We have seen that the feature common to all impact craters is, initially, a floor at a lower level than the surrounding terrain. On Earth, such hollows become repositories for water where it is available. On the dry Moon, such hollows are the repositories of ejecta from later impacts and the products of volcanism. Magma rising to the surface will seek the shortest pathway, which means it will break through to the surface at the lowest altitudes. We saw that in the case of the lunar Orientale basin, the center was the first to fill with lava. Subsequent flows were erupted between the elevated rings. Effectively, once the deepest holes are filled, progressively less deep cavities become the targets for rising magma. However, magma will only rise to a level where its density is less than that of the rocks through which it is passing. If it were to encounter a layer of loosely packed impact breccia on its way to the surface, it would more likely end up being intruded as a sill. Such is probably the case with many lunar floor-fractured craters. The following image shows the craters Lavoisier D (biggest), E and B (smallest) on the western edge of Oceanus Procellarum on the near side of the Moon. Each has a floor that is obviously fractured but the fracturing does not extend beyond the crater floor.

The following cartoon illustrates two possible scenarios for the post-impact modification of lunar impact craters by volcanism. We start at the top of the cartoon with the unmodified lunar crater Harpalus. Taking the left branch, we see that Damoiseau has a flat floor, no visible central peak and floor fracturing. It is possible that the crater never had a central peak or it was covered by ejecta from surrounding craters. It is believed that the floor fracturing, in this case, is the result of intrusion by a sill. Moving on to Alphonsus, we see that the fractures have become the sites of eruption with, typically, dark-haloed material occupying the surrounding floor of the crater. Continuing through Schluter and Posidonius, the extent of volcanic material occupying the floor becomes more continuous until it eventually buries all traces of the fractures from which it was erupted and we have the totally infilled crater Cruger. Taking the right branch, it appears that floor flooding proceeded without accompanying fracturing as a result of sill intrusion. From Aitken through Maraldi we have progressively more infilling till we again get to the Cruger Scenario. At this point further infilling ceases, or as in the case of Plato, further volcanism shifts to a lower level on the exterior of the crater wall. If volcanism ceases entirely then, because of its greater density, the basalt fill starts to sink under its own weight and a series of fractures follow, producing the scene that we witness in the crater Pitatus.

In no way am I implying that the impact cratering event was the initiator of volcanism; the crater floors simply acted as the lowest altitude waiting to be filled. However, in the case of large impacts, the pressure-release accompanying the excavation and rebound of the crater floor, may have been sufficient to initiate the subsequent eruption of material from a pre-existing shallow magma chamber. The following cartoon illustrates the scenarios encountered in this section on cratering.

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