Over the volcano

The eruption of the Eyjafjallajökull last spring was unusual in all respects. Geophysicist Dr Andy Hooper analysed the events in pursuit of better understanding.

Dr Andy Hooper experiences the early phases of the Eyjafjallajökull eruption (Photo: Sigrún Hreinsdóttir)

Last spring, whilst air traffic across the Atlantic Ocean was shut down due to an Icelandic volcano spewing ashes out into the atmosphere, high overhead the TerraSAR-X satellite silently passed by, taking its regular readings, as it had been doing for nearly a year up to that point. In taking fresh measurements every 11 days, the satellite’s imaging radar had shown how the slopes of the Eyjafjallajökull (‘Island Mountain Icecap’) had risen by several centimetres prior to the eruption on its flank on 20 March 2010.

“Normally a volcano builds up pressure, and then an eruption takes place. The pressure is released and the mountain slopes sag back,” geophysicist Dr Andy Hooper (Aerospace Engineering) says about the cycle of life of normally active volcanoes. “But this case was different,” he says, referring to an article he co-authored, titled ‘2010 Eyjafjallajökull explosive eruption’, which was published in Nature. “There was no deflation in the first eruption at the flank. Then it became quiet, and two days later, on 14 April, the explosive phase of the eruption began from the central caldera. This time the lava was much stickier, which means it had been sitting in the volcano for some time.”
Older magma has a greater viscosity and thus traps more gas, which makes the eruption more explosive, throwing ashes high (6 to 9 kilometres) into the atmosphere.

In case one hadn’t noticed: volcanoes fascinate Dr Hooper. During his PhD research in California, he developed a new method for processing interferometric satellite data for monitoring volcanoes. Radar satellites scan the earth with radio waves. Changes in the distance to the Earth’s surface can be calculated according to the phase difference between successive passes. Comparison of the data over time allows researchers to monitor geological changes.

For their recent Nature article, Hooper combined satellite data with GPS measurements taken on the ground by the University of
Iceland’s Nordic Volcanological Center. It helped that Dr Hooper had spent two years there as a postdoc “on top of the volcano”. The reconstruction that he and his Icelandic colleagues presented in the paper is “complex and unusual”, showing the south flank of the volcano rising because of an underground magma channel (a ‘sill’) opening up by a decimetre or so. Two weeks later the eastern flank rose and eventually another sill in the centre began to close. The eruption however continued for another month, with lava flowing from great depths.

“Eruptions cannot be exactly predicted,” Dr Hooper says, “but we can forecast them better.” He believes that eventually more frequent satellite data and better geological models will allow for some sort of weather forecast for volcanoes. Air passengers will be keenly interested.

‘Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption’, Nature, 18 November 2010