BYLINE: Roelof Bezuidenhout
As the 45m long earth-crosser, 2012 DA14, hurtles by and in the wake of the meteor strike in Russia's Ural Mountains last weekend, it is interesting to review the damage caused by a meteorite, half the size of 2012 DA14, which smashed into the Great Karoo 250 000 years ago and blasted out a crater 640m wide and 300m deep.
The missile probably vapourised instantly from friction heat, triggering a shock wave that would have registered over maximum on the Richter scale and spewing debris for kilometres around. A dense cloud of dust is likely to have covered the whole area for at least a few months. Over centuries, the crater slowly filled with mineral-rich water which eventually evaporated, leaving a limestone fill now raised above the sparse veld.
The mudstone ridge that at first surrounded the impact zone has weathered away, leaving the harder limestone protruding above the terrain. From the air, the formation shows up as a round, white patch almost touching the Bull River, at a spot 50km south of Graaff-Reinet.
The impact site, known as Kalkkop, is one of the least-known astrological and geological curiosities in South Africa. Meteorite impact sites that have survived erosion over the millennia are extremely rare, and Kalkkop is only the third to have been positively identified in southern Africa. The others are the larger - and of the same age - Tswaing Crater (formerly Pretoria Saltpan Crater) and the Roter Kamm Crater in Namibia, which contains a 60 ton monster of iron and nickel.
The peculiarities of Kalkkop have been sporadically studied since 1943. At one time it was included on a list of drilling targets for an oil search. But the drill core taken attracted little interest and was thrown away. With it went evidence of Kalkkop's origin. Kalkkop remained just an odd, flat-topped hill in the middle of nowhere - until the early 1990s when geologists working for the Geological Survey started a more comprehensive drilling project.
The new drill core presented a profile consisting of a thick layer of lake-deposited limestone, underlayed by fragmented bedrock of the type often encountered in impact structures. …