Flowing lava can fracture and break, causing tiny earthquakes.
Magma erupting out of volcanoes can fracture and crack even though it is a liquid. This odd-sounding phenomenon — the ‘snapping’ of a flowing mass of molten rock — could help scientists studying volcanoes to better predict when a lava dome might collapse, an event that could be devastating to nearby residents.
Researchers led by Hugh Tuffen of Lancaster University, UK, wanted to better understand the hundreds of tiny earthquakes that accompany a volcanic eruption, and how they relate to the collapse of the lava dome, which typically builds up slowly and calmly, then suddenly explodes.
“It’s a really big challenge to predict when the onset of eruptive behaviour in volcanoes will occur,” says Tuffen. “The best clues we have are the pattern of extremely small earthquakes that take place directly under the lava dome.”
Tuffen’s hunch was that the lava reaching the surface was fracturing and causing the small quakes, with magnitudes of less than 3, in much the same way that larger tectonic rifts cause huge earthquakes, such as the magnitude-8 quake that recently struck in China. No definitive link between fractures and quakes had been shown for volcanic eruptions.
Tuffen's team sampled old, solidified lava from volcanoes in Iceland and California, and then recreated volcanic conditions for the samples. The Icelandic lava was glassy, and the Californian sample was crystalline. They then heated some samples of the rock to 600 ºC, some to 645 ºC and others to 900 ºC, and applied a range of different pressures and strains.
When subject to slow changes in strain, the lava bulged but still flowed, but when harder strain was applied, and more quickly, the lava cracked. This pattern of behaviour was evident at all temperatures tested, report Tuffen and colleagues in this week's Nature1.
This kind of behaviour, known as viscoelasticity, is the same as that seen in Silly Putty, which stretches when pulled gently, but snaps when yanked hard.
It had been thought that this behaviour would not occur in magma above a certain temperature, but Tuffen's data for the samples heated to 900 ºC now prove otherwise.
The Silly-Putty-like behaviour of magma is all down to its viscosity as it reaches the volcano’s lava dome. “There’s a really strong change in the nature of the magma as it rises,” says Tuffen. He is concerned with magma less than 2 kilometres below the volcano’s mouth. Here, the magma thickens, loses gas and for some types of lava the magma starts to form crystals.
These things all act to slow the magma down so that although on a long timescale it behaves as one would expect from a fluid, on a short timescale it can behave like a brittle solid. Tuffen says that the fractures in the flowing magma would be a few metres long and would form quickly, unlike those caused by tectonic plates, for which fault lines many kilometres long are formed over many years.
The cracks reported by Tuffen et al. come as no surprise to some experts. Steve Sparks, a volcanologist at Bristol University, UK, says that the cracking of crystalline magma has been suggested for a long time. “Understanding of this has gradually improved,” he says. He is more impressed by Tuffen’s observation that obsidian, a component of the glassy Icelandic lava, also shows the faulting behaviour.
A related paper by Yan Lavallée at the Ludwig-Maximilian University in Munich and colleagues, also published in Nature2, shows how the cracking of magma can be tracked during dome-building periods. These authors suggest that their models could be used to help predict explosive eruptions, and to help in volcano forecasting.
"It's imperative to know what's happening to the magma because of what's at stake," says Tuffen. With better understanding, predicting when evacuation of areas within range of a potential volcanic eruption are necessary could be much easier, he says.
Tuffen, H., Smith, R. & Sammonds, P. R. Nature 453, 511–514 (2008).
Lavallée, Y. et al. Nature 453, 507–510 (2008).