Monday, 28 March 2011

Meteorite Triggered Ancient New York Tsunami?

Ker Than in New York City
December 31, 2008

A meteorite impact off Long Island 2,300 years ago may have set off a huge tsunami that flooded the New York City region, a new study says (New York City and Long Island map).

It's not known whether any ancient settlements were in the path of the proposed killer waves, but "any significant tsunami today would be devastating and likely to flood places like lower Manhattan," Vanderbilt University geologist Steven Goodbred said.

Tsunamis are typically triggered by seismic events. An undersea earthquake, for example, caused the catastrophic 2004 Indian Ocean tsunami. But meteorite strikes have also been known to spark the killer waves.

In the New York area, "there are no exploding volcanoes and there probably haven't been" for millions of years, said study co-author Katherine Cagen of Harvard University. "The same goes for [major] earthquakes."

Cagen, however, recently found signs of a meteorite impact in sediments taken from several sites along the Hudson River, which forms the border between New York City and New Jersey.

The evidence included deformed rocks; rare microscopic "nanodiamonds"; and microscopic, perfectly round rocks called spherules, which form when molten and vaporized rock are flung into the air by a space impact and then solidify in the temporary vacuum created by the blast.

Nothing as big as a crater has been found, but Dallas Abbott, a Columbia University impact expert, estimates that the space rock would have had a diameter of between about 165 feet (50 meters) and 490 feet (150 meters). Any smaller, and a major wave would not have formed and the rock would have exploded before hitting Earth. Any bigger, and the strike would have created "impact glass"—forged in the extreme heat of an impact blast—which has not been found as of yet.

Abbott presented her team's research this month at the annual meeting of the American Geophysical Union in San Francisco.

Mystery Solved?

The Hudson River samples date back to around 300 B.C.—the same age as some out-of-place gravel deposits discovered by another team of scientists on Long Island in 2003.

The rocky layer is several inches thick and appears to have been transported from a gravel-rich coast a few hundred meters away. The individual rocks are quite large—some as big as fists—so normal waves or wind could not have carried the stones, according to Vanderbilt's Goodbred.

At the time of the gravel discovery, Goodbred suggested that the rocks had been moved by one of two phenomena: a very big storm or a tsunami.

The new meteorite evidence may tip the scales toward the tsunami theory. Even so, Goodbred stressed that the research is still in its early stages.

"There are two unique hypotheses here"—the tsunami and the storm—"and either one or both may yet be proven incorrect," he said.

Geologist Ted Bryant of the University of Wollongong in Australia said an extraterrestrial impact is the most likely explanation for the spherules and nanodiamonds.

Abbott's "discoveries cannot be reproduced by processes naturally occurring in the New York region," said Bryant, who was not involved in the new study.

"Is her hypothesis wild? No. Is it unconventional? Yes," he added. "But so was continental drift when it was first proposed."