<Li> Since many mare basalts fill low - lying impact basins, it was once assumed that the impact event itself somehow caused the volcanic eruption . (Note: current data in fact may not preclude this, although the timing and length of mare volcanism in a number of basins cast some doubt on it . Initial mare volcanism generally seems to have begun within 100 million years of basin formation . Although these authors felt that 100 million years was sufficiently long that a correlation between impact and volcanism seemed unlikely, there are problems with this argument . The authors also point out that the oldest and deepest basalts in each basin are likely buried and inaccessible, leading to a sampling bias .) </Li> <Li> It is sometimes suggested that the gravity field of the Earth might preferentially allow eruptions to occur on the near side, but not on the far side . However, in a reference frame rotating with the Moon, the centrifugal acceleration the Moon is experiencing is exactly equal and opposite to the gravitational acceleration of the Earth . There is thus no net force directed towards the Earth . The Earth tides do act to deform the shape of the Moon, but this shape is that of an elongated ellipsoid with high points at both the sub - and anti-Earth points . As an analogy, one should remember that there are two high tides per day on Earth, and not one . </Li> <Li> Since mare basaltic magmas are denser than upper crustal anorthositic materials, basaltic eruptions might be favored at locations of low elevation where the crust is thin . However, the far side South Pole - Aitken basin contains the lowest elevations of the Moon and yet is only sparingly filled by basaltic lavas . In addition, the crustal thickness beneath this basin is predicted to be much smaller than beneath Oceanus Procellarum . While the thickness of the crust might modulate the quantity of basaltic lavas that ultimately reach the surface, crustal thickness by itself cannot be the sole factor controlling the distribution of mare basalts . </Li> <Li> It is commonly suggested that there is some form of link between the synchronous rotation of the Moon about the Earth, and the mare basalts . However, gravitational torques that result in tidal despinning only arise from the moments of inertia of the body (these are directly relatable to the spherical harmonic degree - 2 terms of the gravity field), and the mare basalts hardly contribute to this (see also tidal locking). (Hemispheric structures correspond to spherical harmonic degree 1, and do not contribute to the moments of inertia .) Furthermore, tidal despinning is predicted to have occurred quickly (in the order of tens of millions of years), whereas the majority of mare basalts erupted about one billion years later . </Li>

What is the basic composition of the lunar maria regions