcanologist at Lancaster University who wasn’t involved with the work. Like a soda slowly going flat, if the covert pockets sit there undisturbed for ages, they may calmly lose their fizz, lowering the odds of an explosion occurring if and when the cap above is removed.
But while 0.25 cubic miles of magma is small compared to many magma reservoirs already known to exist, their eruptions can have wide-ranging effects if they do stay fizzy. The 1875 rhyolite eruption of Iceland’s Askja volcano involved less than a third of that volume, but its explosivity allowed it to dump debris all over the country, and ashfall reached as far as continental Europe.
It is also possible that Earth’s crust harbors even larger magma bodies with only patches of melt, meaning they may also be eluding geophysical surveys. If something reawakens those, then “that’s a completely different kettle of fish,” says McGarvie.
Still, smaller covert magma bodies could be way more abundant, and thankfully, scientists know where to look for them. Places where the crust is being stretched apart—like in parts of Iceland, or around the East African Rift—are “the prime suspects if you’re going to be looking for these covert magmas,” says Liu. They are likely present in or around large calderas too, including Italy’s Campi Flegrei, a beast of a volcano that partly overlaps with the bustling city of Naples.
But how can scientists locate them if they are, at present, essentially invisible?
“That is the question which we cannot answer,” says Lancaster University’s Tuffen. “And that is the question that we need to be able to answer.”
Placing a greater number of scientific instruments at a suspected covert magma spot may prove to be an effective way to detect them. And whether or not that pans out, we already know a covert magma pocket exists below Krafla—which brings enormous potential.
A proposal known as the Krafla Magma Testbed hopes to study that covert magma pocket by revisiting the 2009 well, drilling further into the magma and funneling scientific instruments down this infernal borehole. “We could have this direct window, to be able to watch what the magma’s doing through time,” says Tuffen.
If the project accrues enough funding, it will become the world’s first magma observatory, transforming volcanologists’ understanding of covert magma and shining a permanent light on that shadowy geologic underworld.