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Late K volcanism (was RE: causes of K-T ferns)



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Marcel Bertolucci
>
>
>     Even if the iridium font is volcanic, how so many volcanoes
> could erupt
> so quickly in so short time if not by a greater force, such as an
> impact by
> a meteor, or a small comet or whtaever it could have been?

Ummm... because plate tectonics doesn't operate at the same rate at all
times in Earth History.  Because deep plumes are generated (by reasons still
unexplained, but very very far away from minor trivialities like 10-15 km
diameter pebbles hitting the thin wrinkly cover of Earth).

Rates of volcanism do vary greatly throughout Earth History: sometimes due
to new spreading centers; sometimes due to development of new subduction
zones; sometimes due to changes in directions or orientations of subduction
zones; etc.

The later part of the Late Cretaceous saw many new volcanic orogenic
episodes in different parts of the world: a shift from Sevier to
Laramide-style orogenies in the Cordilleran region of western North America;
Seychelles and Deccan volcanism; and some other smaller events.  These begin
prior to the impactor, so unless the blast of Chixculub could travel
backwards in time, it is a coincidence that both were going on in the latest
K.

>     I´m not saying that the impact is the cause/only cause of extinction,
> but the cause of the big K-T volcanic eruptions.

Most likely not.  After all, the antipodal volcanism suspected on Mars and
Mercury seemed to have been associated by really big impacts (not piddly
little bumps like Chixculub...).  Also, at least some of the important
volcanic episodes are associated with mantle plumes, which are currently
believed to be generated near the mantle-core boundary (and thus 1000s of km
away from the impact site, though bazillions of tons of material much denser
than the brittle silica-rich froth we live on).

That being said, it is becoming a bit annoying that there is evidence
associating impacts and big flood basalt events in the Permo-Triassic, the
Triassic-Jurassic, and the K-T boundaries.  (Anyone know of any good
evidence for impacts associated with the Columbia Flood Basalts or the Snake
River extrusives?).

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796