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Monster Meteor Once Hit Scotland

Larry O'Hanlon, Discovery News

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Earthbound?

April 3, 2008 -- A tad more than a billion years ago, a chunk of rock a third of a mile wide punched into what is now northwest Scotland, says a geologist who has found definitive evidence of the meteor impact. The crater itself, however, remains hidden.

The evidence for the impact comes in the form of microscopic quartz grains with telltale "shock metamorphic" features that can only be created by the intense pressures of a meteor impact.

"I think this is a very good, solid announcement," said meteor researcher Jay Melosh of the University of Arizona. Melosh fields many claims of meteor impacts -- most of which are unfounded. But shocked quartz is impossible to create any other way, so it's definitive even when the crater is missing.

The approximate site of ground zero for this, the largest known meteor to have hit the British Isles, is suspected to be near the Scottish town of Ullapool. The crater is thought to be buried deep under local rocks, still awaiting discovery.

"It's actually a bit of a mystery where the crater is," geologist Ken Amor of Oxford University told Discovery News. Amor published his discovery in the April issue of the journal Geology.

What spurred him to study the grains from Ullapool under a microscope was a hunch he had while leading undergraduates on a field trip. In the sequence of rocks exposed by erosion near Ullapool is a layer called the Stac Fada Member. It has long been interpreted as some sort of volcanic ash or mudflow, like those that formed in the goopy aftermath of the Mount St. Helens eruption.