From Francis Macdonald, California Institute of Technology
Yes, BP’s money can buy wonderful imaging, but it cannot necessarily buy a unique impact crater (3 August, p 19). There are several impact structures on Earth that are just as well-preserved and probably also retain concentric fracturing, but have not been imaged to the same extent.
The article claims that “no other crater on Earth has anything like [the 10 concentric fractures]”. This is not true. Phil Hawke of the University of Western Australia processed geophysical images of the 11-kilometre Yallalie feature in Western Australia. They show concentric fractures extending to a diameter of about 40 kilometres.
Researcher Phil Allen writes: Francis Macdonald is quite correct when he suggests that there are other “multi-ringed” impacts on earth. However, the nature of the fracturing at Silverpit is quite different from craters such as Yallalie. That one has large ring-faults, most of which appear to be part of the normal cratering process such as uplift and slumping. These have a comparatively large spacing with respect to the crater’s diameter. The ring-to-crater width ratio is quite different at Silverpit.
Writer Ralph Lorenz adds: Macdonald makes an important point. To date we have literally only scratched the surface of the terrestrial impact crater population. There may be hundreds more craters beneath the surface that can be discovered seismically, and perhaps a few may have external rings like those at Silverpit.
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