Antarctica s violent past

Antarctica s  violent past scientists drilling into sediment off the coast in Antarctica, found evidence of one of the most violent volcanic eruptions ( New Scientist , Vol 160, No 2161). The explosion, occurred some 25 million years ago, blasted 30 cubic kilometres of ash and volcanic debris 50 kilometres or more into the stratosphere. "We had no idea that Antarctica's volcanic history included an event as dramatic as this one,' says Peter Barret, a geologist from the Victoria University in Wellington, New Zealand.

Barret heads a team of 50 scientists from New Zealand, Australia, Britain, the us, Germany and Italy. They mounted a drilling rig on sea at Cape Roberts, about 140 km from the us base at McMurdo. They said the blast was as big as the eruption of Karkatoa in Indonesia in 1883.

Drilling is being done through the ice, into the sea bed. The scientists have managed to penetrate 200 metres (m) into the sediment this year. At 110 m, they found a deposit of pumice, a frothy form of volcanic glass, 1.2 m thick. "A deposit that thick can only have been made by a large explosion,' says Barret. Two smaller deposits, one about 20 centimetres thick, have also been found. They may be from smaller explosions, but are probably from the same volcano, says Barret.

On the basis of fossil remains of diatoms or single-celled algae, scientists concluded that the deposits were 25 million years old. They are sure that they will be able to date the blast to within 100,000 years by measuring the radioactivity of the volcanic debris itself.

The pumice are full of holes, indicating that the explosion was violent and released a lot of gas. Given the size and coarseness of the particles in the rocks, which are up to a centimetre in diameter, the scientists believe that blast occurred about 100 km from where the ash was deposited.

Scientists expect that the find will tie in with the evidence uncovered elsewhere about the world climate and environment at that time.

They say there are two possibilities: Because the ash went so high, it is possible that stratospheric winds spread a cloud around the globe, which would have caused temperatures to fall. Alternatively, the ash may have remained near the pole, causing the region to cool and leading to a formation of massive ice sheets.

Evidence of glaciation 108 m down the drill hole would support this theory. "When we get a more precise date, we want to compare this event with what was going on elsewhere in the world at that time,' says Barret.