The lunar maria are large, dark, basaltic plains on Earth’s Moon, formed by ancient volcanic eruptions. They were dubbed Maria, Latin for ‘seas’, by early astronomers who mistook them for actual seas. They are less reflective then the “highlands” as a result of their iron-rich composition, and hence appear dark to the naked eye.
Therefore, the roundness of the moon is determined by the positions of the sun, the earth, and the moon.
The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting. The radiometric ages range from about 3.16 to 4.2 billion years old (Ga), whereas the youngest ages determined from crater counting are about 1.2Ga. Nevertheless, the majority of mare basalts appear to have erupted between about 3 and 3.5 Ga. The few basaltic eruptions that occurred on the far side are old, whereas the youngest flows are found within Oceanus Procellarum on the nearside.
While many of the basalts either erupted within, or flowed into, low-lying impact basins, the largest expanse of volcanic units, Oceanus Procellarum, does not correspond to any known impact basin. Since many mare basalts fill low-lying impact basins, it was once assumed that the impact events itself somehow caused the volcanic eruption. Initial mare volcanism generally seems to have begun within 100 million year 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.
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, there are two high tides per day on Earth, and not one.
Since mare basaltic magmas are denser than upper crustal anorthositic materials, basaltic eruption 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.