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Re: Theories on the extinction of dinosaurs



JimC wrote:

>Three Cheers -- Rah, Rah, Ray!!!  I made a similar request before seeing
that you had beat me to it.  The only crater that I can think of that may
possibly rival Chixulub is the VanDerVoort Ring (sp?) in Africa, but I don't
remember its date or other specifics.  I'd be interested in knowing whether
there were any extinctions associated with it.<

Based on the references provided below, the Vredefort Crater in Africa (I
think this is what was meant) appears to be about half the diameter of
Chicxulub (~140 Km v ~300 Km). It's interesting to note that the American
Geophysical Union considers Chicxulub to be possibly one of the largest
craters produced in the inner solar system in the last 4 billion years.
We're not talking about the center of the bell curve here.  References are
regurgitated below.

>From a very informative site about terrestrial impact craters found at
http://cass.jsc.nasa.gov/publications/slidesets/impacts.html  comes the
following: "The Vredefort structure is located near the center of the
Witwatersrand Basin, about 100 km from Johannesburg. It is expressed as a
central core about 40 km in diameter that is composed of old crystalline
rocks. The core is surrounded by a deformed collar of uplifted and
overturned younger sediments and lavas. Much of the structure is buried by
younger flat-lying sediments resulting in the arcuate shape observed in this
image. The diameter of the collar rocks is approximately 80 km, but
reconstructions based on the distribution of shock metamorphic effects
suggest an original crater diameter of 140 km. Formed almost 2 billion years
ago, it is one of the oldest recognized impact structures on Earth. (Space
shuttle photograph S08-35-1294.)

And from an AGU website at
http://earth.agu.org/revgeophys/claeys00/node8.html#SECTION00031000000000000
000
"Chicxulub is certainly one of the largest impact structure on Earth and if
the 300 km size estimate is proven correct, perhaps one of the largest
produced in the inner solar system in the last 4 billion years."

Pat