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ByRobin George Andrews
Published January 17, 2024
On Sunday January 14, at 3 a.m. local time, residents of the coastal Icelandic town of Grindavík were jolted awake by blaring sirens. A swarm of earthquakes indicated that magma was suddenly rising from below, threatening to engulf streets and homes. At 7:57 a.m., lava began gushing out of a newly opened fissure just to the town’s north.
The town was swiftly evacuated, the second evacuation for Grindavík due to volcanic activity since last November. The fissure quickly expanded to 3,000 feet long and poured lava toward the town. Defensive walls—put in place around the time of a December eruption in the area—deflected much of the lava to the town’s west. But the fissure undercut the walls, and some molten rock crept south toward Grindavík’s outskirts.
Then a nightmare scenario unfolded. Around 12:30 p.m., a second, smaller fissure emerged closer to the town’s northeastern edge—and lava bulldozed through several evacuated houses, effortlessly knocking them down and setting them ablaze. If this outpouring of lava continued for several days, as some feared, it could cut right through entire neighborhoods.
“And then it stops,” says Mike Burton, a volcanologist at the University of Manchester.
By January 15 the lava output from both fissures had decreased precipitously, with the smaller one petering out. Seismic activity dropped off, and the ground stopped convulsing so intensely. By the early hours of January 16, all eruptive activity had ceased.
After such a tremendous geologic prelude, Grindavík faced down an initially very intense, but ultimately short-lived eruption. Several homes were destroyed, but thanks to the evacuations no lives were lost.
The eruption may have stopped—or it may have simply paused. A shallow magma reservoir was recently injected beneath this Svartsengi region of Iceland’s Reykjanes Peninsula. It supplied the lava for December’s eruption, took a breather, then did the same for this month’s outburst. And some evidence suggests this pattern is cyclical.
“I think we’re going to see a repeat of these events sometime into the future,” says Þorvaldur Þórðarson, a volcanologist at the University of Iceland. Unfortunately, for the residents of Grindavík, their troubles may be far from over.
The peninsula’s two pyres
Some 800 years ago, between the years 1210 and 1240, sporadic fissure eruptions took place across Iceland’s Reykjanes Peninsula—a period of activity that was followed by a long period of quiescence. Then in March 2021, this activity appeared to begin anew, starting with a spectacular eruption in a valley next to an uninhabited volcanic mound named Fagradalsfjall.
Two additional eruptions broke out nearby in 2022 and 2023, both away from population centers. But everything changed last October, when the Svartsengi region to the south—containing the popular Blue Lagoon, a major geothermal power plant, and the town of Grindavík—began to stir.
A series of increasingly frequent and intense quakes reached a zenith in early November. The ground shook, and in some areas dramatically changed shape, damaging buildings are roads in Grindavík. On November 10, a spike in violent quakes suggested that magma was quickly rising to the surface, and Grindavík’s 4,000 residents were speedily evacuated in the dead of night.
But an eruption didn’t immediately occur. Instead magma appeared to pool and stretch out laterally just beneath the surface, threatening to emerge from anywhere along a 10-mile line, including within the town itself. Then the eruption struck just to the town’s northeast on December 18 which, thankfully, flowed to the north, away from Grindavík.
Like the eruption this month, the December event involved the apparent rise of a lot of magma, followed by a short eruption. But why—and what happens next?
Hurry up and wait
Answering these questions starts with decoding the region’s seismic cacophony. Last November, scientists reasoned that a significant volume of magma had intruded into the Svartsengi region’s shallow crust. The quakes indicated that the crust here was vigorously moving in two different directions, creating space for magma to rush up from below.
That this magma didn’t immediately erupt on November 10 implies that it was struggling to find a way to the surface, or that it wasn’t primed to erupt at that stage.
Clearly something gave way on both December 18 and January 14, when two brief eruptions took place. That suggests a few things, including that “the magma container is quite big, and shallow, and easily broken,” says Burton.
If that magma cache becomes just a little too pressurized—say, from more gas-rich magma flowing into it from below—then some of that magma can be forced to the surface. And when you’re that shallow in the crust, “there isn’t much resistance to eruption,” says Burton.
Both Grindavík eruptions began very intensely, particularly the December event, with substantial volumes of magma gushing out at breakneck speeds. That indicates that the initial driving forces of the eruptions were high. “The more you manage to build up the internal pressure before it ruptures, the quicker things will come out,” says Þórðarson.
But in both instances, the lava output declined dramatically after the first few hours, before stopping altogether. “As it releases the magma, the pathway closes down pretty much as quickly,” says David Pyle, a volcanologist at the University of Oxford. He likens it to a trapdoor. At first the high pressure is pushing the magma out, keeping the door ajar; eventually the pressure drops, and the door slams shut.
A drop in pressure causes the rocks around the ascending magma to snap back and seal that pathway, preventing any more melt from the reservoir from erupting. Things then stabilize—but another internal pressure spike, caused perhaps by the injection of new magma or the escape of more gas from the molten rock itself, could trigger another small but intense eruption.
An unbreakable cycle
Although research into these eruptions is still in the early stages, scientists believe the volcanic activity around Grindavík seems to be a cycle without a clear breaking point.
That magma reservoir “loses such a small fraction of its volume that it doesn’t drain completely away, so it can keep on doing it,” says Burton. If the reservoir is being sufficiently supplied from deep below, as seems to be the case, it could be a long time before the system freezes up enough to stop erupting entirely.
That means it is unlikely that this month’s eruption will be the last to trouble Grindavík. There are no signs that another eruption is imminent, but the presence of magma right below the town and the region’s continued inflation doesn’t bode well.
The December and January eruptions may not even be separate events, but the same eruption interrupted by a brief pause—and now the activity has paused again. Lava could, at some point in the near future, emerge from new fissures or reactivate old ones.
Svartsengi and the more remote Fagradalsfjall may not remain the only two active volcanic centers on Reykjanes. There is no evidence that other regions have magma currently pooling beneath them in the same way—but that could change, as showcased by the multitude of ancient fissure scars crisscrossing much of the peninsula.
The prospect of co-existing with years, perhaps decades, of scattershot fissure eruptions on Reykjanes may seem hopeless. But it isn’t. “The Icelandic Met Office [which handles natural hazards science] have clearly done a brilliant job,” says Pyle. Cutting-edge, 24/7 monitoring has helped keep the population safe.
The defensive barriers built around Grindavík over the winter months proved vital. Partly made using compacted volcanic soil and rock, these barriers diverted much of the lava flow from the first, larger fissure away from the town.
“The barrier worked. But it worked because the eruption didn’t last very long,” says Burton. “It gave them time—and that’s huge in that situation.”
Defensive walls can only do so much, however. A new eruptive fissure may appear with little warning, making it hard to guard against all possible eruptions. Instead, says Þórðarson, the best defense is to be as proactive as possible: mapping out sites that are likely to experience fissure eruptions, charting which way the lava may flow, and having material placed in high-risk areas, ready to erect walls as speedily as possible should an eruption begin.
“We have the tools to do this,” says Þórðarson. But it won’t be easy. The Reykjanes Peninsula has well and truly entered a chaotic, hypervolcanic era—one in which difficult choices will have to be made. “It’s time we accept that new reality,” he says.
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Copyright for syndicated content belongs to the linked Source : National Geographic – https://www.nationalgeographic.com/science/article/iceland-latest-volcanic-eruption-quieting-down-grindavik-lava
