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