Re: Theories on the extinction of dinosaurs

["James R. Cunningham" <jrccea@bellsouth.net>]

Stanley Friesen wrote:

> At 03:54 PM 11/12/99 -0500, TomHopp@aol.com wrote:
> >I tend to agree with Jim. The overwhelming factor had to be that huge imact.
> >The issues of Deccan volcanism and oxygen levels have been invoked more as
> >alternatives than as co-variables.
>
> Not by me, nor by such authors as Archibald.  Citation as co-variables is
> actually quite well established in the literature.

I agree with Stanley again.  I've been of the impression that dinosaurs were
struggling toward the end of the Cretaceous, and the impact was just the straw
that broke the camel's back.

> It is the maximal confluence of the contributory factors that determines the
> timing of the
> extinction peak.

I agree again, but I do think the impact was a very effective and very sudden
final straw.

> The impact would not have been directly comparable.  Shoemaker-LEvy had
> fallen through a larger gravity potential than would the Yucatan impactor.
> This alone would make the S-L impact results larger.

Actually, it might not.  They were faster, but reportedly smaller, so it would
depend on the individual mass of each of the 'pearls'.  Increase the velocity by
about 41% and reduce the mass by half, and the energy would be unchanged 
(that's a
virtual example, not observed data).

> Also, geometric factors (the curvature of the Earth, ...........) would
> tend to partially shield portions of the Earth from the direct impact effects.

On earth, an impactor starts coming alive at about 350,000 feet altitude.  From
that height, the horizon is about 700 miles away, and a line of sight passing 
from
impactor to a horizon graze back to the top of the atmosphere along that line of
sight is about 1400-1500 miles away.  So, measured at the top, about 1/8 of the
circumference of the earth's atmosphere is within the direct line of sight
(4*730/25,000 = 0.12)

> The pattern of terminal Cretaceous extinctions is complex, and defies any
> simple criterion for determining survival.

I would hope we could all agree with this statement.

Jim