Photograph by Eggert Norðdahl, Demotix/Corbis

Read Caption

The Bárðarbunga volcano, cloaked under the Vatnajokull glacier in central Iceland, has started rumbling (top left), raising concerns of an imminent eruption.

Photograph by Eggert Norðdahl, Demotix/Corbis

Q&A: Why Iceland’s Volcanoes Have Vexed Humans for Centuries

Author Alexandra Witze explains why Iceland has so many, and such troublesome, volcanoes.

A swarm of earthquakes recently began rumbling beneath Iceland's second tallest volcano, raising fears about a repeat of 2010, when ash clouds from another Icelandic volcano halted thousands of international flights.

Iceland's namesake ice is one of the reasons the nation's volcanoes keep making the news, says science writer Alexandra Witze, co-author of Island on Fire: The Extraordinary Story of Laki, the Volcano That Turned 18th-Century Europe Dark (now out in the United Kingdom; to be published in the United States this winter). Ice-covered volcanoes can be highly explosive and produce more fine ash than other volcanoes. That ash can be carried high and wide, threatening air travel.

The volcano that's raising fears of an eruption is called Bárđarbunga, located near the center of Iceland beneath the Vatnajökull ice cap, which is Europe’s largest, covering about 3,130 square miles (8,100 square kilometers).

Witze says that Iceland's volcanoes have affected human communities for more than a thousand years and that Bárđarbunga is responsible for the largest eruption anywhere on Earth in the last 10,000 years. (Related Video: Volcanoes 101)

What have been some of the biggest volcanic eruptions in Iceland?

Some of the most famous ones in the last couple of decades have been Surtsey, a brand-new island that rose from the waves off the southern coast in 1963, and Heimaey, another island where in 1973 an eruption began in the middle of the night, in the middle of a town.

But if you go further back in time you can find much more devastating examples. In the year 1104, the volcano Hekla covered more than half the island with pumice. And in 1783, Laki erupted for eight months, pouring out the biggest lava flow in recorded history. Laki also emitted more than 100 million tons of sulfur dioxide, which drifted over Europe to form a choking fog that damaged crops and changed the climate for years.

Is Bárđarbunga particularly active or dangerous?

About 8,000 years ago it generated the largest eruption anywhere in the world for the past 10,000 years. Bárđarbunga, or a fissure very near it, last erupted in 1910.On average, this volcano has erupted [more frequently,] about five times each century for the past 7,600 years. (Watch the volcano’s current activity on an Icelandic Meteorological Office webcam.)

Why does Iceland have so many volcanoes?

Iceland has around 30 volcanoes, depending how you count the active mountains and fissures. There are two main reasons why there are so many. For one thing, Iceland sits on the boundary between two tectonic plates that are spreading apart, with the North American plate moving westward and the Eurasian plate moving eastward. Molten material rises to Earth's surface all along this Mid-Atlantic Ridge.

Second, Iceland sits atop a plume of hot material welling up through the mantle, which boosts the flow of molten material beneath the island even more than elsewhere along the Mid-Atlantic Ridge. All this fresh magma means that all of Iceland was formed in the last 25 million years.

Why are eruptions in Iceland of such concern?

Iceland is right in the middle of a lot of aviation routes that cross the North Atlantic. And it's not too far from mainland Europe, so if the wind patterns are just right its ash can be carried across Scandinavia, Great Britain, and farther into Europe. Volcanic ash can shut down jet engines if it gets sucked into the engine intake.

And of course a lot of Iceland's volcanoes are covered by ice, which makes them particularly unpredictable.

What happens when a volcano erupts under ice?

Eruptions that happen under ice are often more explosive than those that happen on dry land. The meltwater and the magma interact to form small particles of ash that can be blown high into the air.

View Images

Eyjafjallajökull erupts in April 2010.

This happened in Eyjafjallajökull in 2010; for a couple of weeks the magma erupted quietly out of a bare ridge on the mountain, but as soon as the eruption changed and the magma started coming out beneath a glacier, you got this very high plume of ash that started blowing south and east, toward continental Europe.

How thick is the ice over the Bárđarbunga volcano, and does that make it more or less dangerous?

The ice over Bárđarbunga is at least several hundred meters thick. To some extent the weight of overlying ice actually suppresses how explosive eruptions can get. Scientists are studying how climate change might affect volcanoes, on the theory that the less ice there is on top the more they might erupt in the future. But the interaction between volcanoes and ice is very complicated, and it can be really hard to predict what will happen in a given situation. (Related: “Scientist Models Deadly Volcano Eruptions With Harmless Baby Powder.”)

Should people be worried about air travel now or when passing by Iceland in the future?

Don't cancel your plane tickets just yet. If molten material actually makes it through the ice to the open air, there could be an ash plume, but it may not affect air travel as significantly as previous eruptions. Since the air-traffic-stopping eruptions of 2010 and 2011, scientists have gotten a lot better at forecasting how volcanic ash spreads through the atmosphere, and airlines have adopted new rules about what levels of volcanic ash are safe to fly through.

Editor's Note: An incorrect size comparison of the Vatnajökull ice cap has been deleted.