Hotspot volcanoes

Recent discoveries from Iceland’s Fagradalsfjall eruptions change what we know about how volcanoes work

It’s not every day that we learn something that fundamentally changes the way we understand our world. But for Matthew Jackson, an Earth scientist at UC Santa Barbara, and the thousands of volcanologists around the world, such a revelation has happened.

While sampling magma from the Fagradalsfjall volcano in Iceland, Jackson and his collaborators 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.

The findings of geologists are published in the journal Nature.

10,000 years in one month It took a sabbatical, a pandemic and 780 years of underground rock melting to put Jackson in the right place at the right time to witness the birth of Fagradalsfjall, a fissure in the south-west lowlands. west Iceland that split and erupted with magma in March 2021. By then, he said, everyone on the Reykjanes Peninsula was ready for some sort of eruption.

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

But the sleep deprivation was worth it, and the bad mood 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. They were able to get close enough to sample the lava continuously from the start, thanks to the winds blowing away the noxious gases and the slow flow of the lava.

What the geologists, led by Sæmundur Halldórsson at the University of Iceland, were trying to find out was “how deep in the mantle magma originated, how far below the surface it was stored before the eruption and what was happening in the reservoir both before and during the eruption.” Questions like these, while fundamental, are in fact among the greatest challenges for those who study volcanoes, due to the unpredictability of eruptions, danger and extreme conditions, as well as the remoteness and the 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 Kilauea in Hawaii,” he said. “You will have breakouts 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 one month, the Fagradalsfjall eruption showed more compositional variability than Kilauea eruptions have shown in decades. The total range of chemical compositions that were sampled during this eruption in the first month covers the full range that has ever erupted in southwestern Iceland over the past 10,000 years.”

According to the scientists, this variability is the result of subsequent batches of magma flowing into the chamber from deep within the mantle.

“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 and then sinks. We can think of the Earth’s mantle – from the top of the core to below the tectonic plates – functioning much like a lava lamp. ” As heat causes regions of the mantle to rise and plumes to form and move buoyantly toward the surface, he explained, the molten rock from these plumes accumulates in chambers and crystallizes, the gases escape through the crust and the pressure increases until the magma finds a way. to escape.

For the first few weeks, as described in the article, what erupted was the expected “depleted” type of magma that had accumulated in the reservoir, located about 10 miles below the surface. But in April, evidence showed the chamber was being recharged by deeper “enriched” type melts with a different composition from a different region of the 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, indicating 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.”

These lineup changes may not be that rare, Jackson said; 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 built, although it is not yet clear to what extent this phenomenon is representative 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.”