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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?
>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
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]