Re: End of dinosaurs

["King, Norm" <nking.ucs@smtp.usi.edu>]

WOlewiler sent this note (5/29/96; 10:48a):

>    Charles Pellegrino suggests this "co-factor" in dino extinction:
>     During the dino era much of N. America, Europe, Asia, and Africa
>were under water. And this high ratio of water to land surface
>established a mild climate that dinos thrived in.
>     Then volcanic activity at the mid sea ridges subsided. The ridges
>began to sink under their own weight, draining water from the
>continents. As sea levels fell, oceanic and air currents were changed
>drastically by the newly exposed land, with dramatic effects on
>climates. This might have been a "major cofactor" in dinosaur
>extinction.
>     Source: Pelligrino, Charles. Unearthing Atlantis. New York:
>Random House, 1991.

Much of Australia and Antarctica were also underwater then.  Sea levels 
were indeed high during the Cretaceous, and have been in a generally 
falling trend since late in the period.  However, sea levels seem to have 
fluctuated about today's mean for much of the Triassic and Jurassic (of 
course, also part of the "dino's era").  Thus, changes in rate of 
sea-floor spreading seem not to be a "co-factor" in the dinosaur's 
extinction.

The mid-ocean ridges do not sink.  Newly-formed oceanic crust, indeed 
formed through volcanism, has "floated" (isostatically on the mantle) at 
the same level throughout the Phanerozoic, since the molten rock erupted 
along the ridges has always been at about the same temperature at the 
time of eruption (until you get way back into the Precambrian).  However, 
oceanic crust that forms along the rift zones of the ridges will indeed 
sink as it is carried away from its source at the ridge by sea-floor 
spreading.  It sinks to below the level of the ridge crest because it 
contracts, hence becomes denser, as the rock gradually cools.  When 
sea-floor spreading is rapid, oceanic crust is still quite hot and 
"riding high" some distance from the rift.  During times of slow 
speading, the crustal rock becomes cooler at comparable distances, hence 
subsides more, creating more volume in the ocean basins.  As a result, 
water "drains off" the continental crust.  

The long-term sea-level curve shows that dinosaurs lived much of their 
time in conditions of low sea level, hence continental climatic extremes, 
and did quite well at those times. Thus, there is no reason to suspect 
that the rate of sea-floor spreading, which in turn affected sea levels, 
which in turn affected climates, played a role in the dinosaur's 
extinction.

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Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu