From Rob Hough, Alex Bevan and Ken McNamara, Western Australian Museum
We were surprised to read that an asteroid “wiped out huge swathes of life
when it collided with Earth 360 million years ago”
(26 January, p 11).
The resulting Woodleigh crater is apparently “120 kilometres wide” and the impact
happened at “the time of a mass extinction towards the end of the Devonian
period, when 85 per cent of all species were wiped out”. These statements are
not borne out by close scrutiny. Controversy surrounds the crater’s diameter,
and its age has yet to be determined accurately.
The Late Devonian date for Woodleigh is based on dating of clay minerals from
the shocked granitic rocks. But dating clay minerals is not considered a
definitive way of establishing the age of an impact event, since there are many
alternative interpretations for the origin of the minerals. The clays most
likely formed over a long period of time, probably due to a water-driven thermal
event that occurred some considerable time after the impact. Moreover, marine
fossil fauna from the Late Devonian in northern Western Australia shows that
there was not a single mass extinction event, but a number of smaller
extinctions, readily explained by earthbound processes.
Critics of the 120-kilometre-diameter interpretation—and there are
many—suggest that it is nearer 60 kilometres in diameter, based on the
available geophysical data. The Woodleigh impact would indeed have “created one
hell of a bang”. Until we know when it happened, any purported association with
mass extinctions or global environmental effects is pure geofantasy.
Perth
