19 October 2009

Did giant Indian impact kill the dinosaurs?

Cosmos Online
Move aside Chicxulub; an even bigger asteroid impact off the coast of India may have been ground zero for the dinosaurs, U.S. and Indian scientists say in a controversial finding.
Shiva crater

Diagram shows a 3-D reconstruction of the submerged Shiva crater (~500 km diameter) off the coast of Mumbai in India. Credit: Sankar Chaterjee

PORTLAND, OREGON: Move aside Chicxulub; an even bigger asteroid impact in the Shiva basin, off the coast of India, may have been ground zero for the dinosaurs, a team of U.S. and Indian scientists say in a guaranteed-to-be-controversial finding.

The huge depression, around 500 km across, appears to be the largest, multi-ringed impact crater in the world, according to Sankar Chatterjee, a geoscientist at Texas Tech University in Lubbock, who presented his research at the Geological Society of America’s annual meeting in Portland, USA, today.

Earth’s biggest impact

“If we are right, this is the largest crater known on our planet… a bolide of this size, perhaps 40 km in diameter creates its own tectonics,” he said. “It may have been the one to kill the dinosaurs.”

The Chicxulub crater, an impact site beneath the Yucatan Peninsula in Mexico, was caused by an asteroid 8 to 10 km in diameter. This impactor is commonly believed to have caused a mass extinction at the end of Cretaceous Period, 65 million years ago, which wiped out the dinosaurs.

But now Chatterjee and Naresh Mehrotra, a palaeobotonist at the Birbal Sahni Institute of Palaeobotany in Lucknow, India, believe otherwise.

Shocked quartz

Chatterjee called Shiva a geophysical anomaly, adding that there are telltale markers in and around the crater that suggest the basin could be an impact site.

One is the presence of pseudotachylite, a glassy rock which is formed in the process of impact melting. Another telltale, Chatterjee said, is a high level of iridium, an element found in meteorites, but rare on Earth.

“Around the crater, you have lots of igneous complexes,” said Chatterjee. “All are highly rich in iridium. We think these are fluid ejecta formed by impact.”

Shocked quartz was also found in the Shiva basin, Chatterjee said, which is common in impact craters and difficult to create by other means. Even volcanoes can’t generate the kind of pressure needed to form shocked quartz. This asteroid, according to Chatterjee, appears to have been nearly 40 km in diameter – three times the size of the one believed to have killed the dinosaurs.

Also, he said, the crust is thinner beneath the putative impact site. “A whole 30 km of granitic crust is missing. The mantle comes high here and it’s hot. Where did the crust go?” He points to similar impact structures elsewhere. “There are at least two or three peaks. You see this with large, complex impact craters on Mars and the Moon.”

The researchers speculate that the impact would have caused the Earth’s crust here to vaporise instantly, leaving only superhot liquid mantle to well up in place of it. The impact could also have been powerful enough to break the islands of the Seychelles off of the Indian tectonic plate, sending them drifting toward Africa, they said.

Dubious of findings

Chatterjee also suggested that the Shiva asteroid may have been a factor in the formation of the Deccan Traps, an enormous volcanic eruption that some scientists believe may have contributed to the extinction of the dinosaurs.

The dinosaurs survived at least the initial stages of the Deccan volcanism, however, said Chaterjee, pointing to fossil finds of dinosaurs and their eggs between lava flows. “There’s no doubt that dinosaurs were thriving [then],” he said. “But there are other layers where you can see they are gone; extinction by impact seems to be a more logical thing. Deccan Traps was an accomplice, but it was not the main culprit.”

Other key experts told Cosmos Online they were dubious, however, and refused to comment on the record. They suggested that everything observed in the new study could have other explanations, and that extraordinary claims must be supported by extraordinary evidence.


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