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Probably the most dramatic geological event that ever took place on the Atlantic margin of North America occurred about 35 million years ago in the late part of the Eocene Epoch. This was a time when sea level was unusually high everywhere on Earth. The ancient shoreline of the Virginia region was somewhere in the vicinity of where Richmond is today. The late Eocene climate was even warmer and more humid than today's tidewater summers. Tropical rain forests covered the slopes of the Appalachians, and a broad, lime-covered continental shelf stretched to the east of a narrow coastal plain. But with a brief flash of mind-numbing light, that tranquil scene was transformed into a bloody killing field of incredible carnage. From the far reaches of the solar system, a giant bolide swooped through the Earth's atmosphere. It blasted an enormous crater into the continental shelf, and obliterated nearby terrestrial flora and fauna.
What is a bolide? There is no consensus on its definition, but we use it to mean an extraterrestrial body in the 1-10-km size range, which impacts the earth at velocities of literally faster than a speeding bullet (20-70 km/sec = Mach 75), explodes upon impact, and creates a large crater. "Bolide" is a generic term, used to imply that we do not know the precise nature of the impacting body . . . whether it is a rocky or metallic asteroid, or an icy comet, for example.
Nearly everyone knows what an impact crater looks like. For most, that image is derived from Meteor Crater (also known as Barringer Crater) in Arizona, some 38 miles east of Flagstaff, the archetypical example of what cratering experts call a simple crater. It is a shallow, bowl-shaped excavation, 1 km in diameter, with an upraised sub-circular rim, and is extraordinarily well-preserved. Meteor Crater was the first terrestrial crater recognized as an impact structure back in the 1920's. Since then, more than 150 additional impact craters have been identified.
Few of you probably know that most craters wider than 10 km are classified as complex craters, because they exhibit additional features. A good example of a complex crater is King Crater on the farside of the moon. Like simple craters, the outer margin of complex craters is marked by a raised rim. Inside the rim is a broad, flat, circular plain, called the annular trough. Large slump blocks fall away from the crater's outer wall and slide out over the floor of the annular trough toward the crater center. The inner edge of the annular trough is marked by either a central mountainous peak, a ring of peaks (a peak ring), or both. Inside the peak ring is the deepest part of the crater, called the inner basin. The Chesapeake Bay crater has all the characteristics of a peak-ring crater, like King crater.
The general public is intensely interested in bolide impacts, in
particular because a large one, Chicxulub (pronounced
cheek-shoo-lube), has been implicated in the demise of the
dinosaurs. The comet Shoemaker-Levy 9 impacted on Jupiter in 1994, and
riveted us all to our TV screens. Lately, we have come to realize the
potential of impacts for global environmental devastation and the mass
extinction of animal and plant species. Immediate regional effects of
such an impact include a super-hot blast wave, a base surge of hot
debris, gigantic tsunami waves, vaporization of water column and target
rocks, and giant earthquakes. On a global scale, short term effects
include fallout of ejected particles and raging wildfires. Long term
effects include prolonged darkness due to atmospheric debris, acid
rain, and greenhouse warming.
King Crater image courtesy of Ron Greeley, "Planetary Landscapes",
2nd Edition, pp. 99
Shoemaker-Levy artistry courtesy of David Seal, JPL
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