Wednesday, January 03, 2007

Can new research change our knowledge about volcanoes?

Ordinary volcanoes spew lava, erupting magma, from cones or vents. But in the case of a supervolcano, the underground magma chamber bursts out in a titanic explosion with a force thousands of times that of a normal eruption and huge amounts of ash, dust, and poisonous sulphur dioxide are thrown into the atmosphere, leaving a giant crater or caldera.

Such large eruptions of greater than 100 cubic kilometres of magma are generally rare and random events worldwide.

But geologist Darren Gravley of Auckland University and his colleagues have shown that one of the largest supervolcano eruptions on record, at Taupo 250,000 years ago, was twice as big as previously thought.

They have published in the Bulletin of the Geological Society of America evidence that the eruption in the Taupo Volcanic Zone was actually two supervolcanoes 30km apart which erupted within days or weeks of each other.

It is the first time such a close pairing of supervolcano eruptions has been documented and provides scientists with a new understanding of the potential linkage between geographically separate caldera volcanoes. Each eruption belched out more than 100cu km rock and volcanic ash, creating what are now known as the Mamaku and Ohakuri volcanic deposits.

"It's possible one of these triggered the other," said Gravley. But exactly how the triggering might have worked is uncertain.

What is clear from the university's explorations is that they were created very close in time – a surprising discovery because most caldera or "supervolcano" eruptions in any one region tend to be tens of thousands of years apart, according to accepted theories.

Among signs the rocks from the two eruptions were piled on one another is the lack of erosion on the first volcanic deposits – which is striking, considering the eruptions would have been followed by heavy rains.

Previous studies that looked only at the radioisotope dates of the volcanic rocks from the eruptions missed the timing details, Gravley said, as they had a margin of error of 10,000 years.

"You've got to look at the physical evidence," said Gravley. "It's really getting into the nitty-gritty. From the stratigraphy (rock layers) it's clear two were erupting at the same time. That just blows away any (regional frequency) studies out of the water."

The bad news is that double eruption represents a whole new way that supervolcanoes can threaten humanity.

Caldera researcher Gerardo Aguirre, of Mexico's National University in Juriquilla, said caldera eruptions were far less frequent than other volcanoes.

But when they did erupt, "the consequences for the surroundings and in general for the world would be enormous, because these explosive eruptions are many orders of magnitude bigger than a more common eruption from a volcano, such as Mount St Helens or Vesuvius."

Last year, other research at Taupo – on the more recent Taupo supervolcano of only 26,500 years ago – changed accepted theories that it takes hundreds of thousands of years for the reservoir of molten rock, or magma, beneath a supervolcano to build up to an eruption. They showed the period between super-eruptions can be much shorter, perhaps a few tens of thousands of years.
Dr Bruce Charlier, from Britain's Open University, showed the build-up at Taupo was no more than 40,000 years – a relatively short time period in geological terms.

"Our findings mean that we have to reassess our understanding of the speed at which the volcano can reactivate, and this has important implications for volcanic monitoring and hazard mitigation at Taupo and similar volcanoes worldwide."

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