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The 2010 eruption of Iceland's Eyjafjallajökull volcano caused transatlantic travel chaos. Now, all eyes are on Katla, which has been eerily active in recent months. Could Europe be facing another volcanic catastrophe?
All is quiet outside Iceland's Katla volcano - for now
Katla is widely considered to be Iceland's most-monitored volcano - and with good reason.
Icelandic scientists have warned that an eruption could trigger mass flooding, as well as a plume of fine ash that could cause a repeat of the travel disruptions that rocked the airline industry during the Eyjafjallajökull volcano's period of restlessness last year.
Now, new seismic data out of Iceland suggests that Katla could be roaring back to life.
A post published last week on Iceland's Meteorological Office (IMO) website pointed to an increase in seismic activity at Katla starting in July, including an "intense swarm" of earthquakes on October 5. Overall, hundreds of micro-earthquakes have been registered in Katla over the past few months.
But experts still aren't sure if that means Katla is getting ready to blow - and if it does erupt, whether they can expect a major event. The IMO was quick to point out that there was no cause for immediate alarm.
"There are presently no measurable signs that an eruption of Katla is imminent; however, given the heightened levels of seismicity, the situation might change abruptly," the office said in a statement on its website.
Katla's last major eruption was almost a century ago, in 1918. That eruption lasted less than a month, making it shorter than 2010's eruption of the Eyjafjallajökull volcano. Yet Katla averages two eruptions every 100 years, raising fears that it could be about to burst.
The eruption of Iceland's Eyjafjallajökull volcano in 2010 produced a huge ash cloud
No crystal ball
When it comes to making predictions about Katla, experts today have little to go on. Though they have high-tech scientific equipment to measure modern-day eruptions, those instruments were lacking in 1918.
"We don't know how it behaved exactly at that time," said Gunnar Gudmundsson, a geophysicist at the Icelandic Meteorological Office, in an interview with Deutsche Welle.
But Gudmundsson noted that historical accounts dating back to the 1700s at least indicate that more sizeable earthquakes preceded volcanic eruptions. And those rumblings were felt in nearby communities, such as Vík, a village located at the foot of the glacier that covers Katla.
The earthquakes registered by the IMO at the eastern part of Katla's caldera, or upper edge, last week were over magnitude 3, but none passed a magnitude 4 on the Richter scale.
Gudmundsson also told Deutsche Welle that the increased activity seen at Katla had since diminished and that overall, earthquakes measured at Katla in recent months were both small and shallow.
Predicting the scale of the next eruption is equally difficult.
The last time Katla blew, in 1918, the eruption was quite powerful. There has also been ongoing speculation that the impact from the next eruption could even dwarf that of the Eyjafjallajökull volcano in 2010.
But Gundmundsson said that although a Katla eruption would always result in explosive activity due to the interaction of magma with glacial ice, history doesn't provide many clues on what to expect.
"They have been different, these eruptions from Katla," he said. "Some are large, and some are small."
Icelandic officials have evacuation plans in place in the event of a Katla eruption
A seasonal affair?
Moreover, the spike in seismic activity might not necessarily be a sign that Katla is gearing up for a major eruption in the near future - it could also be the result of something as simple as the changing of the seasons.
Reidar Trønnes, a professor at the Natural History Museum at the University of Oslo, explained that ice melt over the summer results in higher seismic activity during the fall months. These "seasonal bursts of activity," he said, are well-documented.
"I've not heard any specific news from Iceland that this is signifying anything more than ordinary autumn activity," Trønnes told Deutsche Welle.
Katla is located on the southern edge of Iceland, where it shares magma channels with the Eyjafjallajökull volcano, the culprit behind the massive ash cloud over Europe in 2010.
Trønnes acknowledged that Katla is long overdue for an eruption - considering its two-eruptions-a-century average - but advised that predicting when it might happen should be done with caution. He noted that the eruption sequence witnessed in southern Iceland in past years could have affected Katla's pattern.
"I'm very skeptical to these predictions that Katla should erupt seriously now," he said. "Because I think that it might be that the whole Katla behavior is more erratic, more irregular, than we have come to believe."
A plan for Katla
A major eruption of the Katla volcano could cause a repeat of last year's travel chaos
Some experts also think that Katla's behavior in decades' past suggests that 1918 wasn't actually the volcano's last eruption. Dave McGarvie, a senior lecturer in volcanology at the Open University in Scotland, told Deutsche Welle that there is also evidence pointing to smaller-scale eruptions in 1955, 1999 and even this year.
In July, the area around Katla was hit with a glacial flood and afterwards, cracks were observed in the ice layer over the volcano - an observation that suggested a small eruption could have taken place.
McGarvie acknowledged the seasonal character of the recent earthquakes at Katla, but said the location of that seismic activity has changed. Autumn earthquakes have typically been observed along Goðabunga, Katla's western flank, but they have started pushing farther east, toward the volcano itself.
That could indicate that Katla is "winding up to its next eruption."
"The actual pattern is a bit different to what we've seen in the past, and that's what's gotten people thinking," he said.
McGarvie, who visited Vík, the village near Katla, in early September of this year, said the volcano was a cause of heightened concern there - and a major topic of conservation in local cafes.
He noted that the village was busy shoring up its glacial flood defenses during his visit. But McGarvie also underscored Icelanders' deep trust in the country's scientists and authorities:
"They will have a plan for whatever Katla decides to do," he said.
Author: Amanda Price
Editor: Cyrus Farivar