A layer of quartz grains found in an east Georgia kaolin mine have been traced to the impact of a giant asteroid that crashed near the mouth of the Chesapeake Bay 35 million years ago.
The 54-mile-wide (86-kilometer-wide) crater left by the meteor, the sixth-largest in the world, has previously been identified as the source of a rare mineral called Georgiaite. Small, glassy, olive-green beads of it have been found for decades in more than a dozen middle Georgia counties.
They are a type of impact-produced natural glass called tektite.
Ed Albin, a tektite expert at Fernbank Science Center in Atlanta, steered research into the substance toward the open kaolin mines that dot the center of the state. Sandwiched between layers of clay in an abandoned kaolin mine in Warren County, a team of researchers headed by University of Georgia graduate student Scott Harris, found traces of the impact.
Microscopic analysis, reported in the current issue of the journal Geology, revealed a 3-inch-thick layer of "shocked quartz" — a form of the mineral produced only under intense pressure like that of an impact — that dated to 35.5 million years ago, when a space rock slammed into the Earth about 120 miles (192 kilometers) southeast of present-day Washington.
The asteroid, believed to be about 2 miles (3.2 kilometers) wide, was traveling at tens of thousands of miles an hour when it struck on what is now the Chesapeake's Eastern Shore.
Scientists estimate that the debris from the blast rose more than 30 miles (48 kilometers) into the air, spreading melted bits of sand and sediment — including Georgia's tektites and the newly discovered layer of shocked quartz — for thousands of miles.
Although the Warren County discovery is the first known deposit of its kind, scientists say the diagnostic layer of glassy quartz grains — which probably blanketed much of the East Coast — could, with the discovery of deposits in other locations, become a valuable yardstick for geologic measurement.