There has been a long-lived controversy over the origin of a well-defined linear set of eight cryptoexplosion structures in the southern midcontinent, United States. The linearity suggests that these structures must be related to either (1) impact of a string of bolides derived from breakup of a large parent bolide, or (2) volatile-rich, explosive ultramafic magmatism associated with structural weaknesses in the crust. That these structures are of various ages, are at or near intersections of major regional tectonic features, and have closely associated ultramafic magmatism, rules out the impact-string hypothesis. The only remaining hypothesis is that these cryptoexplosion structures were produced by explosive ultramafic volcanic activity. An important corollary to this hypothesis is that some shock-metamorphic features, including shatter cones and shocked quartz, can be produced by explosive ultramafic igneous activity.

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