http://news.yahoo.com/s/livescience/20101027/sc_livescience/66footwaveshitnewyorkinancientasteroidsplashdown
A cosmic impact two millennia ago may have
sent tsunamis deluging what is now the Big Apple, scientists suggest.
Many of the giant sea waves known as tsunamis
are caused by underwater earthquakes and volcanoes - for example, the devastating 2004 Indian
Ocean tsunami was triggered by a quake off the northwestern
coast of Sumatra. Still, the causes of nearly 10 percent of
all tsunamis nowadays remain uncertain.
Cosmic impacts have been known to cause
tsunamis in the past. For instance, scientists have found evidence that the Chicxulub
impact in Mexico, which may have ended the age of
dinosaurs, triggered gigantic waves.
Now researchers have evidence suggesting
that an asteroid roughly 200 yards (183 meters) wide crashed off the coast of New Jersey and sent tsunamis surging toward
what is now New York City some 2,300 years ago. [Video - Recreating
an Ancient Tsunami]
"Our models suggest the tsunamis were up
to 20 meters (66 feet) high when they entered the Hudson River," said researcher Dallas Abbott, a geologist at Columbia University's
Lamont-Doherty Earth Observatory in New York.
New York City lies at the mouth of the Hudson.
When the scientists drilled out tubes of sediment from the New York and New Jersey area, they discovered layers of unusual
debris that, they suggest, were laid down by tsunamis.
"We have layers up to maybe 30 centimeters
(11.8 inches) thick," Abbott said. "They get thinner upriver, where they're more like 6 centimeters (2.3 inches) thick."
Within these potential tsunami layers is
evidence of a cosmic impact, including shocked minerals and microscopic
carbon beads loaded with "nano-diamonds," which are "all things only impacts can do," Abbott said. One candidate for the crater
that was produced by this impact, she said, lies in the undersea Carteret Canyon, located roughly 90 miles (150 km) off the
coast of New Jersey.
Other scientists have raised alternate explanations
for these layers. For instance, volcanic eruptions or gigantic landslides on the other side of the Atlantic might have caused
the giant waves. Or these anomalous layers of sediment that Abbott and her colleagues are investigating may not have been
caused by tsunamis at all - hurricanes can generate huge pulses of water known as storm surges, whose effects on sediment
could resemble those of tsunamis.
"Of course, that doesn't explain the evidence
of impact that we've found," Abbott said.
For
a cosmic impact, one smoking gun would be a specific form of deformed rock known as shocked quartz. The rock is generated
by the intense heat and pressure of a collision
with an extraterrestrial object. "But if there was an oceanic impact,
the oceanic
crust doesn't really have quartz to shock," Abbott
said.
It there were tsunamis, it remains unclear
if ancient Native Americans witnessed them.
"One possible reason why Indian tribes only
moved into the area relatively recently is that the people who were once there were all wiped out," Abbott said. "If you look
at the predicted wave heights, there would have been few places to hide."
Abbott and her colleagues plan to detail
their most recent findings Nov. 3 in Denver, at the annual meeting of the Geological Society of America.
