Fundamental changes in what we know about how volcanoes work

Fundamental changes in what we know about how volcanoes work

Fundamental changes in what we know about how volcanoes work

Fagradalsfjall volcanic eruption at night

The Fagradalsfjall volcano in Iceland erupts at night.

Recent findings from Iceland’s Fagradalsfjall eruption are changing what we know about how volcanoes work.

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

While sampling magma from the Fagradalsfjall volcano in Iceland, Jackson and his colleagues uncovered a process that was far more dynamic than anyone had suspected in the two centuries that scientists have been studying volcanoes.

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

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

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

10,000 years in one month

Thanks to a sabbatical, a pandemic and 780 years of melting underground rock, Jackson was in the right place and time to witness the birth of Fagradalsfjall, a fissure in the lowlands of southwest Iceland that split and exploded with magma in March 2021. By At that time, everyone on the Reykjaneshalvøya was ready for some kind of eruption, he said.

“The earthquake swarm was intense,” he said of the roughly 50,000 aftershocks — some 4 and higher — that shook the earth for weeks and kept most of Iceland’s population on edge.

The lack of sleep was worth it, though, and boredom soon turned to fascination as lava bubbled up and spewed from the hole in the ground in the relatively empty Geldingadalur region. Scientists and visitors alike flocked to the area to see the newest part of the Earth’s crust forming. From the start, they were able to get close enough to sample the lava continuously, due to the lava’s slow flow and abundant wind that blew the noxious gases away.

Fagradalsfjall Iceland volcanic eruption

Volcanic eruption of Mount Fagradalsfjall in Iceland.

Led by Sæmundur Halldórsson at the University of Iceland, the geologists tried to find out “how deep in the mantle the magma originated, how far below the surface it was stored before the eruption, and what happened in the reservoir both before and during the eruption.” Questions like these, although they are fundamental, are in fact some of the greatest challenges for those studying volcanoes, due to the unpredictability of the 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 spills out during the eruption.” As a result of this well-defined two-step process, he added, those studying volcanic eruptions do not expect to see significant changes in the chemical composition of the magma as it flows out of the Earth.

“This is what we see at Mount Kilauea in Hawaii,” he said. “You will have outbreaks that last for years, and there will be minor changes over time.

“However, in Iceland there were more than a factor of 1,000 higher rates of change for key chemical indicators,” Jackson continued. “In one month, the Fagradalsfjall eruption showed more compositional variation than Kilauea eruptions showed in decades. The total range of chemical compositions sampled by this eruption in the first month spans the entire area that has ever erupted in southwest Iceland during the last 10,000 years.”

Fagradalsfjall volcanic eruption at night

Night view of a volcanic eruption at Fagradalsfjall in Iceland.

This variation is the result of successive batches of magma flowing into the chamber from deeper in the mantle, according to the researchers.

“Imagine a lava lamp in your mind,” Jackson said. “You have a hot light bulb at the bottom, it heats up a blob and the blob rises, cools and then sinks. We can think of the Earth’s mantle – from the top of the core to below the tectonic plates – acting much like a lava lamp.” He went on to explain that as heat causes regions of the mantle to rise and shocks form and move fluidly upward toward the surface, molten rock from these shocks collects in chambers and crystallizes, gases escape through the crust, and pressure builds to magma finds a way to escape.

“Just when I think we’ve come close to figuring out how these volcanoes work, we get a big surprise.” — Matthew Jackson

As described in the paper, what erupted in the first weeks was the expected “depleted” type of magma that had accumulatedg in the reservoir, which lies approximately 16 km below the surface. In April, however, evidence showed that the chamber was recharged by deeper, “enriched” melts of a different composition. These were obtained from another region of the rising mantle plume beneath Iceland. This new magma had a less modified chemical composition, with a higher magnesium content and a higher proportion of carbon dioxide gas. This indicated that fewer gases from this deeper magma had escaped. In May, the magma that dominated the flow was the deeper, enriched type. These rapid, extreme changes in magma composition at a plume-fed hotspot, they say, “have never before been observed in near real time.”

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

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

For the researchers, this result presents a “key limitation” in how models of volcanoes around the world will be built. However, it is not yet clear how representative this phenomenon is for other volcanoes, or what role it plays in triggering an eruption. For Jackson, it’s a reminder that the earth still has secrets to give.

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

Reference: “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, 14 September 2022, Nature.
DOI: 10.1038/s41586-022-04981-x

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