Distribution and Origin of Lunar Craters

Abstract

I HAVE elsewhere1 expressed agreement with Mr. Moore's view that the lack of shaking-down effects provides an important clue to the origin of lunar craters, and at the same time expressed my own view that the structure of the crater Porter is inexplicable in terms of explosive evisceration, a process which I have never favoured, though simple dish-craters in maria could be produced by a bubble-burst through a thin plastic veneer. The fact that craters appear to have been the product of slow and gentle processes does not, in my view, invalidate my hypothesis. Certainly the large craters stem from subsidence, and, as on Earth, the initiating upwards push may not necessarily have been explosive in every case. The blisters seen in some craters probably reflect gentle pressures without break-out. It remains possible that many of the large cirques were initiated by point-focus explosion, forming axial vents and confocal fractures, not craters. This seems most likely in the case of early structures, since, once fractured, the retaining crust would allow slow gas leakage rather than explosive discharge. It is interesting to note that at least one carbonatite complex reveals some evidence of a recessive stage involving cauldron subsidence2. Moore's suggestion of gentle uplift and subsidence, repeated several times, fits in well with patterns of history in terrestrial volcanic foci, but, as on Earth, one must expect no uniformity of detailed history, and I do not think point-focus explosion can be ruled out, entirely, because of the objection made.