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A photo of a warning sign blocking the road to Bardarbunga, an Icelandic volcano.

A warning sign blocks the road to Iceland's Bárđarbunga volcano.

Photograph by Sigtryggur Johannsson, Reuters

Dan Vergano

National Geographic

Published August 21, 2014

Earthquake swarms are shaking up a large ice-capped volcano in Iceland, raising worries of an eruption that could trigger flooding and send ash clouds into the atmosphere.

The 2010 eruption of the Eyjafjallajökull volcano triggered floods and ash plumes that halted air travel to Europe. (Related: "Iceland Volcano Erupts Under Ice, Triggers Floods.")

Icelandic officials report that the minor quakes have occurred since Monday near the Bárđarbunga volcano, the country's second highest mountain at 6,560 feet (2,000 meters). It lies in the remote central region of Iceland under the largest glacier, Vatnajökull. The ice above the volcano's central caldera is about 2,300 feet (700 meters) thick. (Related: "Pictures: Iceland Volcano Erupts, Under Ice This Time.")

Map of Iceland showing the Bárđarbunga volcano, the country’s second highest mountain at 6,560 feet (2,000 meters), within the remote central region of Iceland under the largest glacier, Vatnajökull.
HANS H. HANSEN, FIXLANDA EHF; NGM MAPS

"Presently there are no signs of eruption, but it cannot be excluded that the current activity will result in an explosive subglacial eruption," says the Icelandic Meteorological Office in a statement. Such an eruption may lead "to an outburst flood (jökulhlaup) and ash emission."

No one lives near the volcano, but with nearly one temblor striking per minute, officials have evacuated tourists from the area north of it, where floodwaters would flow. The officials have also elevated air travel warnings to "orange" levels—which they define as meaning the "volcano shows heightened or escalating unrest with increased potential of eruption."

"Most potential volcanic eruptions don't lead to eruptions," says volcanologist Agust Gudmundsson of the University of London in the United Kingdom. "But if this one leads to an eruption, it could be a moderate-sized one, which will look quite large to most people."

Swarm Physics

The largest of the quakes around the volcano was magnitude 4.5 in size, a mild one that struck on Monday. But the continuing pattern of quakes and their drift to the northeast raises concern among geologists, Gudmundsson says.

The intrusion of magma may stall, if there is not enough pressure. But it is too soon to tell.

Right now a finger of magma, or molten rock, which rose from the Earth's mantle more than 12 miles (20 kilometers) underground, is filling a fracture in the crust beneath the volcano. The plume appears to be traveling horizontally to the east at a depth of more than 3.1 miles (5 kilometers). Its temperature is more than 2,200 degrees Fahrenheit (1,200 degrees Celsius).

"It's like molten glass, it's viscous stuff," says geologist Marianne Guffanti of the U.S. Geologic Survey. As it intrudes into surrounding rock, it breaks the rock and triggers earthquakes.

Iceland monitors seismic activity, stream flow from glaciers, and other indicators for signs of an eruption—a highly regarded observation effort that was honed by the 2010 eruptions. "The intrusion of magma may stall, if there is not enough pressure," Guffanti says. "But it is too soon to tell. It is a very hidden process."

Fire and Ice

Ice caps the volcano, which complicates the efforts at prediction, says Gudmundsson. If the magma erupts beneath the ice, the result would be sudden floods and a buildup of water vapor and pressure under the ice cap that could lead to a tremendous explosion, sending ash as high as the stratosphere. That's what happened in 2010.

Whether the eruption would exactly mimic the one in 2010 is hard to say, because the basaltic magma beneath Bárđarbunga is of a different variety, and the grain size of ash depends on complex interactions with water vapor. Smaller grains travel higher and pose more of a threat to jet engines.

A sudden jökulhlaup flood from the region would sweep north within an hour of the eruption and take nine hours to reach Iceland's northern coast, Gudmundsson says.

Bárđarbunga is more than 124 miles (200 kilometers) from Iceland's capital, Reykjavík, so it poses little threat to populated parts of the island. Aside from some farms on the coastline, no one lives in the barren region threatened with flooding. But its remoteness made evacuation of tourists from the region a reasonable precaution.

Evacuation Precaution

If the magma were to travel far enough horizontally to erupt outside the ice-capped part of the region, then the eruption would look more like lava flows on Hawaii, Gudmundsson says, "only larger."

Like Hawaii, Iceland sits atop a hot spot in the Earth's crust, where magma readily travels from deep in the Earth's mantle to the surface. But it also straddles the Mid-Atlantic Ridge, the boundary along which the European and North American tectonic plates are spreading apart at a rate of about 0.8 inches (2 centimeters) every year. Magma wells up in that rift to fill the gap and produce new crust.

The conjunction of two sources of magma accounts for the prodigious volcanism that has built the entire island of Iceland—a place where significant eruptions are seen about once every five years.

The largest eruption seen worldwide in the last 10,000 years likely took place at Bárđarbunga roughly 8,000 years ago, Gudmundsson says. "There is plenty of magma beneath this volcanic system."

Follow Dan Vergano on Twitter.

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