Hotspot volcanoes

Fundamental changes to what we know about how volcanoes work

Fagradalsfjall volcano in Iceland erupting at night.

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

Learning something that fundamentally changes the way 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 sampling magma from the Fagradalsfjall volcano in Iceland, Jackson and his colleagues discovered a much more dynamic process than anyone had assumed in the two centuries that scientists have studied volcanoes.

“Just when I think we’re close to understanding how these volcanoes work, we have 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 findings of the geologists were published on September 14 in the journal Nature.

10,000 years in a month

Thanks to a sabbatical, a pandemic, and 780 years of melting rock underground, Jackson was in the right place and at the right time to witness the birth of Fagradalsfjall, a fissure in Iceland’s southwestern lowlands that s split open and exploded with magma in March 2021. At that time everyone on the Reykjanes Peninsula was ready for some sort of eruption, he said.

“The swarm of earthquakes was intense,” he said of the roughly 50,000 tremors of magnitude 4 and above that shook the earth for weeks and kept most of the Icelandic population in suspense.

However, the sleep deprivation was worth it, and the bad mood quickly turned to fascination as lava bubbled and splashed from the hole in the ground of the relatively empty region of Geldingadalur. Scientists and visitors flocked to the area to see the new section of the shape of the earth’s crust. Right off the bat, they were able to get close enough to sample the lava continuously, due to the slow flow of the lava and the strong winds that blew the noxious gases away.

Volcanic eruption of Mount Fagradalsfjall in Iceland.

Led by Sæmundur Halldórsson of the University of Iceland, geologists were trying to find out “how deep in the mantle magma was born, 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 among the greatest challenges for those who study volcanoes. This is 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 slowly over time and the magma becomes well-mixed,” Jackson explained. “And then it flows during the eruption.” As a result of this well-defined two-step process, he added, those who study volcanic eruptions do not expect to see significant changes in the chemical composition of magma as it flows out of the rock. earth.

“That’s what we see at Mount Kīlauea, Hawaii,” he said. “You will have rashes 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. “Within one month, Fagradalsfjall’s eruption showed greater compositional variability than Kīlauea’s eruptions for decades. The total range of chemical compositions that were sampled during this eruption in the first month spans the full that has ever erupted in southwestern Iceland in the last 10,000 years.

Fagradalsfjall volcano eruption at night

Night view of a volcanic eruption at Mount Fagradalsfjall in Iceland.

This variability is the result of subsequent batches of magma flowing into the chamber from deep within the mantle, the scientists say.

“Imagine a lava lamp in your mind,” Jackson said. “You have a hot bulb at the bottom, it heats a blob and the blob rises, cools, then sinks. We can think of the earth’s mantle – from the top of the core down to below the tectonic plates – functioning a bit like a lamp He went on to explain that as heat causes regions of the mantle to rise and plumes to form and move buoyantly to the surface, the molten rock from these plumes accumulates in chambers and crystallizes, gases escape through the crust, and pressure builds until the magma finds a way to escape.

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

As described in the article, what erupted during the first few weeks was the expected “depleted” type of magma that had accumulated.g in the reservoir, located about 16 km below the surface. However, in April, evidence showed that the chamber was recharged by deeper “enriched” type casts with a different composition. These came from a different region of the rising mantle plume below Iceland. This new magma had a less altered chemical composition, with a higher magnesium content and a higher proportion of carbon dioxide. This indicated that less gas from this deeper magma had escaped. In May, the magma that dominated the flow was of the deeper and enriched type. These rapid and extreme changes in magma composition at a plume-fed hotspot, they say, “have never been observed before in near real time.”

However, Jackson said these lineup changes may not have been so uncommon. It’s just that the opportunities to sample eruptions at such an early stage are not common. For example, before Fagradalsfjall erupted in 2021, the most recent 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 southwest Iceland.

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

For scientists, this result presents a “key constraint” in how models of volcanoes around the world will be constructed. 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 Earth still has secrets to tell.

“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 be on my mind: it may not be not be the full story of the eruption.”

Reference: “Rapidly moving 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