SVENSSON1 has drawn attention to a most interesting cryptic structure—one more in the growing list of such cryptoexplosion structures. I would, however, question how much weight should be attached to the recognition of shock lamellae in quartz grains as a criterion of meteorite impact involvement. I have previously endorsed Bucher's scepticism regarding numerous so-called astro-blemes2, and since that publication Snyder and Gerdemann3 have put forward what seems to me to be an overwhelming case for the endogenous origin of the numerous cryptoexplosion structures east of the Appalachians. Currie and Shafiqullah4 have recovered potassic trachyte and alnöite from the Brent Crater, Ontario. Both these accounts confirm my conclusions, and the Brent discovery supports the idea that some sort of highly explosive alkalic vulcanism produced these structures. It is apparent that shatter cones and coesite, both found in association with cryptic structures of this group, are not restricted to meteoritic cryptoexplosion structures. One may well question whether shock lamellation in quartz has any more validity: surely it has been reported from the Brent and Holleford craters, which are clearly of endogenous origin ? (A fortuitous association of alnöite with an astro-bleme is extremely unlikely, for alnöites are very rare igneous rocks associated, in most cases, with localized, highly explosive diatreme activity, involving carbonatites, and such endogenous eruptivity produced the Pretoria Salt Pan which appears just like many of the cryptoexplosion craters attributed to meteorite impact explosion.) Geologists who, like myself, have worked extensively on carbonatites5 must realize that the supposed restriction on the energy involvement involved in volcanic explosions (accepted without question by many authorities because it is repeated so often in the literature) is without foundation. The sort of phase change reaction that must be involved in the production of the immense carbonatite explosion breccia fillings of some carbonatite vents, and of the associated cone sheets—a reaction producing a colossal explosion at a point focus at depth—does not seem to be limited in any way by the strength of the overlying thickness of crustal rocks. Such an explosion, involving adiabatic expansion, could, physicists assure me, involve a virtually unlimited amount of energy, and so we are left with the possibility of two mechanisms, one internal and one of extraterrestrial origin, capable of producing these cryptoexplosion structures. No one would argue that there is much conflicting evidence: and this conflict applies to the Rieskessel, New Quebec Crater, Manacouagan Lake, Clearwater Lakes, Bosumtwi Crater, and even to Wolf Creek Crater. One thing, however, is clear from the literature: there are at present no valid criteria that afford strong support for either theory, except the actual association of meteoritic material; and even where that is present, as at Wolf Creek6,7, one can reasonably harbour just an iota of doubt7.
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Jun 1, 1974
R B Hargraves + 1
Open Access