Fundamental changes to what we know about how volcanoes work

Fagradalsfjall Volcanic Eruption at Night
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Fagradalsfjall volcanic eruption at night

The Fagradalsfjall volcano in Iceland erupts at night.

Recent discoveries of Iceland’s Fagradalsfjall eruptions are changing our knowledge of how volcanoes work.

Learning something that fundamentally changes how we understand our world doesn’t happen very often. But for the University of California, Santa Barbara earth scientist Matthew Jackson, and the thousands of volcanologists around the world, such a revelation has just happened.

While extracting magma from Iceland’s Fagradalsfjall volcano, Jackson and his colleagues discovered a far more dynamic process than anyone had anticipated in the two centuries that scientists have studied volcanoes.

“Just when I think we’re getting close to asking how these volcanoes work, we get a big surprise,” he said.

Fagradalsfjall is a Tuya volcano that formed in the last Ice Age on the Reykjanes Peninsula about 40 km from Reykjavík, Iceland.

The geologists’ findings were published in the journal on September 14th Nature.

10,000 years in one month

Thanks to a sabbatical, a pandemic, and 780 years of melting subterranean rock, Jackson was in the right place and time to witness the birth of Fagradalsfjall, a fissure in the lowlands of southwest Iceland that ruptured with magma and exploded in March 2021 At that time, everyone in the Reykjanes Peninsula was poised for some kind of eruption, he said.

“The earthquake swarm was intense,” he said of the roughly 50,000 tremors — around magnitude 4 and higher — that shook the earth for weeks and kept most of Iceland’s population in suspense.

The sleep deprivation paid off, however, and the quirk soon turned to intrigue as lava bubbled and spattered from the hole in the ground of the relatively empty region of Geldingadalur. Both scientists and visitors flocked to the area to see the newest form of the earth’s crust. Because of the slow lava flow and strong winds that blew away the noxious gases, they were able to get close enough to continuously study the lava right from the start.

Fagradalsfjall Iceland volcanic eruption

Mount Fagradalsfjall volcanic eruption in Iceland.

Led by Sæmundur Halldórsson from the University of Iceland, the geologists were trying to find out “how deep in the mantle the magma formed, how far below the surface it was stored before the eruption, and what was happening in the reservoir before and during the eruption.” Questions like these while fundamental, are actually some of the greatest challenges facing those studying volcanoes due to the unpredictability of eruptions, the danger and extreme conditions, and the remoteness and inaccessibility of many active sites.

“The assumption was that a magma chamber fills up slowly over time and the magma gets well mixed,” Jackson explained. “And then it empties as the eruption progresses.” As a result of this well-defined two-step process, he added, those who study volcanic eruptions don’t expect significant changes in the chemical composition of magma as it flows from the Earth.

“We see that at Mount Kīlauea in Hawaii,” he said. “You will have eruptions that last for years and there will be minor changes over time.

“But in Iceland, there were more than a factor of 1,000 higher rates of change for key chemical indicators,” Jackson continued. “In a month, the Fagradalsfjall eruption showed more compositional variability than the Kīlauea eruptions in decades. The full range of chemistries sampled from this eruption over the course of the first month encompasses the full range that has ever erupted in southwest Iceland in the last 10,000 years.”

Volcanic eruption Fagradalsfjall at night

Night view of a volcanic eruption on Fagradalsfjall mountain in Iceland.

This variability, according to the scientists, is the result of subsequent batches of magma pouring into the chamber from deeper layers of the mantle.

“Imagine a lava lamp,” Jackson said. “You have a hot bulb at the bottom, it heats up a blob and the blob rises, cools, then sinks. We can envision the Earth’s mantle — from the top of the core to below the tectonic plates — functioning much like a lava lamp.” He further explained that when the heat causes molten rock from these plumes to collect in chambers, they crystallize that regions of the mantle rise and plume and float upward to the surface, gases escaping through the crust and pressure building until the magma finds a way to escape.

“Just when I think we’re getting close to how these volcanoes work, we get a big surprise.” — Matthew Jackson

As described in the publication, the expected “depleted” type of magma that had accumulated erupted in the first few weeksMister in the reservoir located about 16 km below the surface. By April, however, it was evident that the chamber was being recharged by deeper, “enriched” melts of a different composition. These were sourced from a different region of the ascending mantle cloud beneath Iceland. This new magma had a less modified chemical composition, with a higher magnesium content and higher proportion of carbon dioxide gas. This indicated that fewer gases had escaped from this deeper magma. By May, the magma that dominated the flow was the deeper, enriched type. These rapid, extreme changes in magma composition at a cloud-fed hotspot have “never been observed before in near real time.”

However, Jackson said these compositional changes may not be that rare. It’s just that opportunities to sample eruptions at such an early stage are not common. For example, prior to the eruption of Fagradalsfjall in 2021, the most recent eruptions on Iceland’s Reykjanes Peninsula occurred eight centuries ago. He suspects this new activity signals the start of a new, possibly centuries-long, volcanic cycle in southwest Iceland.

“We often don’t have records of the early stages of most eruptions because they get buried by lava flows from the later stages,” he said. This project, the researchers say, allowed them to see for the first time a phenomenon that was thought possible but had never been observed directly.

For the scientists, this result represents an “important limitation” on how models of volcanoes are built around the world. However, it is not yet clear how representative this phenomenon is of other volcanoes or what role it plays in triggering an eruption. For Jackson, it’s a reminder that the earth still has secrets to reveal.

“So when I go out to sample an ancient lava flow, or when I read or write articles in the future,” he said, “it will always come to mind: This may not be the full story of the eruption.”

References: “Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland” by Sæmundur A. Halldórsson, Edward W. Marshall, Alberto Caracciolo, Simon Matthews, Enikő Bali, Maja B. Rasmussen, Eemu Ranta, Jóhann Gunnarsson Robin, Guðmundur H Guðfinnsson, Olgeir Sigmarsson, John Maclennan, Matthew G Jackson, Martin J Whitehouse, Heejin Jeon, Quinten HA van der Meer, Geoffrey K Mibei, Maarit H Kalliokoski, Maria M Repczynska, Rebekka Hlín Rúnarsdóttir, Gylfi Sigurðsson, Melissa Anne Pfeffer, Samuel W. Scott, Ríkey Kjartansdóttir, Barbara I. Kleine, Clive Oppenheimer, Alessandro Aiuppa, Evgenia Ilyinskaya, Marcello Bitetto, Gaetano Giudice and Andri Stefánsson, September 14, 2022, Nature.
DOI: 10.1038/s41586-022-04981-x

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