Sunday, June 15, 2008

Roadside crater should have made more of an impact

HOW could evidence of a major asteroid impact have been missed when it was in plain sight all along? The telltale signs of a huge impact site were sitting alongside a busy road 8 kilometres north-east of Santa Fe, New Mexico.

Unusual cracks radiating from the tips of cone-shaped structures in rocks along the Santa Fe National Forest Scenic Byway were first spotted by independent geologist Tim McElvain. He called in experts, who identified the projections as shatter cones, distinctive structures that form when shock waves from a high-speed impact fracture the underlying rock.

The shatter cones are the biggest ever found, says Christian Koeberl at the University of Vienna in Austria. "To see 2-metre-high shatter cones on the side of the road took my breath away," he told New Scientist. "Who knows how many hundreds of thousands of people had driven by there without recognising them."

Koeberl, McElvain and colleagues went on to find additional shatter cones and other evidence of an impact over an area of 5 square kilometres, though the impact crater itself has long been obliterated by weathering and tectonic activity. They estimate the total area affected by the impact was 6 to 13 kilometres across, and that the asteroid responsible struck between 300 million and 1.2 billion years ago (Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2008.03.033)

he discovery suggests that many undiscovered impact sites are "still hiding right next to us", says Koeberl.

Comets and Asteroids - Learn more about the threat to human civilisation in our special report.

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