BIG SPLASH THEORY Friday, June 19, 2009

Big splash theory says meteors hit regularly

A LARGE asteroid or comet, the kind that could kill a quarter of the world's population, smashed into the Indian Ocean 4800 years ago, producing a tsunami more than 180 metres high - about 13 times as big as the one that hit Indonesia almost two years ago.

The startling claim is made by a group of researchers, including Australians, who cite as evidence a newly discovered crater, 29 kilometres in diameter, 3800 metres below the surface of 1600 kilometres south-east of Madagascar.

Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the past 10,000 years. But the self-described "band of misfits" that make up the Holocene Impact Working Group say astronomers simply have not known how or where to look for evidence of such impacts along the world's shorelines and in the deep ocean.

Scientists in the working group say the evidence for such impacts during the past 10,000 years is strong enough to overturn estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to 1 million years, as astronomers now calculate, catastrophic impacts could happen every few thousand years.

The researchers, who formed the working group after finding one another through an international conference, are based in the US, Australia, Russia, France and Ireland. This year the group started using Google Earth, a free source of satellite images, to search the globe for chevrons - enormous wedge-shaped sediment deposits, that are composed of material from the ocean floor - which they interpret as evidence of past giant tsunamis.

Ted Bryant, a geomorphologist at the University of Wollongong, was the first person to recognise the palm prints of mega-tsunamis. Large tsunamis of 10 metres or more were caused by volcanoes, earthquakes and submarine landslides, Dr Bryant said, and their deposits have different features.

Deposits from mega-tsunamis contained unusual rocks with marine oyster shells, which could not be explained by wind erosion, storm waves, volcanoes or other natural processes, he said.

"We're not talking about any tsunami you've ever seen. Aceh was a dimple. No tsunami in the modern world could have made these features." Dr Bryant identified two chevrons found about six kilometres inland from the Gulf of Carpentaria. Both point north.

When Dallas Abbott, an adjunct research scientist at Lamont-Doherty Earth Observatory, in New York, visited a year ago, he asked her to find the craters.

To locate craters Dr Abbott uses sea surface altimetry data. Satellites scan the ocean surface and log the exact height of it. Underwater mountain ranges, trenches and holes in the ground disturb the Earth's gravitational field, causing sea surface heights to vary by fractions of a centimetre. Within 24 hours of searching the shallow water north of the two chevrons she found two craters.

She obtained samples from deep sea sediment cores taken in the area by the Australian Geological Survey.

"We think these two craters are 1200 years old," she said. The chevrons are well preserved and date to about the same time.

The New York Times