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"Oddball" Scottish Rocks Formed By Meteorite

Anil Ananthaswamy in London, England
for National Geographic News
March 31, 2008
 
Geologists have uncovered evidence of the biggest meteorite crater ever found in Britain and Ireland.

The study findings solve a long-standing puzzle about a layer of rock that stretches for about 31 miles (50 kilometers) in northwest Scotland.

The layer, 65 feet (20 meters) thick in places, is sandwiched between gigantic layers of red sandstone and siltstone, which form the so-called Torridonian sedimentary rocks.

Volcanic Suspicions

Some scientists have suggested that the layer hails from a volcano—but there is no evidence of volcanic activity in the region.

Also, the rocks below the layer hint at something other than volcanic deposition, scientists say.

"Some of the underlying strata have quite obviously been ripped up, and curled over, and in some places rolled up almost into a ball," said study co-author Kenneth Amor of the University of Oxford, in England.

The layer also contains green bits of shattered rock fragments that had once been molten. Similar pieces have been found in the impact layer of the Ries Crater in southern Germany.

"The layer has always been a bit of an oddball," said geologist Caroline Smith, curator of the meteorite collection at the Natural History Museum in London, England. Smith was not part of the study.

(Related news: "Mysterious Meteorites Stymie Scientists" [March 12, 2008].)

Huge Impact

To co-author Amor, these features smacked of a high-energy meteorite impact. So he and colleagues from the University of Oxford and the University of Aberdeen, in Scotland, analyzed samples from the layer for signs of a crash.

They found a mineral called shocked quartz.

Shocked quartz has microscopic fractures that form only under high pressure—for instance, from the shock wave of a meteorite impact.

"Most people working in the field would accept shocked quartz as a diagnostic test for meteorite impact," Amor said.

The researchers also found higher amounts of iridium and chromium 53 in the samples than in the surrounding rocks. The levels of both these elements are consistent with those found in meteorites.

The scientists concluded that the layer is part of an "ejecta blanket," or a layer of debris that settles after a meteorite impact.

Based on the length of the outcrop, the team estimated that the object striking Earth must have been at least 1,650 feet (500 meters) wide and formed a crater at least 3.75 miles (6 kilometers) across.

(Related news: "Crater From 1908 Russian Space Impact Found, Team Says" [November 7, 2007].)

Amor and his colleagues published their research in the April issue of the journal Geology.

Convincing Evidence

The impact occurred about 1.2 billion years ago, when Scotland was still attached to North America and the Atlantic Ocean had not yet formed.

Amor suspects that the actual crater is probably beneath The Minch, a strait that separates the Isle of Lewis from the Scottish mainland.

Alexander Deutsch is a meteor expert at the University of Münster in Germany.

"The data seems to be at the edge of being convincing," he said.

But he agreed that given the thickness of the layer, the crater may have had a size between that of the Ries Crater, 15 miles (24 kilometers) and Sweden's Siljan crater, 34 miles (55 kilometers) across.

Smith of the Natural History Museum is impressed by the analysis of the rogue layer.

"[The new analysis] is convincing evidence indeed that [the crater] is an ejecta layer from a large impact," she said.
 

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