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The mother-of-all extinctions in the history of life on Earth occurred at around 252.6 million years ago give or take 200,000 years thanks to an improved method for dating minerals in ancient rocks by Ian Metcalfe, deputy director of the University of New England Asia Centre. So massive was this extinction that nearly 96 percent of all marine species, at least half the main group of animals, many plant species, and up to 85 percent of all genera disappeared forever. The trilobites, which successfully survived for over 300 million years as a voracious ocean-floor scavenger, also became extinct.
What caused the massive extinction — an event known to the scientists as the "Great Dying"?
Prior to 23 February 2001, a few scientists came up with several imaginative explanations.
At first, some scientists thought the extinction had something to do with a severe change in climate. Certainly scientists found evidence for a change in climate at this time. It seems conditions were warming-up forcing shallow seas and swamps on land to dry up, and possibly helping to create the vast desert-like expanses in the interior of the great supercontinent. But the scientists could not explain exactly how the climate changed. Was there a high concentration of carbon dioxide in the atmosphere combined with a sudden energy burst from the early, but growing Sun?
Other scientists suggested the Earth had probably passed through one of the spiral arms of the Milky Way galaxy where an aging supergiant star lying near the vicinity of our Sun suddenly exploded, showing the Earth for a brief moment with deadly cosmic and gamma radiation.
Then on 23 February 2001, scientists announced the discovery of what appears to be a very large ancient crater not far from the east coast of South Africa (1). To make the discovery even more interesting, geological tests conducted to estimate the age of the crater all suggest the event had occurred at the end of the Permian period give or take a few million years.
Not long after this discovery, scientists noticed yet another large crater buried off the northwest corner of Australia. Measuring perhaps 193 kilometres in size, the Bedout crater (pronounced Beh-doo) as it is now called, is impressive to say the least. It makes the Arizona crater of more recent times (about 40,000 years ago) look like the impact of a speck of sugar in a tea cup compared to the massive sugar cube of a collision this ancient asteroid must have produced at this time in the Permian period. Whether the Bedout crater is part of the same massive crater near South Africa when Australia was closer to the African continent is hard to tell at the moment. Clearly both sites are large and appeared to have occurred at around the end of the Permian period.
Evidence to support what appears to be an impact site outside the Australian continent has been determined after studying the crystalline structures making up the melted rock layers drilled out by oil companies several decades ago on a ridge at the Bedout site. Apparently the oil companies had kept core samples from the area until it was analysed more closely by the team of scientists led by researcher Luann Becker of the University of California, Santa Barbara, USA (2). Now Becker is confident the crystalline structures showed evidence of a "shocked" pattern characteristic of an impact by a large rock in the area (either a meteor or something much larger such as an asteroid).
Because of its enormous size, scientists are still debating whether the Bedout crater is in fact a crater. As Peter D. Ward, a professor of geological sciences at the University of Washington in Seattle, USA, said:
'It's not yet persuasive that it's even a crater.' (3)
Despite the continuing controversy, the general consensus is that a large asteroid of no less than 11 kilometres in width (others claim perhaps even 16 kilometres wide, or roughly the size of Mount Everest) had indeed crashed to Earth towards the end of the Permian period resulting in massive volcanoes springing up in what is now India. The shock waves resulting from this massive impact travelled to near the opposite side of the Earth where the world's biggest volcanoes and earthquakes suddenly erupted over Siberia. For nearly a million years after the impact, lava poured from these volcanoes (estimated to be in the thousands) in such great quantities that if the Earth was absolutely smooth, the lava would have covered the entire planet up to 10 metres thick.
But since the Earth is not absolutely smooth, it is likely that at least a quarter (if not more than half) of the Earth's solid surface had suddenly become covered with red hot molten rock. So it would seem plausible that any life that tried to rely heavily on its ability to crawl at the bottom of the ocean looking for food had soon perished.
As for the rest of life on the great continents of the world, breathing the rarefied air at this time would probably have had their populations either wiped out or seriously curtailed due to the massive increase in the levels of carbon dioxide and other toxic gases released from the interior of the Earth by the volcanoes following the impact.
The link between mass extinctions, volcanoes and asteroid impacts and the likelihood this might be a common occurrence is being supported by some scientists including the researcher Ms Becker when she said:
'We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time.' (4)
Further studies are continuing to prove or disprove Becker's interesting claim. But one thing is certain: this extraordinary event is the closest life on Earth has ever got to being totally snuffed out by a piece of rock drifting through space.
## SPECIAL UPDATE ##
4 June 2006
A scientific team led by Dr Ralph von Frese and Laramie Potts of Ohio State University have found yet another crater dated to around 250 million years ago. This one is big. So big that scientists say they have found the Earth's biggest known crater. At 300 miles (482 km) wide, it is twice the size of the Yucatan peninsula's Chicxulub crater that killed off the dinosaurs nearly 64 million years ago.
The crater is hidden beneath East Antartic ice in the Wilkes Land region, south of Australia. It was noticed only after NASA satellites measured the gravitational field over the surface of the Earth where an anomaly was found in this region. The anomaly consists of a circular ridge and a massive central plug of material suddenly risen up into the crust. Scientists believe this is a classic tell-tale sign of a massive impact resulting in the denser mantle bouncing up into the crust above.
A combined false colour image of gravity and radar measurements of the Wilkes Land region (Image from Ohio State University).
An impact of this sort suggests an asteroid of 30 miles (48 km) wide could have created it. Life would not have been happy at this time. As von Frese said:
'All the environmental changes that would have resulted from the impact would have created a highly caustic environment that was really hard to endure.'
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