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Re: (meteorobs) Iridium from Impacts vs Volcanism



At 10:31 AM 7/25/99 -0400, Asaf wrote:
>
>I was once in a lecture on the topic. More Iridium is
>given (I think) from Asteriods, but that wasn't why
>they think that Dinosaurs got extinct from asteroids
>rather than volcanoes. Today scientists believe that
>the extinction was caused by an Asteroid. I don't
>remember the main reason, though.

The July/August issue of The Sciences contains an excellent review of two
recent books on this topic, and a discussion of current thought.  The thin
layer of clay at the K/T boundary where the iridium was first found also
contains shocked quartz, tektites, and minute glass spheres, all features
created by large impacts, not volcanism.  That the iridium in that layer
had its origin in an asteroid impact is no longer much debated.  Still
debated is whether the K/T impact was the cause of a massive spike in
extinctions... or whether the extinctions happened at all, at least in the
rapid time frame called for by the impact theorists.  

"...the extent of the catastrophe is far from clear.  Some investigators
note that the number of dinosaur species in North America began to decline
millions of years before the impact, whereas some marine and plant species
survived long after.  Other investigators wonder why some of the most
vulnerable species were among the survivors.  Still others question how the
impact could have ignited global wildfires." - Robert Zimmerman, "This Is
The Way The World Ended", The Sciences, July/August 1999, p. 40.

Volcanism, though not the source of K/T iridium, is far from dead as the
p[otential mechanism of mass extinction.  In his Sciences article,
Zimmerman reviews Vincent Courtillot's "Evolutionary Catastrophes: The
Science of Mass Extinction".  Courtillot details his dating of the origin
of the Deccan Traps, a vast volcanic feature in India, to the K/T boundary
time.  Seven of the twelve known traps are related chronologically to mass
extinctions.  Courtillot blames the K/T boundary mass extinction on a
combination of factors, in chronological order:  a rapidly lowering sea
level, the Deccan Traps, and then the Chicxulub impact.

The debate is still alive, though the impact theory has rapidly become the
canonical theory.  As in most big science, there is a vested interest in
promoting the impact theory of mass extinctions - if impacts are not as
dangerous as we think, why should we (taxpayers) spend all this money on
NEO search projects, NASA asteroid-intercept missions and the like?
Zimmerman points out that even if there were a variety of reasons, both
terrestrial and cosmic, for the K/T extinction, impacts are still
cataclysmic events, and the science generated is still good science.

Finally, a plug for The Sciences:  The Sciences is published bimonthly by
the New York Academy of Sciences, and the articles discuss various areas of
science, typically in an integrative style, drawing in science's effect on
society and the the effect on other areas of knowledge.  The editor of The
Sciences is Peter G. Brown, son-in-law of the late Maurice King, meteor
photographer and longtime mentor of Meteor Group Hawaii.  Subscriptions can
be had for $21 from the NYAS, 2 East 63rd Street, New York, NY 10131-0164.  

JB 
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