Volcanic obsidian lava flows for a year
An explosive volcanic eruption in Chile has given scientists the first direct insight into the strange motion of rocky lava known as obsidian flow.
Rather than a red river of molten rock oozing down a mountainside, this lava is a thick, rubble-strewn mass of rock.
Dr Hugh Tuffen, from the University of Lancaster, and his colleagues took the first measurements of this lava flow.
They found that it kept moving more than a year after the eruption.
Their results were published in the journal Nature Communications.
To carry out their study, the team visited a volcano in the south of Chile called Cordon Caulle in January 2012.
"There had just been an enormous eruption [in June 2011] that was still continuing," Dr Tuffen told BBC News.
This explosive eruption produced rhyolite - the rock that creates the "incredibly sticky magma" that results in these strange obsidian flows.
"This was our chance to get right in there and see an obsidian flow moving for the first time," he said. No-one had previously witnessed an obsidian flow moving.
As well as witnessing the ongoing volcanic eruption, the researchers recorded the lava - a heaving mass of black rock more than 30m (100ft) thick - as it "slowly creaked its way down the mountain".
They captured footage and images of the flow. Piecing these images together produced a 3-D model of the obsidian flow's changing shape.
"What was exciting was that it kept on going," said Dr Tuffen.
The obsidian flow has a thick insulating crust that keeps it warm
"Even after the eruption had stopped - it was still flowing away many months afterwards."
Initially, the lava advanced several metres per day, then as its thick rocky crust cooled, it slowed. But, according to Dr Tuffen's data, over a year after the eruption, the lava was still advancing at a rate of 1.5m to 3m per day.
Crucially, the lava flow altered as pieces of the rocky mass collapsed and the hotter lava leaked out in what the scientists referred to as "breakouts".
This, Dr Tuffen said, revealed "lots of secrets about how lavas advance".
"So now we can make new models that can encompass all the different sorts of lavas on the planet and how they move."
Dr William Burton, of the US Geological Survey, said: "What these guys have done is observe processes along the margins that may govern how these things flow - the breakouts.
"Knowing what they learned in this paper may help predict where this flow is going to go next.
"So if it is going close to a town you might be able to use some of the lessons they learned to [work out changes in its movement]."
Dr Burton added that the most likely place for the next rhyolitic volcanic eruption was near Mono lake, in northern California - a popular ski area.
Prof Peter Sammonds, an earth scientist from University College London, said the imaging techniques the team had developed would be "powerful" for the future study of volcanoes.
Breakouts or collapses at the edges of the flow can alter its motion
"The modelling of these lava flows is quite important," he said. "These patterns can seen as a general feature [of volcanoes] - it doesn't simply relate to this one.
Volcanoes that have produced rhyolitic eruptions are found all over the world, and have been some of the biggest on Earth.
These include the US "supervolcano" eruptions at Yellowstone National Park in Wyoming, and Novarupta in Alaska, which erupted in 1912 and was the biggest of the 20th Century.
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