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SHATTER CONES PROVE... what? Controversy surrounds the shatter cone "fingerprints in the rock" which have been used by geologists to prove that an impact event occurred at Vredefort. The evidence is strongly in favour of an impact event by a bolide or space body that hit the earth - an asteroid. But some scholars maintain that the same evidence can be used to construct a very different story. Shatter cones are spherical or polygonal chunks of rock, examples of which occur in the inner crater rim of the Vredefort Dome. Shatter cones are also found wherever there has been a major blast or impact on the earth. They indicate that the original rock was shattered - rather like a car's windscreen being hit by a stone. Their origin has been put down to either impacts of large space objects or to catastrophic blasts from within the earth. Shatter cones are "fractal" in character, that is, they repeat the same patterns ad infinitum so that the patterns one sees with the naked eye also appear at the microscopic level. Nicholaysen and Reimold (1999) found in a study of the Vredefort shatter cones that "Shatter cone and MSJS surfaces are often covered with glassy films; we evaluate whether these fracture phenomena are linked to the formation of pseudotachylitic (friction) melt. Our field and petrographic observations can be interpreted as consistent with the generation of shatter cones/MSJS relatively late in the formation of the Vredefort structure. This scenario contrasts sharply with the widely held view that shatter cones are formed during the early ``compression'' phase of a shock event that affected horizontal strata." (Abstract: Journal of Geophysical Research, Volume 104, Issue B3, p. 4911-4930. Nicolaysen, L. O.; Reimold, W. U.) Baratoux and Melosh (2003) proposed a new model for the formation of shatter cones. They followed an older theory that shatter cones could be caused by fragmentation in mixed forms of rock, which they modelled numerically. They found that the formation of shatter cones was consistent both with impacts and explosions. "The conditions required for the formation of shatter cones are explored numerically and are found to be consistent with the pressure range derived from both explosion experiments and the analysis of shock metamorphic features in impact structures." (Abstract: Submitted to Earth And Planetary Science Letters March, 28th, 2003. "A New Model for the Formation of Shatter Cones : Consequences for the Interpretation of Shatter Cone Data in Terrestrial Impact Structures". D. Baratoux, H.J. Melosh).
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Among the controversies:
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