Iceland's Bárðarbunga volcano system was racked by four large earthquakes at the end of October. The rumbles, reported as magnitudes 3.9, 3.2, 4.7, and 4.7, are a sign that pressure is building up in the volcano's magma chamber."Pressure in the system is increasing, that's for sure," says Sara Barsotti, the volcanic hazards coordinator at the Icelandic Meterological Office. This will eventually lead to an eruption, but Barsotti says it’s hard to say when that could happen, or how big the eruption might be.

This volcano could be particularly dangerous if it erupts, since it’s sitting partly beneath Europe’s largest ice cap, Vatnajökull. Volcanoes that erupt under ice can be highly explosive and produce lots of fine ash. When Iceland’s Eyjafjallajökull volcano erupted in 2010, its ash clouds wreaked havoc with air travel worldwide.

Earthquakes happen when waves of energy move through a solid, says Ben Edwards, a geology professor at Dickinson College in Pennsylvania. But they don't necessarily mean a volcano is going to explode immediately. Measuring 6,591 feet above sea level in Iceland's Vatnajökull National Park, Bárðarbunga is one of the largest, most active volcanoes on the island.

"Once a volcano starts erupting, we can track it better than when it's not erupting," Edwards says. "You kind of have to wait until nature does what it's going to do."

An Active History

Though Bárðarbunga's remote location may make it seem harmless, an eruption could have effects beyond Iceland. The volcano’s last eruption, which started in August 2014 and ended in February 2015, was the largest the country had seen in 200 years and spewed air pollution across Western Europe.

The imminent eruption could have varying impacts, depending on where it happens in the system. One-third of the volcano system is covered by ice, Barsotti says. Eruptions under ice can trigger explosions by turning large volumes of ice into steam, and as magma hits the steam it explodes into fine particles that can shoot high into the atmosphere.Or, lava from an eruption could melt the ice and cause flooding, like that brought on by the Eyjafjallajökull eruption .

When might this eruption happen? Barsotti says it's hard to answer the "one-million-dollar question," but Iceland’s Met Office will keep monitoring the situation. According to the Catalogue of Icelandic Volcanoes, this system erupts once every 50 years on average, and the last eruption started in 2014.

Iceland's restless volcanoes make it a perfect place to gather data on all kinds of eruptions.

Current Events

Bárðarbunga isn’t the only volcano lurking under the ice. Geophysicist Páll Einarsson of the University of Iceland told the Iceland Monitor in February 2017 that four of the island's volcanoes were showing increased activity and gearing up for eruption. The other three are Katla, Hekla, and Grímsvötn. Of these, Katla is considered the most dangerous.

When a volcano erupts, lava is the least of a community's problems, Edwards says. What's more dangerous is the ash propelled into the atmosphere, as air travelers learned from Eyjafjallajökull in 2010. Worse, in 1783, the eruption of Laki blanketed parts of Europe in toxic ash and changed climate patterns worldwide for several years.

As a nation that sits on the Mid-Atlantic Ridge, Iceland is no stranger to volcanoes. When Eldfell erupted in 1973, the country safely evacuated about 5,000 people from the island within one day. The eruption continued for six months and eventually, most of the residents returned.

"Iceland's ready," Edwards says. "Icelanders are prepared."