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Big volcanoes wake up fast

Crystal chemistry suggests magma changes quickly before a huge eruption

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2:16pm, February 1, 2012
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Long-slumbering volcanoes can jolt to life faster than students drinking Red Bull, a new study suggests.

Studies of millennia-old rocks that erupted at Santorini, Greece, show that the chemical composition of its magma changed just a few decades before the volcano blew its top around 1600 B.C. That blast came after 18,000 years of relative calm.

“All this happens at a very late stage relative to this long period of repose,” says Tim Druitt, a volcanologist at the Blaise Pascal University in Clermont-Ferrand, France. “There’s kind of a rapid wake up.”

Druitt and his colleagues describe the awakening in the Feb. 2 Nature.

Santorini’s Bronze Age eruption is one of the most famous in history. When the volcano blew, it sent a tsunami racing across the eastern Mediterranean that wiped out dozens of coastal towns. So much magma erupted that Santorini collapsed to form a lagoon. (Some scientists argue this is the source of the legend of Atlantis.) What was left of the volcano’s caldera forms the islands surrounding the lagoon, which is now dotted by a small peak created by more recent volcanic activity.

But what exactly set the volcano off after such a long quiet period has remained a mystery. “A fundamental question that we know very little about is what happens to reactivate these systems,” says Druitt.

To try to answer that question, his team decided to look at chemical elements within crystals that erupted from Santorini. Many of the crystals are zoned — layered with different chemical compositions that form as new pulses of magma arrive from deep within Earth’s mantle, mix and cool. Druitt and his colleagues looked at concentrations of the element magnesium in different crystal zones. Over time magnesium diffuses slowly through a crystal, moving about 10 micrometers a year at the temperature of the Santorini magma, so the scientists could calculate how quickly the crystals had formed. 

Essentially all of them turned out to be geological babies. “This made me sit up because you’ve got about 18,000 years since the last major eruption, and yet all the crystals seem to be a few decades old,” says Druitt. “That’s a hint that something is happening very late on before the eruption.”

Several cubic kilometers of magma may have welled up from the depths, then melted and mixed with other rocks just below the surface and primed Santorini for its mammoth eruption. It’s still not clear what may have caused more magma to rise in the first place, says David Pyle, a volcanologist at the University of Oxford in England.

Other scientists have shown rapid changes in magma happening before big eruptions, such as before that of California’s Long Valley Caldera 760,000 years ago. But Druitt’s team has put together one of the most compelling cases, says Mary Reid, a volcanologist at Northern Arizona University in Flagstaff.

Researchers keep a close eye on volcanoes that have erupted recently, Druitt says, but in light of the recent findings, scientists also might want to add big volcanoes that look inactive to their watch lists.

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