Hidden Underground Magma Pools Fueled Iceland’s Volcanic Eruptions, Study Suggests

For more than eight centuries, Iceland’s Reykjanes Peninsula was volcanically quiet. People built roads, towns, a geothermal power plant, the country’s major international airport and its most popular tourist destination: the Blue Lagoon geothermal spa. But in just the last few years, that quiescent period ended in dramatic fashion as a string of major eruptions sent lava flowing toward many of these sites.

“We’re going through a volcanic crisis,” Matthew J. Roberts, the managing director of the Icelandic Meteorological Office, tells the New York Times’ Sarah Hurtes. “This is perhaps the most intense period of volcanic activity affecting an urban environment at the moment.”

The Reykjanes Peninsula has now seen eight eruptions since March 2021. And on July 30, Iceland’s Meteorological Office issued a warning about another one “in the coming days.” With more lava flows imminent, a new paper published last week in the journal Nature suggests scientists have found the subterranean source of the recent volcanic activity.

According to the study, hidden pools of magma in the Earth’s crust during the first recent eruption carved out an easy route for molten rock to continue erupting to the surface. Learning more about these pathways is important for managing the hazards of living in an active volcanic zone and understanding how, when and why eruptions take place.

Iceland is a geologically unique locale where two combined forces lead to rampant volcanic activity. First, the island is located on the Mid-Atlantic Ridge, an underwater mountain range that runs down the center of the Atlantic Ocean and separates the North American and Eurasian tectonic plates as they spread slowly apart from each other. Second, a plume of hot, rocky material from deep in the Earth rises to the surface right around where Iceland meets this ridge. The interaction of these two phenomena led to the creation and ongoing volcanism of the island nation.

So, Icelanders have long known to expect volcanic eruptions. But the source of lava in the populous and previously calm Reykjanes Peninsula remained a mystery. In 2022, a study found that magma was rising to the peninsula’s surface directly from the Earth’s mantle, without interacting much with the crust. To further investigate the sources of lava in the area, the authors of the new paper collected samples from recent eruptions on a near daily basis and tracked how their chemistry changed over time. Researchers call this type of data set a “time series.”

“Taking a volcanic time series is like regularly drawing someone’s blood to understand their medical condition,” writes James Day, a geochemist at the Scripps Institution of Oceanography and lead author of the new study, in the Conversation. “In this case, though, the blood is red-hot lava.”

To examine the samples, they used a sensitive instrument called a mass spectrometer, which revealed the ratio of two elements: rhenium and osmium. Osmium can decay radioactively from rhenium, but the two behave differently when rocks melt. As a result, the Earth’s crust has proportionally more rhenium than the deeper layers of the planet, such as the mantle.

But Day and his colleagues found surprising amounts of rhenium in the volcanic samples, a clear signal that lava from some of the eruptions had spent time pooling in the crust.

“We fully expected to see mantle signatures in the lavas throughout the eruption,” says Day in a statement. “You can imagine our astonishment when we were sitting in front of the mass spectrometer measuring the early samples and saw obvious signals of crust within them.”

They determined the original eruption on the Reykjanes Peninsula in 2021 came from magma that started in the mantle, then pooled in a chamber in the crust, maybe for as long as a year. When in this pool, the magma melted the crustal rocks around it, which contained minute amounts of water, carbon dioxide and other gases, in addition to rhenium. These additions eventually caused pressure to build up enough that the magma erupted to the surface.

In the following eruptions, the lava rocks contained less rhenium, meaning magma was able to use the existing pathways to easily get to the surface without melting much more crustal rock.

In all, researchers say, this means visitors and residents of the Reykjanes Peninsula can expect eruptions and lava flows for a long time to come.

“It seems that we are now witnessing the earliest part of a major eruption episode. That’s a recurrent phenomenon on the peninsula, with 800 years of pause or quiescence, followed by 100 or 200 years of intense eruptions, followed by another quiet period,” study co-author Valentin Troll, a geologist at Uppsala University in Sweden and lead author of another recent study on the source of this volcanic activity, tells CNN’s Jacopo Prisco. “Scientifically, we are lucky to be able to observe this, but from a societal point of view we are not, because it happens in a very populated part of the country with a lot of infrastructure.”

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Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.