Lunar volcanism in space and time.

Lunar volcanic deposits are dominated by areally extensive mare units occurring in regionally low areas predominantly on the lunar near side. Data obtained from lunar orbit and earth-based observations have been used to extend the detailed characterizations derived from Apollo and Luna sample return missions to other parts of the moon. Mare volcanism occurred over a period of about 1.3 billion years (b.y.), from about 3.8 to about 2.5 b.y. ago, although the absolute age of the youngest flows is not known. An early Ti-rich mare phase (Apollo 11 and Apollo 17 type basalts) flooded large areas of the eastern portion of the lunar near side in the early Imbrian Period (about 3.5–3.8 b.y. ago). An intermediate age, less Ti-rich phase (Apollo 12 and Apollo 15 type basalts) flooded widespread areas of the moon predominantly in the middle to late Imbrian Period (about 3.0–3.5 b.y. ago). Finally, a second Ti-rich phase (unsampled by Apollo and Luna) flooded portions of Mare Imbrium and the western maria in the early Eratosthenian Period (about 2.5–3.0 b.y. ago). Features associated with mare deposits and processes include lobate scarps, sinuous rilles, domes, cones, dark halo craters, collapse craters, kipukas, lava terraces, mare ridges, and volcanic complexes. The very fluid nature of lunar lavas, their voluminous and extensive flows, and the dominance of volcanic landforms of nonexplosive origin all strongly indicate that lunar eruptions and deposits are most similar to terrestrial basaltic flood and Hawaiian eruptions. Volcanic morphologic features, deposit volumes, extrusion rates, and association of many sources with major crustal fractures all suggest that mare lavas originated at subcrustal depths and worked their way to the lunar surface through a passive but fractured lunar crust. Lavas preferentially infilled the existing low regions of the moon, apparently in a hydrostatic mode. Since lunar impact basins produce most of the topographic lows, mare deposits are concentrated in and around these features. The lunar crust acted essentially as a passive platform on which mare lavas were emplaced. Far side mare deposits are patchy but are concentrated on the floors of the largest basins. Thicker far side crust and hydrostatic emplacement of mare lavas may explain the lack of extensive far side deposits even in deep basins. Individual maria had a history of filling largely determined by the geometry and state of degradation of the basins in which they occur. Mare deposits are generally less than about 2 km in thickness but probably approach 6–8 km locally in the center of some mare basins. The process of filling took place over extensive periods of time and produced complex deposits. Although some near-surface fractionation of mare lavas must have occurred, there is little photogeologic evidence for environments in which extensive fractionation would take place; lavas appear to have erupted at high rates, to have been very fluid, and to have spread out into thin flows; evidence is lacking for shallow magma reservoirs and for lava lakes on the scale of a mare basin. The portion of lunar mare deposits emplaced on the surface of the moon represents less than 1% of the total volume of the lunar crust. There is little unequivocal morphologic evidence for extensive highland volcanism, although intense bombardment may have erased such evidence from early lunar history. The majority of upland plains appear to be related to impact processes. Morphologic evidence for local highland volcanism is found primarily in the form of spectrally distinct domes.