MANY lunar craters have lava flows associated with them. It is widely accepted that, in most cases, the craters are the result of meteoric impact and not volcanic activity but, if so, the occurrence of lavas needs explaining. It is commonly supposed that the lava flows are material that was melted during the impact process itself but this is not acceptable as a general explanation, for two reasons. First, in several cases the ages of the lavas have been found, by crater counting techniques, to be significantly younger than the craters themselves1 and so they cannot have been formed at the time of impact. Second, some of the lavas show morphology typical of terrestrial lava flows and it is possible to deduce from this that the lava came from sources similar to those of terrestrial volcanoes—that is, lava was supplied at a fairly steady rate over periods of several days and was not simply splash-out from a meteoric impact2. It therefore seems that these lava flows are a product of the presence of an impact crater and yet have occurred as normal volcanic effusions long after the formation of the craters.