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Gornitz, V., 1973: Igneous vs. impact processes for the origin of the mare lavas. Moon, 6, 357-379.
In spite of chemical and petrological data furnished by the early Apollo missions, disagreement has persisted as to the ultimate origin of the mare lavas — were they true igenous magmas or impact melts? Examination of Lunar Orbiter and Apollo photographs of Tsiolkovsky, Mare Orientale and Humboldt crater, as examples of mare-filled impact structures, has suggested the answer.
It has been found that the mare lavas possibly stem from internal melting because a considerable time interval has elapsed between the time of basin excavation and basaltic extrusions. This was most effectively shown by crater counts on the ejecta blanket and mare filling of Mare Orientale. The central mare filling is distinctly younger than the ejecta covers, as shown by the lower crater densities on the mare surface as compared with the ejecta. Furthermore, many craters on the ejecta blanket of Orientale were flooded by lava long after the impact had occurred.
Mare-type lavas are not only confined to large circular impact basins, but also fill irregular depressions, like Mare Australe, where evidence for different flooding episodes has been observed.
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