similarity between the volcanic craters in the Campi Phlegraei in Italy and the craters on the Moon. Edmund Neison published a book on the Moon favoring the volcanic hypothesis for crater formation in the 1870s (Hoyt, 1987). Speculation on the mechanism of crater formation continued with the publication of The Moon: Considered as a Planet, a World, and a Satellite by James Nasmyth and James Carpenter in 1874, which considered a number of possible volcanic mechanisms. Nasmyth and Carpenter did consider the possibility of steam explosions, but they noted that the apparent lack of water on the moon made it difficult to see how that could account for the “immense display of volcanic action which the surface exhibits”. However, a recent reanalysis of rocks returned from the moon has indeed found some trace amounts of water, not much, but potentially enough to make a difference geologically. Furthermore, since they assumed that the Moon had accreted over time from a primordial nebula and initially was a molten ball, they said “it does not appear clear how expansive vapours could have lain dormant till the Moon assumed a solid crust, as all such would doubtless make their escape before any shell was formed, and at an epoch when there was ample facility for their expansion”. However, if the Moon was initially created cool, this objection does not apply. These authors also considered the possibility of underground explosions, especially since the craters are mostly perfect circles. But they didn’t have a way to explain “such a very local generation of a deep-seated force” and even if they did, they didn’t see how an explosion would produce raised crater walls (Hoyt, 1987). But circular maar craters up to several kilometers in diameter are known to form on earth due to steam explosions that originate underground. Also, subterranean nuclear explosions have shown that raised crater rims do form from such explosions, and the physics involved is now understood. So, neither of those objections seems any more to be very convincing. Grove Gilbert worked for the US Geological Survey and had the chance to visit Barringer Crater, which he concluded was formed as a maar volcanic crater after failing to find any magnetic evidence for a large amount of buried iron in the crater, and after calculating that the volume of material in the rim matched the volume of the hole. He also considered the formation of lunar craters, and believed that the smaller craters may have likewise been maar craters, but he couldn’t imagine that a similar process could form craters on the moon with over a hundred of times larger diameters than most maar craters formed recently on earth (Hoyt, 1987). However, he knew neither about evidence for immense thermal expansion cracks nor for accelerated radioactive decay, including how much energy would have been available to cause enormous craters during such an episode. Indeed, the question of how lunar craters could be so large compared to any known terrestrial examples was a frequent objection to volcanic cratering hypothesis, particularly from the 1800s and beyond. A likely reason for this objection is that, by this time, the doctrine of uniformitarianism was gaining scientific respectability, and craters that are over 1000 km in diameter such as are seen on the moon are wildly inconsistent with slow and gradual geologic processes over the ages, particularly if they are due to internal forces on the moon. For instance, Ralph Baldwin reasoned that volcanic processes that produced craters of a maximum size of only a few kilometers in diameter in recorded history, such as Tambora in 1815, could not explain lunar craters that are sometimes over 1000 km in diameter (Spudis, 2010). In a sense, he was right. Craters that large would require many orders of magnitude more heat and energy than are available today, such as the heat produced by accelerated radioactive decay. If they were fully aware of the massive volcanoes on Mars and the giant thermal expansion cracks on the moon and Mars, and the energy required for their formation, would they have been so quick to reject volcanism as a cause of many of the craters on the Moon? 2. “Impact Signatures” Reconsidered A number of features have been proposed to be impact signatures, or features that allegedly can only be explained by an asteroid or cometary impact but not by a volcanic explosion. Some of those proposed features include shock metamorphism, planar deformation features, shatter cones, and impact melts (Grieve et. al., 1996). The impact theory states that these features form only under the extremely high transient pressures of the shock of a high speed impact. However, every one of these features is also present in underground nuclear explosions, as in the Sedan nuclear test crater and other craters formed by nuclear explosions. (Figure 8). So these features are not necessarily impact signatures, but rather signatures of high energy explosions that generate high local pressures more generally. So it is not clear from a physics standpoint why an impact-induced explosion with enough energy to form a 2-km diameter crater would generate a substantially higher shock pressure than a volcanically-induced steam explosion with enough energy to form a 2-km diameter crater. Indeed, alleged impact signatures need to be tested by systematically surveying every similar maar crater for “impact signatures” such as shock metamorphism and shatter cones. If some impact signatures are found at some clear maar craters such as MacDougal and Elegante craters in the El Picante volcanic field in Mexico near the Arizona border, then it needs to be admitted that these are not impact signatures at all. Indeed, it is a prediction of this hypothesis that some “impact signatures” will be found at some of these maar craters Figure 8. Sedan Nuclear Test Crater STERNBERG Craters and cracks 2023 ICC 17
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