Will a Volcanic Eruption Be a Burp or a Blast?

Last December, a gloopy ooze of lava began extruding out of the summit of La Soufrière, a volcano on the Caribbean island of St. Vincent. The effusion was slow at first; no one was threatened. Then in late March and early April, the volcano began to emit seismic waves associated with swiftly rising magma. Noxious fumes vigorously vented from the peak.

Fearing a magmatic bomb was imminent, scientists sounded the alarm, and the government ordered a full evacuation of the island’s north on April 8. The next day, the volcano began catastrophically exploding. The evacuation had come just in time: At the time of writing, no lives have been lost.

Simultaneously, something superficially similar but profoundly different was happening up on the edge of the Arctic.

Increasingly intense tectonic earthquakes had been rumbling beneath Iceland’s Reykjanes Peninsula since late 2019, strongly implying that the underworld was opening up, making space for magma to ascend. Early in 2021, as a subterranean serpent of magma migrated around the peninsula, looking for an escape hatch to the surface, the ground itself began to change shape. Then in mid-March, the first fissure of several snaked through the earth roughly where scientists expected it might, spilling lava into an uninhabited valley named Geldingadalur.

Here, locals immediately flocked to the eruption, picnicking and posing for selfies a literal stone’s throw away from the lava flows. A concert recently took place there, with people treating the ridges like the seats of an amphitheater.

In both cases, scientists didn’t just accurately suggest a new eruption was on its way. They also forecast the two very different forms these eruptions would take. And while the “when” part of the equation is never easy to forecast, getting the “how” part right is especially challenging, especially in the case of the explosive eruption at La Soufrière. “That’s a tricky one, and they nailed it, they absolutely nailed it,” said Diana Roman, a volcanologist at Carnegie Institution for Science.

Volcanologists have developed an increasingly detailed understanding of the conditions that are likely to produce an explosive eruption. The presence or absence of underground water matters, for instance, as does the gassiness and gloopiness of the magma itself. And in a recent series of studies, researchers have shown how to read hidden signals—from seismic waves to satellite observations—so that they may better forecast exactly how the eruption will develop: with a bang or a whimper.

Something Wicked This Way Comes

As with skyscrapers or cathedrals, the architectural designs of Earth’s volcanoes differ wildly. You can get tall and steep volcanoes, ultra-expansive and shallowly sloped volcanoes, and colossal, wide-open calderas. Sometimes there isn’t a volcano at all, but chains of small depressions or swarms of fissures scarring the earth like claw marks.

Lava flows from the Geldingadalur volcano have been relatively languid and predictable.

Photograph: Anton Brink/Anadolu Agency/Getty Images

Eruption forecasting asks a lot of questions. Chief among them is: When? At its core, this question is equivalent to asking when magma from below will travel up through a conduit (the pipe between the magma and the surface opening) and break through, as lava flows and ash, as volcanic glass and bombs.

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