After three essentially safe volcanic eruptions in a remote, valley-filled swath of Iceland’s Reykjanes Peninsula, a fourth has now begun. The latest eruption, which was anticipated for weeks, has imperiled a power plant, the Blue Lagoon spa resort and, most significantly, the coastal town of Grindavík. On December 18, after 10 P.M. local time, a nearly four-kilometer-long fissure tore open the ground just to the town’s northeast, pouring ribbons of lava into the night.
The situation is ongoing, somewhat unpredictable and varyingly perilous. “The fissure has opened close to the worst-case-scenario position,” says Tom Winder, a volcano seismologist at the University of Iceland. The eruption, which could last for days, weeks or months, could completely avoid the town or force lava into it.
As of the afternoon of December 19, the eruption’s intensity has declined, and molten rock is not flowing toward Grindavík. Things could suddenly shift, however, and scientists are on high alert. “It’s an active crisis,” says Samuel Mitchell, a volcanologist at the University of Bristol in England. So how did we get here?
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Iceland is a complex jigsaw of volcanoes, and each section of the island has a different style of volcanism. Although Iceland’s southwestern Reykjanes Peninsula has its fair share of volcanic mountains and hills, it arguably specializes in fissure-style eruptions, where lava spurts and oozes out of newly formed cracks in the ground.
Between 1210 and 1240, fissure eruptions happened sporadically across the peninsula, a period known as the Reykjanes Fires. Then, after nearly 800 years of volcanic silence, a maelstrom of earthquakes that started in early 2020 and lasted 15 months implied that an eruptive awakening was nigh. Finally, in March 2021 a series of fissures opened near the remote volcanic mountain Fagradalsfjall, and lava filled up an uninhabited valley for the next six months. By the time two smaller additional eruptions briefly followed nearby—one in August 2022 and another this past summer—scientists were relatively certain that a new multidecadal period of eruptions had arrived.
They had hoped the inevitable fourth fissure eruption would be comparably remote, but in late October things took a concerning turn. Scientists observed a spike in seismic activity and ground deformation atop a volcanic system named Svartsengi to the southwest of the previous three eruptions. The seismicity was clustered around a miniature mountain called Þorbjörn, which is near the Blue Lagoon spa, a geothermal power station and Grindavík, home to 3,500 people.
Magma seemed to have gathered a few kilometers below the surface. It wasn’t necessarily going to erupt, however. This area had inflated several times in the past few years, and each occasion had ended without consequence.
But on November 10 a crescendo of quakes—some fairly violent—shook the area, and the ground convulsed dramatically; that supply of magma rapidly ascended and then pooled just 800 meters or so below Grindavík, prompting the town’s speedy evacuation. An eruption in the coming days was deemed highly likely; lava would make an incursion either within Grindavík, just offshore or around a line of ancient craters to the town’s northeast. “Things remained unsettled for at least 10 days after the initial crescendo,” Winder says. The pattern and locations of quakes and the deforming ground indicated that the large sheet of magma was shifting and spreading out at very shallow depths.
Over the next few weeks, this geological havoc tailed off, and the small possibility that there would be no eruption—because the magma had cooled too much, had lost its upward momentum or simply couldn’t find an escape route—started to seem a little likelier. Trouble soon appeared to be brewing again, however: the ground beneath Svartsengi quickly resumed inflating. That strongly suggested “it was not safe for residents to return to anything like ‘business as usual’ in Grindavík,” Winder says.
The pandemoniac nature of the crisis meant that forecasting what would happen next proved incredibly difficult. But on December 18, starting at about 8:00 P.M. local time, there was a sudden swarm of earthquakes—an indication something was about to give. And at 10:17 P.M., a fissure carved a path through the frosted earth, lava exploded outward, and the sky turned into a canvas of vermillion hues—all stunningly captured by a webcam. The peninsula’s fourth eruption had begun.
The fissure emerged about 3 km northeast of Grindavík, close to that ancient crater line, which suggested the magma found its escape through a zone of preexisting weakness in the crust. The worst-case scenario—an eruption within the town itself—had not happened. But during the first few hours of the eruption, officials were on high alert.
Lava was gushing from the fissure at up to 200 cubic meters per second—“around 10 times greater than the maximum rate during any of the prior Fagradalsfjall eruptions,” Winder says. And the fissure itself was rapidly expanding to the south, becoming 4 km long by midnight. “It started unzipping very, very quickly,” Mitchell says. “The length of that fissure was astounding.” If that continued, or if the lava turned to flowed south, Grindavík would be hit within a matter of hours.
The hope was that the eruption’s output would simmer down, and the fissure would stop proliferating. “It is often the case with these eruptions that they are the most powerful at the beginning,” said Kristín Jónsdóttir, a volcanic hazards expert at the Icelandic Meteorological Office, to local news. But this isn’t a hard-and-fast rule: the peninsula’s 2021 eruption started unleashing considerably more lava over a month after it debuted.
Ideally, lava would flow only to the uninhabited north, away from any major infrastructure. By the morning of December 19, there were some encouraging signs that the situation was improving: the eruption had become less intense, and the fissure had stopped expanding. As with most fissure eruptions at this stage, “order was coming out of chaos,” Mitchell says.
Serendipitously, the fissure had not extended south past a watershed—essentially a topographic feature that channels fluids, particularly water but also lava. “As it stands, the fissure stopped propagating southward just north of this point, meaning lava is currently flowing to the north and east, away from the town,” Winder says.
The peninsula may have narrowly dodged a proverbial bullet. “Over the coming days, the most likely scenario is that the eruption rate reduces significantly, and effusion localizes to a smaller section of the fissure system,” Winder says. “This will most likely be close to the middle and where it first opened up.” Each eruption is idiosyncratic, however, so scientists cannot be certain how the eruption will evolve. It could peter out sooner than many think—or, if it’s continually replenished with magma rising from below, it may last weeks or months.
If the eruption “goes on for a long time, it can cover a wide area,” says Mike Burton, a volcanologist at the University of Manchester in England. Some of that lava may be deflected south and ultimately threaten Grindavík. Although it could also flow toward the power plant to the west, a series of walls constructed since the volcanic crisis began in November should help deflect some of that molten rock. A major road to the north could be severed if the lava keeps flowing that way—that is far from the direst scenario, however.
It may sound counterintuitive at first, but an eruption unnervingly close to the town is a better outcome than no eruption. Waiting for more than a month for the lava to show “took a mental toll on the people of Grindavík,” Mitchell says. Without that forecasted release of magmatic pressure, a return home would feel inherently risky, as if residents were walking atop exploded ordinance. Now that the eruption has got going, “there’s probably almost some relief for those people now,” Mitchell says. But of course, “there’s still uncertainty as to what’s to come.”