New method may help anticipate large volcanic eruptions

Volcanic eruptions are not easy to anticipate. Now, a new paper proposes a way to provide early clues by evaluating magma movement far beneath volcanoes.

The Bárdarbunga volcanic system in Iceland began to erupt from a fissure on Aug. 29, 2014. By the time it quit six months later, it had created an almost 33-square-mile lava field, the largest in Iceland in 200 years. For many scientists, the size of the eruption was unexpected.

“Our work identifies the underlying mechanisms at work to cause such a behavior at a volcano,” said paper co-author Ronni Grapenthin from the University of Alaska Fairbanks Geophysical Institute.

“Since we’re using very basic physics, this isn’t tailored to just this one volcano, but instead very transferrable to other volcanoes, hopefully helping us to anticipate such eruptions better in the future,” he said.

These kinds of fissure eruptions are not uncommon. Over the past 10,000 years, some of the largest effusive eruptions, where lava steadily flows out, were fissure eruptions in Iceland. The volumes of basaltic lava that seeped out ranged from approximately 6 to 10 square miles. However, this kind of eruptive behavior is hard to predict because the signals, such as small increases in earthquake activity and magma movements, can be subtle.

“What the 2014-2015 Bárdarbunga eruption showed is that you can have very large eruptions, in terms of the lava volume, with only small or insignificant precursors, making them very hard to anticipate,” Grapenthin said.

This kind of behavior seems counterintuitive and is at odds with widely used scientific models for interpreting volcanoes.

To understand more, scientists investigated the conditions required for eruptions to begin and progress towards caldera collapse — when a significant part of a volcano collapses and large-volume eruptions occur.

The scientists developed a new method that takes into consideration three effects that influence how magma accumulates and forces its way to the surface.

First, if magma is less dense than the rock surrounding it, it wants to float. Accumulating magma can have a large upward-directed buoyancy force. This force can break surrounding rock, allowing magma to flow upward.

Second, the host rock can deform and flow, like a heated metal. Solid rocks can yield to the magma, creating space for new magma without fracturing or showing signs of activity at the surface.

Lastly, magma can create sturdy pipe-like pathways by eroding away part of surrounding rocks where it flows.

Taking these behaviors into consideration, the researchers re-examined data from the Bárdarbunga eruption. They found that their method can account for the sequence of events that led to the 2014-2015 Bárdarbunga caldera collapse and its associated unrest and eruption.

The researchers hope that scientists and agencies that monitor volcano activity around the world, including in Alaska, can use this information to better anticipate when a large eruption could occur with only minor precursory activity.

In Alaska, the effects of eruptions are far-reaching. Many air passenger and cargo routes follow or cross the chain of volcanoes that stretches from Cook Inlet into the western Aleutian Islands.

“The impact of any eruption can be felt far beyond the state, so we want to make sure to understand the processes as good as we can and provide a realistic assessment of the situation,” Grapenthin said.

In order to do this, Grapenthin said scientists need high-quality, continuous data and observations.

“In Alaska, we have many well-instrumented volcanoes and several efforts in motion to expand and modernize the existing monitoring networks,” he said. “It will be important to sustain these efforts into the future so we will be able to understand the subtle changes at volcanoes.”

The paper, “Unexpected Large Eruptions from Buoyant Magma Bodies Within Viscoelastic Crust,” was published in Nature Communications in May. Freysteinn Sigmundsson from the University of Iceland was the lead author.