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Re: Extinction (was: Religion)

There are just two points I'd like to comment on before bailing
out of this discussion myself:

I. Do *big* terrestrial craters correlate with *big* extinction events?

Stan wrote:
>That is far from demonstrated.  You need solid (not merely
>suggestive) evidence for impacts in association with other
>major extinctions to accomplish that.  That is not now present.    

I agree that the data is in need of reinforcing studies, but this
lineup of the big 6 with known huge craters at 4 of them is a good
start. As a matter of fact, the only other known terrestrial crater 
in the 100km club is Lake Acraman in Australia(150 km, 595mya +-32),
but the data to correlate that to a precambrian extinction is to thin
even for me, although I wouldn't be surprised ;-) Sorry to repost this,
but this is for the newcomers to the discussion:

BOUNDARY                     crater      time of
(mya=million years ago)      diameter    impact       location
Eocene/Oligocene(34mya)        100km      39+-9      Popagai, Russia

Cretaceous/Tertiary(65mya)    >200km      65         Chicxulub, Mexico

Carnian/Norian(223mya)         100km     220         Manicouagan, Quebec

Permian/Triassic(250mya)      >300km**   250         S. Atlantic

Frasnian/Fammenian(367mya)     100km     365         Taihu Lake, China

Ordovician/Silurian(439mya)    No known crater

** The Permian/Triassic is not a confirmed crater. The date given is
   from fold belt metamorphism surrounding the proposed structure on 
   Africa, S. America, and Antarctica. [from Rampino]

The 4 known impact structures listed above are the biggest phanerozoic
craters. As the dates for these structures have been refined in recent
years, their alignment with known mass-extinction events has become
evident. It might be a statistical fluke but methinks not. Also, there
is indirect evidence for impacts at the other two mass extinctions
(P/Tr and Ordo/Silu) in the form of Iridium layers and shocked quartz.

II. Lies, Damn lies and Statistics
QUESTION: From a purely statistical analysis, how often should one of 
          the *really* BIG ones hit?

ANSWER: An extraterrestrial body capable of causing a 150km diameter
        crater should hit every ~100 million years. 
(after Gehrels, 1985):

Asteroid       # of objects   Impact probability    Impact energy
diameter (km)                   (impacts/year)      (* 5*10^20 ergs)

 10 km                 10           10^-8               10^9
  1 km              1 000           10^-6               10^6
0.1 km            100 000           10^-4               10^3

Note that 5*10^20 ergs = 13 000 tons TNT equivalent, or the energy 
released by the Hiroshima A-bomb.


Terrestrial Impact Structures, R. A. F. Grieve, Ann. Rev. Earth 
Planet Sci., vol 15: 245-270, 1987.                                        

Gehrels, T. 1985 Asteroids and comets. _Physics Today_ 38, 32-41. [an
excellent general overview of the subject for the layman]

Shoemaker, E.M. 1983 Asteroid and comet bombardment of the earth. _Ann.
Rev. Earth Planet. Sci._ 11, 461-494. [very long and fairly
technical but a comprehensive examination of the subject]

Shoemaker, E.M., J.G. Williams, E.F. Helin & R.F. Wolfe 1979
Earth-crossing asteroids: Orbital classes, collision rates with
Earth, and origin. In _Asteroids_, T. Gehrels, ed., pp. 253-282,
University of Arizona Press, Tucson.

Cunningham, C.J. 1988 _Introduction to Asteroids: The Next Frontier_
(Richmond: Willman-Bell, Inc.) [covers all aspects of asteroid
studies and is an excellent introduction to the subject for people
of all experience levels. It also has a very extensive reference
list covering essentially all of the reference material in the field.]

Earth's Near Death Experience, J. Alper, Earth Magazine, Jan. 1994.
[discusses Rampino's theory about the possibility of a massive double
impact at the Permo-Triassic]

Geological and Biological Consequences of Giant Impacts, D. J. McLaren,
W. D. Goodfellow, Ann. Rev. Earth Planet. Sci., vol 18: 123-171, 1990.
[lots of evidence for impacts correlating with almost all known mass
extinction events]

Astrogeological Events in China, Xu Dao-Yi et al, Geological Publishing
House, Beijing, China, 1989.[This has the photos of the two boundary
clays at the P/Tr section in Meishan, China with Iridium/siderophile data]