Barringer Meteor Impact Crater Arizona

One of the most famous and visited impact craters in the United States is the Barringer meteor impact crater in Arizona, the first structure almost universally accepted by the scientific community as a meteorite impact structure. The crater is 0.75 miles (1.2 km) across, and has an age of 49,000 years before present. Its acceptance as an impact crater did not come easily. The leading proponent of the meteorite impact model was Daniel Barringer, a mining company executive who argued for years against the

Barringer meteor impact crater, Arizona (Fran├žois Gohier/Photo Researchers, Inc.)

powerful head of the U.S. Geological Survey, Grove K. Gilbert, who maintained that the crater was a volcanic feature. For years Daniel Barringer lost the argument because in his model the crater should have been underlain by a massive iron-nickel deposit from the meteorite, which was never found since the meteorite was vaporized by the heat and energy of the impact. By 1930, however, enough other evidence for the origin of the crater had been accumulated to convince the scientific community of its origin by impact from space.

Barringer crater is a small crater with a roughly polygonal outline partly controlled by weak zones (fractures and joints) in the underlying rock. The bedrock of the area is simple, consisting of a cover of alluvium, underlain by the thin Triassic Moenkopi sandstone, about 300 feet (90 m) of the Permian Kai-bab limestone, and 900 feet (270 m) of the Permian Coconino sandstone. The rim of the crater is 148 feet (45 m) higher than the surrounding desert surface, and the floor of the crater lies 328 feet (100 m) lower than the surrounding average desert elevation. The rim of the crater is composed of beds of material that originally filled the crater but was thrown or ejected during the impact. The beds in the rim rocks are fragmented and brecciated, and the whole sequence that was in the crater is now upside down lying on the rim. The underlying beds are turned upward as the crater is approached, reflecting this powerful bending and overturning that occurred when the interior rocks were thrown onto the rim during the impact.

Although the large mass of iron and nickel that Daniel Barringer sought at the base of the crater does not exist, thousands of small meterorite fragments have been collected from around the outer rim of the crater, from as far away as four miles (7 km) from the crater. The soil around the crater, for a distance of up to six miles (10 km) is pervaded by meteorite dust, suggesting that the impacting meteorite vaporized on impact, and the debris settled around the crater in a giant dust cloud. Estimates of the size of the meteorite that hit based on the amount of meteorite debris found are about 12,000 tons (10,884 tonnes).

Several lines of evidence indicate an impact origin for Meteor crater. The widespread brecciation, or fragmentation, of the rim and presence of iron-rich shale from weathering of the meteorite is consistent with an impact origin. More important, high-temperature glasses and minerals are preserved that form almost exclusively during the high pressures of meteorite impact. In some places the rock has been melted into impact glasses that require temperatures and shock pressures obtained only during impact of an object such as a meteor. Some rare minerals that form only at high pressures have been found at Barringer crater, including high-pressure phases of quartz known as coesite and stishovite, and also small diamonds formed by high pressures associated with the impact.

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