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AN
IMPACT CLOSE TO HOME
Lying seven miles southwest of Greensburg, Louisiana, a 1.2 mile-wide crater was identified in 1999 as part of a mapping project by the U.S. Geological Survey. The cause of the crater remained hidden, however, until Paul Heinrich, a Louisiana Geological Survey research associate, identified proof that the cause came from outer space. Known as the “Brushy Creek feature” to Heinrich and his associates, the crater itself is about 60 feet deep, though it was likely several hundred feet deeper before loose sediment washed into it over time. Heinrich spent several months collecting samples of sediment and rock from various points around the crater. The samples were brought back to the Department of Geology and Geophysics, where Stephen Benoist, a research associate in the Paleomagnetism Laboratory, verified that the samples contained shocked quartz. Shocked quartz, which contains fractures that cannot be seen with the naked eye, is only produced by a high velocity impact with the Earth. With this information, Heinrich determined that a meteor impact caused the crater, which is the first such geological feature in Louisiana.
Heinrich has different scenarios regarding the size of the meteorite based on its composition. If it consisted of iron, the meteorite would be about 230 feet in diameter. If it were stoney in composition, the meteorite would be about 490 feet in diameter. The blast caused by such projectiles would have been equivalent to 25 or 28 megatons of TNT. According to Heinrich, the impact devastated the area. Any living thing within a 15-mile radius would not have survived. In Baton Rouge, the sound of the blast would have been the equivalent to the noise generated by afternoon rush hour traffic. Though the impact would have registered 5.2 on the Richter Scale, it would have caused minimal damage outside of the epicenter radius. Heinrich says that statistics show that this type of event happens every 2,000 to 6,000 years, but most meteorites are too small to have a significant impact and fall into the ocean. According to the Planetary and Space Science Center at the University of New Brunswick, which works with NASA and the U.S. Geological Survey to keep the official count, there are 171 confirmed impact craters. Other craters in the U.S. outside of Louisiana are the Wetumpka Crater in northeast Alabama and the Arizona Meteor Crater. ON
THE WEB:
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Driving
along Louisiana Highway 37 in St. Helena Parish, most people probably
notice the scenic meanderings of the Amite River or a deer leaping
through the forest and across the highway. The area is home to
many plant and animal species, as well as campgrounds and Native
American archaeological sites. There is one other thing, perhaps,
that many people never take notice of, too large to see while
driving through in a car or even hiking in the area.
“The
data shows that the meteor that hit St. Helena Parish is younger
than 11,000 to 12,000 years old, which would have put it in the
late Pleistocene or Holocene Epoch,” says Heinrich. “Had
we been here during the impact, it would’ve definitely gotten
our attention.”