Geoscience Reference
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impact did not create the Manson crater (as they also but incorrectly
concluded for Sudbury and Vredefort), the discovery of shocked
quartz with planar deformation features at Manson, as well as the
overall form of the structure, showed that it should be added to the
lengthening list of terrestrial craters. Recent seismic studies show
that it has a structural central peak (not visible at the surface) nearly
3 km high.
4
As the attention of the crater hunters turned increasingly to
North America, Manson loomed as the natural candidate. Their
interest seemed to be justified when the first argon-argon age deter-
mination from Manson returned an age of 65.7 ± 1.0 million years, a
range that included the age of the K-T boundary.
5
As Manson drew
more attention, however, its small size continued to cast doubt. The
10-km impactor predicted by the Alvarezes on the basis of the
worldwide iridium levels would create a crater five times Manson's
size; one no larger was unlikely to have been able to produce the
observed impact effects. Doubt increased when the Izett group ob-
tained additional drill core samples from Manson and dated (by the
argon-argon method) unaltered feldspar grains that appeared to have
crystallized from the impact melt and which therefore should give
the structure's true age. The age came back not at 65 million years,
but at 73.8 ± 0.3.
6
The discovery that Manson is normally mag-
netized and thus cannot belong to K-T Chron 29R confirmed that
it is not of K-T age. To pin the matter down, Izett and colleagues, in
a neat piece of work, journeyed to nearby South Dakota, where, at
the stratigraphic level equivalent to an age of 73.8 million years, they
found a zone of shocked minerals.
7
Thus as more evidence has accu-
mulated, Manson has been confirmed as an impact crater, but one
that formed at a time different from the K-T.
How
TO
R
ECOGNIZE AN
I
MPACT
C
RATER
If the K-T crater was located on a continent and easy to spot, it
would have been discovered long ago; if it exists, it must either be
so eroded or covered by younger sediments, as at Manson, that it is
detectable only through the use of geophysical methods. Unfortu-
nately, geologists could not turn to earthly examples to learn how to
recognize such huge and obscure structures, for it is rare to find ter-
restrial craters larger than 100 km. Those that do occur, as at Sud-
bury and Vredefort, are ancient, distorted, and eroded. Other bodies
in the solar system, however, provide ready examples. As exempli-
