Not every day we learn something that fundamentally changes how we understand our world. But for University of California, Santa Barbara Earth scientist Matthew Jackson, and thousands of volcanologists around the world, the revelation has occurred.
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 past two centuries scientists have been studying volcanoes.
“Just when I think we’re close to figuring out how these volcanoes work, we get a huge surprise,” he said.
The results of the geologists were published in the journal temper nature.
10,000 years a month it took leave, an epidemic, and 780 years of plutonic melting to put Jackson in the right place and time to witness the birth of Fagradalsfjall, a fissure in the lowlands in southwest Iceland that split and erupted with magma in March 2021. By then, as He said, everyone on the Reykjanes Peninsula was ready for some kind of volcanic eruption.
“The earthquake swarm was intense,” he said of 50,000 or so tremors—magnitude 4 and higher—that shook the earth for weeks and kept most of Iceland’s population on edge.
But the sleep deprivation was worth it, and the weirdness turned into magic as lava erupted and splashed out of the hole in the relatively empty land of Gildadallur. Scientists and visitors alike flock to the area to see the newest part of what the Earth’s crust is like. They were able to get close enough to sample lava continuously from the start, thanks to winds that blew noxious gases away, and a slow lava flow.
What the geologists, led by Somundur Halldorsson at the University of Iceland, attempted to discover was “how deep the magma originated in the mantle, how much was stored below the surface before the eruption and what was going on in the reservoir before and during the eruption.” Questions like these, while basic, are actually some of the biggest challenges for those who study 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 the magma chamber fills slowly over time, and that the magma mixes well,” Jackson explained. “And then it is dried during the eruption period.” 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 Earth.
“This is what we see at Mount Kilauea in Hawaii,” he said. “You’re going to have a volcanic eruption that lasts for years, and there will be subtle changes over time.
“But in Iceland, there were more than 1,000 factors with higher rates of change for key chemical indicators,” Jackson continued. “In one month, the Fagradalsfjall eruption showed more compositional variance than Kilauea eruptions have shown in decades. The total range of chemical compositions sampled in this first month eruption spans the full range that ever erupted in southwest Iceland in the ten thousandths. last year.”
According to scientists, this asymmetry is caused by subsequent batches of magma flowing into the chamber from deeper in the mantle.
“Imagine a lava lamp in your mind,” Jackson said. “You have a hot lamp at the bottom, a bubble heats up and the point rises and cools and then sinks. We can think of the Earth’s mantle—from the top of the core to the bottom of the tectonic plates—working like a lava lamp.” He explained that as heat causes the mantle regions to rise and form plumes and move forcefully upward toward the surface, the molten rock from these plumes builds up in chambers and crystallizes, and gases escape through the crust and pressure builds up until the magma finds an escape route.
During the first few weeks, as shown in the paper, what erupted was the expected “spent” type of magma that had accumulated in the reservoir, located about 10 miles (16 km) below the surface. But by April, evidence showed that the chamber had been recharged with a deeper type of “enriched” melt with a different composition obtained from a different region of the rising mantle plume under Iceland. This new magma has a less modified chemical composition, with a higher magnesium content and a higher proportion of carbon dioxide, indicating that fewer gases escaped from this deep magma. By May, the magma that controlled the flow was of the deepest and richest type. These rapid and extreme changes in magma composition at a plume-feeding hotspot have, they say, “never been observed before in near real time.”
These changes in makeup may not be so rare, Jackson said. It’s just that chances of sampling eruptions at such an early stage are not uncommon. For example, before the 2021 Fagradalsfjall eruption, the most recent eruptions occurred on the Reykjanes Peninsula in Iceland eight centuries ago. He suspects that this new activity signals the beginning of a new, centuries-old volcanic cycle in southwestern Iceland.
“We often don’t have a record of the early stages of most eruptions because they were buried by late-stage lava flows,” he said. This project allowed them, according to the researchers, to see a phenomenon that was thought to be possible but that was not seen directly.
For scientists, this result represents a “major limitation” in how models of volcanoes around the world are built, although it is not yet clear how representative this phenomenon is for other volcanoes, or what role it plays in causing eruptions. For Jackson, this is a reminder that Earth still has secrets to lose.
“So when I go out to sample ancient lava flows, or when I read or write papers in the future, it will always be on my mind: This may not be the whole story of the eruption,” he said.