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Re: impacts are cool!

> Overkill.  Complete, total, absolute overkill.  An impact with
> consequences like that would have killed every green thing on the planet
> and left it completely sterile, except perhaps for hydrothermal vent
> communities a kilometer below the ocean surface.  The Asteroid Doomsday
> scenario is illogical, because it is completely contradicted by the
> evidence.  Whatever happened at the K-T border caused widespread havoc
> among both land and aquatic ecosystems, but _some_ life _did_ survive. 
> Don't forget that.  Something Did Survive.  IMHO, that simple fact
> conclusively disproves Asteroid Doomsday.

     It proves that the "asteroid doomsday", if it occured, didn't kill
everybody.  However, the evidence that large impacts do occur on earth is
not only logical, but supported by quite a bit of evidence; including
craters or the remnants of craters.  The force of these impacts and the
amount of energy released is suggested by shocked quartz and stishovite
(a form of quartz). Are you questioning interpretations of
the destrcutive potential of imapcts, or the fact that they happed at all?
The evidence that mass extinctions didn't kill everything certainly pretty
compelling by the simple fact that this message is being written, but so
is the evidence that the impacts occurred.  Either the impact scenarios
described are erroneous, or (more likely to me), they wouldn't be quite as
universally devastating to life as you predict.
     An important point that is often missed when considering who survives
mass extinctions is the NUMBERS of individuals involved.  As someone
(sorry, I forget who) recently pointed out, small endotherms and
ectotherms (such as crocodilians) need relatively little food in order to 
get by, and so might survive a catastrophe relatively well.  However this
also means they can support relatively lage populations.  In other
words, a "nuclear"  winter COULD freeze to death 99% of those poor,
delicate little frogs; but that still leaves an awful lot of relatively
lucky and/or tough frogs behind to get the ball rolling again.  
     In the case of plants I think you are ignoring that seeds and spores
can survive harsh conditions for some time without growth.  Even if you
DID kill or sterilize every single adult plant on earth, that wouldn't
spell then end of plant life on Earth.  Spores and seeds produced
befroe hand would still be around.      
LN Jeff