One of Iceland's most extensive volcanoes is showing signs of restlessness. As reported by the Iceland Monitor, tremors beneath the southern Katla volcano reached magnitudes of about 4.6 at the beginning of this week, followed by another series hitting around 3.3M. These tremors could indicate that magma is rising up through the crust and fracturing it, but according to geophysicist Gunnar Guðmundsson of the Icelandic Met Office (IMO), there are no concrete signs yet of magmatic ascension.
Small effusive episodes do occur at Katla on a semi-regular basis, producing iridescent lava flows that are not particularly hazardous to anyone nearby, but it’s long overdue for a major eruption. At present, though, there’s no immediate danger of it happening.
“On average the time between eruptions is 50 years but now the volcano hasn‘t [significantly] erupted in 98 years,” Kristín 
Jónsdóttir, earthquake hazards coordinator at the IMO, told RÚV. “There will be an eruption, it‘s only a question of when.”
The mighty Katla has erupted at least 20 times in the last 1,000 years or so, but it has not erupted violently for about a century. Its last event, in 2011, was not powerful enough to break through the ice cap, the vast Mýrdalsjökull glacier – but perhaps next time, it will.
If it does, it is likely a huge, sustained ash column – something akin to the one that Eyjafjallajökull produced in 2010 – will be generated. More dangerously for Iceland itself, there is a possibility that the glacier will suddenly melt and cause catastrophic flooding to the valleys below.
The 2011 event caused flooding bad enough to sweep away a major bridge at Múlakvísl, without any fatalities. A more paroxysmal eruption, however, could produce something more devastating.
Katla, like all Icelandic volcanoes, is being closely monitored, and all technical equipment is currently being checked to ensure it is operating correctly.
Image in text: Katla explosively erupting in 1918. RicHard-59/Wikimedia Commons; Public Domain
Trying to predict when a volcano will erupt is notoriously tricky – second, perhaps, to determining when an earthquake will strike a particular fault line. Volcanoes come in a variety of shapes and sizes, from the supervolcanic cauldrons (“calderas”) and the mountain-like peaks to the enormous shield volcanoes and hellish fissures.
Each has its own eruption style, with viscous, gassy magmas tending to produce the most explosive eruptions and fluid, superhot magmas often bursting out into the sky or over the volcano’s flank without harming a single soul. Iceland’s volcanoes tend to produce outbursts of the latter, but the ice above them adds an extra dimension.
When lava is mixed in with ice in a turbulent fashion, the trapped ice melts and rapidly expands, causing a series of explosions. These explosions could unleash more lava from beneath, which causes additional explosions, and so on.
This type of eruption can sometimes be harmless, but if there’s enough lava and ice, it can generate vast, ash-filled plumes – just like the one that shut down European airspace six years ago. Katla could engage in this sort of eruption style, but as always, only time will tell if it will.
Another Icelandic volcano, Hekla, was thought by some to be priming itself for an eruption. It is six years overdue for an explosion, but, as others pointed out, its eruption recurrence rate is so unpredictable that it is as likely to erupt any day now as it is to erupt in a year or two from now.
Glaciers often act as caps to Iceland's volcanoes - and if they break, things become quite violent. Giantrabbit/Shutterstock
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