No. 106: Jul-Aug 1996
100 years ago. Back in 1900, a geologist risked his reputation by suggesting that Arizona's Meteor Crater was an impact structure. It had to be volcanic or perhaps due to a steam explosion!
50 years ago. In 1950, a geologist risked his reputation by suggesting that large impact structures existed; that is, bigger than 10 kilometers in diameter.
0 years ago. Today, geologists converse blithely about 100-kilometer structures buried beneath the Yucatan and Chesapeake Bay. They are, however, exceedingly chary about long chains of impact structures.
Those eight craters in a row. Geologists are questioning whether the eight structures stretching from Kentucky to Kansas (mentioned in SF#105) are all impactcaused. In a letter to Astronomy. A. Goldstein asserts that only three are impact craters; the other five are cryptoexplosion structures; that is, due to internal activity of some sort. However, Goldstein adds that there are actually three additional structures on this long line in Kentucky.
(Goldstein, Alan; "Multiple Strike Stricken," Astronomy, 24:20, July 1996)
Comment. Even if eight of the eleven structures on the line are cryptoexplosive in origin, one has to wonder why these are all lined up. A long line of weakness in the crust?
Meanwhile, in Africa. 1994 radar images from the Space Shuttle have revealed a chain of three suspicious circular structures in the Sahara of northern Chad. Largely buried in sand, each is about 12 kilometers in diameter. Only one of these structures has been studied at ground level; and it is of impact origin, as confirmed by upturned strata and grains of shocked quartz. The other two could be volcanic say the skeptics. It will take an expedition to Chad to clarify things.
(Kerr, Richard A.; "Impact Craters All in a Row?" Science, 272:33, 1996)
A final comment -- at last! All impactcrater chains are relevant to the claim of H.R. Shaw that even the earth's largest impact structures are found in swathes (chains). In his wonderfully heretical book Craters, Cosmos, and Chronicles: A New Theory of the Earth, Shaw mentions several such swathes. One of his chains includes Chicxulub (beneath Yucatan), Manson (Iowa), Avak (Alaska), and three more in Russia. These are giant craters stretched out over much of the planet, not pipsqueak craters athwart a couple states!
50 years from now. In 2050, perhaps someone will wonder why Shaw's vision was rejected so emphatically today.
Reference. Impact craters and other topographic anomalies are cataloged in ETC in the catalog: Carolina Bays, Mima Mounds. Ordering information can be found here.