Absence of Large Shield Volcanoes and Calderas on the Moon: Consequence of Magma Transport Phenomena?

Large shield volcanoes and calderas are not observed on the Moon. Theoretical assessments show that melt ascending from the mantle will stall at rheological traps or at the shallower density trap at the base of the low-density lunar crust. Magma will then only reach the surface along dikes propagated from these deep reservoirs, and such events will be infrequent and characterized by very large-volume ruptions. These conditions favor the formation of sinuous rilles, single long flows, and widespread flow units, rather than abundant small eruptions from a central edifice. The absence of large shield volcanoes and calderas on the Moon is interpreted to be related to the fact that shallow neutral buoyancy zones and magma reservoirs are not favored on the Moon. Shield volcanoes with basal diameters in excess of 50 km are common on the Earth (Peterson and Moore, 1987), Mars (Cart, 1974) and Venus (Roberts and Head, 1990; Schaber, 1991; Pike, 1978) (Figure 1). On Earth, these features are generally built from a large number of small-volume basaltic eruptions and their associated dikes. They are characterized by a broad shield-shaped topographic profile with low flanking slopes, summit calderas, and associated flows, and they commonly have flanking rift zones, from which additional flows emanate. Summit calderas also are common on large volcanic edifices on Mars and Venus. On the Moon, about fifty small shield volcanoes are observed inthe size range 3-20