This epoch occurred from 3.85b years ago to about 3.8b years ago. It overlaps the end of the Late Heavy Bombardment of the Inner Solar System.

Early Imbrian

Named after sulphurous in Greek, the era lasted until about 3.5 Gya. It was an era of extensive volcanism, which released large amounts of sulphur dioxide (SO2) into the atmosphere which combined with water to create a sulphuric acid-rich environment that allowed the formation of hydrated sulphates (notably kieserite and gypsum).

Theiikian

3.7 to approximately 3.0 Gya. It is marked by the formation of extensive lava plains. The formation of Olympus Mons probably began during this period. Catastrophic releases of water carved out extensive outflow channels around Chryse Planitia and elsewhere. Ephemeral lakes or seas may have formed in the northern lowlands.

Hesperian

Runs from 3.2b years ago to 1.1b years ago and named after the crater which displays characteristics typical of craters of this age, including a surface not significantly eroded by subsequent impacts. The massive basaltic volcanism of the Imbrian period ceased during this era, and the youngest lunar lava flows identified from images are tentatively placed near the end of this period.

Eratosthenian

This epoch occurred between 3.8b-3.2b years ago, during which the mantle below the lunar basins partially melted and filled with basalt. This is thought to have occurred because the impacts of the previous era thinned the overlying rock – either causing the mantle to rise because of reduced pressure, or the top melting as heat flowed upwards because of reduced insulation. Most lunar samples come from this epoch.

Late Imbrian

3.0 Gya to present. Regions have few meteorite impact craters but are otherwise quite varied. Lava flows, glacial/periglacial activity, and minor releases of liquid water continued during this period.

Amazonian

Runs from 3.92b years ago to 3.85b years ago when numerous major basins were formed by large impact events. Ejecta from this form the upper part of the densely cratered terrain found in lunar highlands. Magnetic anomalies found in some era basins suggest that the Moon possessed a core dynamo magnetic field at that time.

Nectarian

Named for iron in Greek, the era lasted from 3.5 Gya until the present. With the decline of volcanism and available water, the most notable surface weathering process has been the slow oxidation of the iron-rich rocks by atmospheric peroxides producing the red iron oxides that give the planet its familiar colour.

Siderikan

Runs from 4.533b years ago (the time of the initial formation of the Moon) to 3.92b years ago, when a large basin was formed by a large impact. Rocks from this era are rare in the lunar rock suite.

Pre-Nectarian

The interval from the accretion and differentiation of the planet about 4.5 billion years ago (Gya) to the formation of the Hellas impact basin, between 4.1 and 3.8 Gya. Most of the geologic record of this interval has been erased by subsequent erosion and high impact rates. The crustal dichotomy is thought to have formed during this time, along with the Argyre and Isidis basins.

Pre-Noachian