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The Moon's surface records its complex history: heavily cratered highlands from early bombardment; dark volcanic plains (maria) from lava flooding basin interiors; continued impact cratering. Lunar rocks reveal composition variations, ages from 3.8+ billion years ago, and absence of a strong magnetic field today, constraining internal structure and thermal evolution.
From your study of the Moon-Earth system, you understand that the Moon likely formed from debris ejected by a giant impact early in Earth's history. That violent origin set the stage for the Moon's subsequent geological evolution — a history that is remarkably well preserved on its surface because, unlike Earth, the Moon has no atmosphere, no water, and no plate tectonics to erase the record. The Moon's surface is essentially a geological archive stretching back over 4 billion years.
The Moon's terrain divides into two visually distinct regions. The bright highlands (terrae) are the older, more heavily cratered surfaces, composed primarily of anorthosite — a light-colored rock rich in the mineral plagioclase feldspar. These highlands represent the Moon's original crust, which solidified from a global magma ocean that existed shortly after formation. As this ocean cooled, lighter minerals like plagioclase floated to the surface while denser iron- and magnesium-rich minerals sank, creating a compositional layering that persists today. The highlands are saturated with impact craters from the Late Heavy Bombardment, a period roughly 3.8 to 4.1 billion years ago when the inner solar system experienced an intense flux of asteroid impacts.
The dark maria (singular: mare, Latin for "sea") are younger volcanic plains that fill many of the large impact basins on the Moon's near side. After giant impacts excavated deep basins, heat from radioactive decay in the Moon's interior eventually melted rock at depth, and basaltic lava erupted through fractures to flood the basin floors. These eruptions occurred mainly between 3.8 and 3.1 billion years ago, with some flows as recent as about 1 billion years. The maria are notably concentrated on the near side of the Moon, likely because the crust is thinner there (about 60 km versus 100+ km on the far side), making it easier for magma to reach the surface.
Radiometric dating of lunar samples returned by the Apollo missions established an absolute chronology that anchors crater-counting age estimates across the entire solar system. By counting craters on surfaces of known age, scientists calibrated a relationship between crater density and surface age that is now applied to Mars, Mercury, and other bodies where no samples have been returned. The Moon's geological activity has been winding down for billions of years as its small size allowed it to lose internal heat relatively quickly, but it has not ceased entirely — recent evidence from orbital measurements suggests minor outgassing and possible very young volcanic deposits, hinting that the Moon's deep interior may retain small pockets of residual heat.