Bay born in a blink Scientist theorizes that huge meteorite created Chesapeake

THE BALTIMORE SUN

THE CHESAPEAKE region may seem like an eternal place, an endless dance of light and water, mists and grasses, swamps and forests.

But as scientists learn more about the forces that shape the Earth, it's becoming clear just how accidental and transient the estuary and the land that embraces it really are.

It's long been understood, of course, that this corner of the middle Atlantic has been sculpted by the rise of the Appalachians to the west, meltwater from glaciers to the north and the sediments piled up by its great rivers.

But in recent years scientists have come to recognize that the bay's geography has changed more swiftly, and more radically, than previously supposed.

Take the Appalachians. According to the standard view, the mountain range rose when North America collided with North Africa, part of the continual slide of continents over the face of the globe.

Now, some scientists think that North America has wandered much farther, and that the ancient mountain chain might actually have risen, for the first time, a half-billion years ago when North America bumped into the west coast of South America.

Then there's the bay itself.

C. Wylie Poag, a paleontologist with the U.S. Geological Survey at Woods Hole, Mass., suggests that the bay formed around a 55-mile-wide crater, gouged in an eye blink by a meteorite 1.8 miles in diameter.

This interplanetary rock slammed into what is now the lower Chesapeake and part of the Atlantic Ocean, he says, about 35 million years ago.

The impact site was then under water: The coastline lay near Richmond, Va.

But as ocean waters receded over the millenniums, Dr. Poag speculates, the rivers of the region drained into the crater, creating and nourishing the bay.

When the meteorite hit, a huge stretch of ocean evaporated under the tremendous heat generated by its plunge through the atmosphere.

"For a moment, a blink of the eye, there would have been no water there," Dr. Poag said. Then the ocean would have poured in, converged and finally bounced back in a 300-foot tidal wave.

Crustal rock was vaporized and pulverized, leaving a field of rocky debris, called breccia, found in one rock stratum throughout the region.

The blast itself probably devastated an area 620 miles in diameter -- terrain populated by squirrels, bats, small horses, birds, and numerous less familiar animals.

If they'd existed at the time, Washington and Baltimore would have been wiped out, either by the initial blast or the 300-foot tidal wave that followed.

Humans weren't bothered by the impact. They hadn't evolved yet, and wouldn't for at least another 31 million years.

The rocky projectile struck traveling at about 45 miles per second -- about six times faster than the fastest spacecraft ever built -- and exploded with 17,000 times the power of the largest nuclear weapon ever made.

"You would hear a tremendous sound," Dr. Poag said. "There would be an earthquake. The shock wave would have traveled probably all the way through the earth. There would have been a tremendous blast of heat and wind.

"In the near vicinity there'd be fallout of ejecta -- boulders, blocks of material kilometers in size all the way down to grains of sand."

Some scientists still have their doubts. But researchers presented new evidence for the crater two months ago at a meeting in Washington.

Geologists have found traces of mineral rock melted and shattered by the kind of tremendous pressure and heat that could have been produced only by a meteorite impact.

Well inside the outer rim, they see evidence of another characteristic of meteorite craters: a circular central plateau called a "peak ring," created when the underlying base rock rebounds after the shock of the collision.

A similar rebounding effect can be seen in high-speed photographs of droplets striking the surface of a liquid.

Steven M. Stanley, professor of earth sciences at the Johns Hopkins University, said the crater would help explain a puzzle. About 35 million years ago, something showered millions of square miles of eastern North America with tiny beads, called tektites, made of rock that melted and cooled very quickly.

"It's like Baltimore's Shot Tower," Dr. Stanley said. "They would drop lead through screens, creating little drops that, as they fell, turned spherical before hitting a tub of water. It produced nice shot for shotguns."

A similar, though somewhat larger, meteorite impact is thought to have hit what is now the Yucatan Peninsula 65 million years ago.

It may have thrown up enough debris to blot out the sun for months or years, wiping out plant and animal life and causing the extinction of the dinosaurs.

Not only has change come faster to this region than once thought, but the region itself may have traveled much farther.

Since the 1960s, scientists have have come to believe that the planet's continents drift around its surface, riding on eight large and many smaller "plates" -- vast jagged chunks of the Earth's crust.

They creep around, it turns out, at about the speed that your fingernails grow.

The traditional view holds that North America, as it drifted, repeatedly bumped up against Northwest Africa, then rebounded. These intercontinental fender-benders, it is thought, gave rise to the Appalachians.

Not exactly, says Ian W. D. Dalziel (pronounced DEE-el) of the Institute for Geophysics at the University of Texas at Austin.

Sure, he concedes, North America collided with North Africa in relatively recent times -- say 260 million years ago, about the time most of the planet's land masses glommed together to form a super-continent called Pangaea.

But he and a number of his colleagues think the Appalachians formed in a collision between what is now eastern North America and western South America about 486 million years ago.

"This would not have been a situation like India slamming into Asia and creating the Himalayas," Dr. Dalziel explained. "With the Appalachians, it was more like a bunch of colliding island arcs."

This hypothesis, he said, is "a totally radical departure from what has been the traditional view."

Say Charm City existed 460 million years ago. Residents of Canton could have walked out their doors, headed roughly west and wound up in the general vicinity of Lima, Peru.

A map of the land masses at the time makes it look as if the continents have been tossed in a front-loading washer with the machine set on spin cycle. North America is topsy-turvy, with California to the north and Canada to the east.

Geologists first suspected something was wrong with the standard view, Dr. Dalziel said, after they noticed striking similarities between rocks in Antarctica and others in the southwestern United States. That suggested the two places were once joined.

This heretical notion inspired Dr. Dalziel and his colleagues to reconsider the history of North America's peregrinations.

He called the older, standard theory of repeated collisions between North America and North Africa "accordion tectonics."

It was, he said, "everything sliding along railroad tracks. The world really doesn't move like that."

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Douglas Birch is a reporter for the Sun

Colliding continents

Anew theory of continental drift, the movement of the Earth's crustal plates over its 4.5 billion year history, holds that the North American continent has rambled farther than previously thought. Geologists figured that western Norht Africa and eastern North America kept colliding and pulling apart over the past billion years or so. Now, a University of Texas scientist says that North America drifted up the coast of South America half a billion years ago. At one point, the Baltimore area sat due east of Lima, Peru. About 35 million years ago, a meterite about 1.8 miles in diameter svreamed out of the sky over what is now the lower Chesapeake Bay and exploded with 17,000 times the force of the largest nuclear weapon ever built. Geologists only recently discovered the remains of its crater, which scientists think may have later led to the formation of the Bay.

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