London, May 22 : Scientists have suggested that mini earthquakes observed at volcanoes might be caused by flowing lava, which can fracture and crack even though it is a liquid.
According to a report in Nature News, researchers led by Hugh Tuffen of Lancaster University, UK suggested this.
They 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," said Tuffen. "The best clues we have are the pattern of extremely small earthquakes that take place directly under the lava dome," he added.
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.
Tuffen's team sampled old, solidified lava from volcanoes in Iceland and California, and then recreated volcanic conditions for the samples.
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, reported Tuffen and his colleagues.
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," said Tuffen.
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.
According to Tuffen, 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.
"It's imperative to know what's happening to the magma because of what's at stake," said Tuffen. "With better understanding, predicting when evacuation of areas within range of a potential volcanic eruption are necessary could be much easier," he added.