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It's not news
SOME CORRECTIONS REGARDING
THE VREDEFORT IMPACT CRATER
CCNet, 11 October 1999
From Andrew Glikson <andrew.glikson@anu.edu.au>
Research School of Earth Science,
Australian National University
Canberra, ACT 0200
"SOUTH AFRICAN CRATER
IDENTIFIED AS WORLD'S LARGEST IMPACT STRUCTURE"??? (CCNet 7.10.99)
I read in CCNet 7.10.99 a report citing "South African geologists
have identified a crater in central Free State province to be the
oldest and largest in the world caused by the impact of a comet or
an asteroid, scientists said." and "the Vredefort crater, which is
between 250 and 300 kilometers in diameter, was long thought to be
of
volcanic origin." (quoted from SpaceDaily, 5 October 1999).
These cited "news" are formulated in terms giving an impression as
if
a recent discovery has been made - yet the history of the
investigation of Vredefort structure goes back at least 50 years!
An impact origin for the Vredefort Structure has been suggested as
early as 1947 by R.A. Daly (J. Geol. 55:125-145), and uniquely
diagnostic shatter cone structures were reported by R.B. Hargraves
(1961; Trans. Geol. Soc. S. Africa 64:147-154), R.S. Dietz (1961; J.
Geol 69:499-516) and W.I. Manton (1965; Ann N.Y. Acad. Sci.
123:1017-1048). An extraterrestrial impact has been questioned by
L.O. Nicolaysen and colleagues of the University of Witwatersrand,
who suggested an origin by "cryptoexplosion" triggered by
hydrostatic
pressure buildup (for example, Nicolaysen and Ferguson, 1990,
Tectonophysics 171:303-335). However, apart for the shatter cones,
the identification of Vredefort as of extraterrestrial impact origin
has been confirmed years ago by a wide range of observations,
including coesite and stishovite (high-pressure quartz polymorphs)
(Martini, 1978, Nature 272:715-717), planar deformation features in
quartz (Grieve et al., 1990; Tectonophysics 171:185-200), Iridium
anomalies in granophyre/pseudotachylite (French and Nielsen, 1990,
Tectonophysics 171, 287-301), and Re/Os anomalies (Koeberl et al.,
1996, Geology 24:913-916). This was followed by a reinterpretation
of
the Vredefort as a 180-300 km-diameter multiring structure
(Therriault et al., 1993, LPS XXIV, 1419-1420) and U-Pb zircon
dating
at 2024+/-5 (Kamo et al., 1995, Proc. Geol. S. S. Africa, 566-569).
The jury is still out whether Vredefort is the largest known impact
structure known to date on Earth, in view of the 340 km-diameter
circular magnetic and gravity anomaly associated with the late
Jurassic Morokoweng impact structure in the south Kalahari (Corner
et
al., 1997, EPSL 146:351-364; Hart et al., 1997, EPSL 147:25-35)..
Unfortunately, the inherently unreviewed nature of Internet
bulletins
results in possible inaccurate information creeping into this media.
In order to minimize this risk and maintain minimum scientific
standards, I suggest that CCNet requests authors to provide relevant
scientific references to accompany their reports, allowing readers
to
substantiate their accuracy.
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MODERATOR'S NOTE: The AFP story covered by SpaceDaily (5 October)
and
posted by CCNet (7 October) about the latest research on the
Vredefort
impact crater was, I guess, mainly based on a press release provided
by
the geologists involved in this impact study. As is sometimes the
case,
it is not science journalists but ambitious researchers who are
tempted
to use sensationalist (and sometimes even misleading) language in
press
releases in order to draw attention to their work. While it is true
that the unreviewed nature of most Internet science networks often
lead
to inaccurate information being circulated, the openness of the WWW
also allows for quick and effective self-regulatory messures so that
false or misleading information can be immediately corrected. In
fact,
I have come to the conclusion that this new form of open peer review
is
not only more democratic but, more importantly, much more effective
in
assessing the soundness of scientific data and argumentation.
Benny J Peiser
http://abob.libs.uga.edu/bobk/ccc/cc101199.html
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ABOUT THE CONTROVERSIES
Debate about the cause and effects of
the Vredefort blast has never ceased. Although the majority of earth
and space scientists today accept an asteroid impact as the cause of
the Vredefort structure, some theorists past and present have
maintained that it was not simply that. Connections between impacts
and volcanism have been suggested - the one causing the other.
One such theory is that shock waves
from an impact may travel around the earth and, when they collide on
the opposite side of the planet, the crust ruptures and a massive
volcanic outpouring occurs.
A related theory is that a blast from
inside the earth (a mysterious but massive "cryptoblast" or
Veerneshot) could eject matter which lands elsewhere, creating
craters with some volcanic features.
The controversies have by no means
been put to rest by scientific consensus around the impact origins
of the Vredefort Dome.
Also, it may not be the largest
impact crater on Earth, just the largest so far positively
identified and measured. None of this reduces the interest of the
area - in fact it enhances the Dome as a scientific phenomenon,
fully meriting its World Heritage status.
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