Chesapeake Bay Crater Offers Clues to Ancient Cataclysm
for National Geographic News
|November 13, 2001|
About 35 million years agothe dinosaurs are dead, but the Appalachian Mountains are still covered in tropical rain forestsa rock from space that was more than a mile wide and moving at supersonic speed crashed into the Atlantic Ocean off North America.
Traveling at about 70,000 miles (113,000 kilometers) an hour, the asteroid or comet (bolide) splashed through several hundred feet of water and several thousand feet of mud and sediment.
Billions of tons of ocean water were propelled into the air as high as 30 miles (48 kilometers) and vaporized. Millions of tons of debris and rocks were ejected into the atmosphere. The incident incinerated everything along the East Coast, triggered gigantic tsunamis, and decimated marine life in the surrounding area.
For millions of years the crater lay buried in the southern part of the Chesapeake Bay and surrounding peninsulas, more than a thousand feet (300 meters) beneath sand, silt, and clay. Scientists discovered it in 1983 and have been studying it ever since.
For the past two summers, researchers have been extracting core samples from deep inside the crater. The core material is beginning to provide the answers to a lot of questions about the crater and its effects in the region.
"We're finding things that are giving us an idea of the heat and power at the time of impactpartially melted algae fossils, completely shattered rocks, lots of basin fragments, fractured and tilted seabeds," said David Powars, a hydrologist at the U.S. Geological Survey (USGS).
Powars and C. Wylie Poag, a USGS colleague, presented some of the results of their research earlier this month in Boston at the annual meeting of the Geological Society of America.
"It's pretty exciting," Powars said. "In every direction there's new data. Wylie and I are like two kids with candy."
Unusual Geologic Conditions
The new findings offer greater insight into geological and hydrological conditions of the Chesapeake area and eastern Virginia that have puzzled scientists for more than a century.
The land is sinking, erosion patterns are unusual, and earthquakes have occurred in this region not known for earthquakes. The groundwater in some areas of the region is salty, and three rivers in the area have abruptand highly unusual90-degree turns.
One thing the core samples show, according to Powars, is that the roughly circular crater is much bigger and deeper than originally thought.
When it hit, the asteroid or comet "fractured the crystalline bedrock below to at least a depth of 7 miles (11 kilometers) and a width of 85 miles (137 kilometers). This was a big hit," he said.
Earlier estimates had suggested the crater was about a mile deep (1.3 kilometers) and 53 miles (85 kilometers) wide.
The impact of the bolide disturbed the normal layering of sediment, rocks, and aquifers, and water from many of the wells in the region is salty or brackish.
Utility companies in eastern Virginia are funding some of Powars' work because the findings have a direct bearing on the search for fresh groundwater needed to supply growing populations in the region.
Linked With Extinction?
Scientists have long suspected that the heat from the impact incinerated every living thing within hundreds of miles. The core drilling has revealed a zone of silt that is devoid of signs of indigenous life, lending credence to that hypothesis, said Poag.
Poag believes the impact could have influenced an extinction that occurred about 33 million years ago.
The early Oligocene extinction dramatically affected land mammals. Forest dwellers declined as forested habitat became less abundant, while hoofed animals flourished as a result of increasing temperate grasslands. A number of predators also became extinct at this time, mainly because of changes in vegetation.
The asteroid or comet that struck the area that later became the Chesapeake Bay may be evidence of a 2 million-year-long comet shower that scientists think may have occurred between 36 and 34 million years ago.
An even bigger crater in Popigai, northern Siberia, was created at about the same time. Scientists also have found traces of helium 3, an isotope associated with extraterrestrial objects, in sediment layers in Massignano, Italy, and other places dating to 35 million years ago.
"Global paleo-temperatures during the early Eocene show the world was getting cooler," said Poag. "Around 35 million years ago, at the end of the Eocene, there was a warm pulse, and for a short period of time the Earth warmed. Then beginning at around 34 million years ago the Earth cooled very rapidly. At 33.4 million years ago there was a major extinction event."
The aftermath of the bolide collision, Poag said, would have been prolonged darkness and acid rain caused by the fallout of rocks, dust, and particles that were blasted into the atmosphere, along with the residual effects of raging wildfires.
These conditions, he suggests, would cause the abrupt cooling of Earth, leading to the extinction that eventually occurred.
The National Geographic Society provided funding for this research in the past.
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