Lunar Probe
Surface guide

Lunar Maria: The Moon's Ancient Basalt Plains

Why the dark patches face Earth, how lava filled impact basins and what basalt reveals about the lunar interior.

Key takeaways

  • The maria are basaltic volcanic plains, not ancient seas.
  • Many occupy large impact basins that formed before the lava flooding.
  • Mare volcanism occurred in multiple episodes and compositions over a long span of lunar history.
  • The strong near-side concentration of maria reflects major asymmetries in crust and thermal evolution.
Lunar landscapes preserve overlapping records of impacts, volcanism and space weathering.

What the dark plains are

The maria are broad plains of basalt, a volcanic rock formed from mantle-derived melts. Their lower albedo makes them appear dark against the highlands.

Early telescopic observers interpreted them as seas, which is why names such as Mare Tranquillitatis and Mare Imbrium survive. The terminology remained after their volcanic nature became clear.

Impact basins came first

Many major maria fill ancient impact basins. The impacts created deep, fractured depressions; later basaltic magma rose through the crust and spread across low terrain.

This sequence explains why mare geology often combines impact structures around the basin margins with younger volcanic surfaces inside.

Repeated lava flooding

Mare surfaces are not single lava sheets. Mapping shows overlapping flow units with different ages and compositions.

Some flows are relatively titanium-rich while others are not. These chemical differences trace variation in mantle source regions and melting conditions.

Why the near side has more maria

The near-side crust is on average thinner than much of the far side, and heat-producing elements are concentrated in particular near-side provinces. These differences likely made magma ascent easier in some regions.

The asymmetry is therefore tied to the Moon's large-scale internal evolution, not simply to where impacts happened.

What samples tell us

Apollo and Luna returned mare basalts with radiometric ages that calibrate volcanic history and crater-count chronology. Different landing sites sampled different basalt types.

Chang'e 5 added younger volcanic material than the Apollo collections, extending direct age calibration into a poorly represented period.

How to observe maria

Maria are easy naked-eye landmarks and useful anchors for telescope navigation. Their broad dark surfaces contrast with bright crater rays and highlands.

Low Sun angles reveal wrinkle ridges, crater rims and subtle relief that disappear visually near full Moon when shadows shorten.

See the landscape in context

Use the Lunar Surface Atlas, Feature Finder and Crater Morphology Explorer to connect the explanation with real terrain.

Sources and further reading