A warmer world could be a more explosive one. Global warming is having a much more profound effect than just melting ice caps – it is melting magma too.
Vatnajökull is the largest ice cap in Iceland, and is disappearing at a rate of 5 cubic kilometres per year.
They say that, as the ice disappears, it relieves the pressure exerted on the rocks deep under the ice sheet, increasing the rate at which it melts into magma. An average of 1.4 cubic kilometres has been produced every century since 1890, a 10% increase on the background rate.
In Iceland there are several active volcanoes under the ice. The last big eruption was in 1996 at Gjàlp, and before then in 1938 – a gap of 58 years. But Pagli and Sigmundsson say that the extra magma produced as the ice cap melts could supply enough magma for similar eruptions to take place every 30 years on average.
Predicting the eruptions precisely will be tricky, though, as the rate of magma migration to the surface is unknown.
The situation in Iceland does not necessarily mean magma will be melting faster around the world. Vatnajökull sits atop a boundary between plates in the Earth’s crust, and it is this configuration that is allowing the release in pressure to have such a great effect deep in the mantle.
But the thinning ice has another effect on volcanoes which will be more widespread.
As the amount of weight on the crust changes, geological stresses inside the crust will also change, increasing the likelihood of eruptions. “Under the ice’s weight, the crust bends and as you melt the ice the crust will bounce up again,” explains Bill McGuire of University College London in the UK, who was not involved in the study.
Pagli say places likely to be at increased risk of eruption due to ice-melt include Antarctica’s Mount Erebus, the Aleutian Islands and other Alaskan volcanoes.
The shifting stress might even cause eruptions in unexpected places.
“We think that during the Gjàlp eruption, magma reached the surface at an unusual location, mid-way between two volcanoes, because of these stress changes,” says Pagli.
McGuire thinks the Vatnajökull study is based on “perfectly reasonable” physics. However, he says that climate change presents an even more explosive threat. “It’s not just unloading the crust that triggers volcanic activity but loading as well.”
He and his team are looking into the effects that rising sea-levels – also a consequence of melting ice caps – will have on volcanoes. “We are going to see a massive increase in volcanic activity globally,” he told New Scientist. “If we look back at previous warm periods, that is what happened.”
Journal reference: Geophysical Research Letters (in press)
Climate Change – Want to know more about global warming: the science, impacts and political debate? Visit our continually updated special report.