Lunar Probe
Surface guide

Lunar Geology: How to Read the Moon's Surface History

A field guide to highlands, maria, basins, craters, volcanic landforms and relative age.

Key takeaways

  • The lunar surface records crust formation, enormous impacts, prolonged volcanism and continuous micrometeoroid gardening.
  • Highlands are generally older and more heavily cratered than maria.
  • Crater density provides relative age, while returned samples calibrate absolute chronology.
  • Orbital maps and samples must be interpreted together because each provides a different scale of evidence.
Lunar landscapes preserve overlapping records of impacts, volcanism and space weathering.

Start with the two most obvious terrains

The bright highlands and dark maria provide the first useful geological division. Highland terrain is generally ancient, heavily cratered and rich in plagioclase-bearing crustal rocks. Maria are basaltic plains formed by lava flooding low regions, often inside older impact basins.

Brightness alone is not a complete geological map. Composition, age, slope and space weathering all influence appearance, so orbital spectroscopy and sample data are needed to go beyond visual classification.

Impact basins reorganised the crust

The largest impacts excavated enormous basins, fractured the crust and deposited ejecta across wide regions. Multi-ring basins such as Imbrium and South Pole-Aitken are regional geological structures, not simply oversized craters.

Later lava commonly flooded low basin interiors, which is why impact and volcanism are linked in the history of many maria.

Volcanism lasted a long time

Basalt flows differ in composition and age. Some mare units are much younger than others, showing that mantle melting continued long after the earliest crust formed.

Sinuous rilles, pyroclastic deposits and volcanic domes reveal processes beyond broad flood basalts. Returned samples from new regions continue to extend the age range represented in laboratories.

Reading relative age from superposition

When one crater cuts across another feature, it must be younger than what it disrupts. Ejecta blankets, lava flows and secondary craters create similar relative-age relationships.

Crater counting formalises this principle statistically. Surfaces exposed for longer generally accumulate more craters, but resurfacing, secondary impacts and terrain differences must be considered.

Ground truth from samples

Radiometric ages of returned rocks allow scientists to connect crater density with absolute time. This calibration is critical because crater-count chronologies are then applied to unsampled lunar regions and, with additional assumptions, other planetary bodies.

The limitation is geographical. Apollo and Luna sampled a relatively small set of locations, so every well-documented new sample-return site improves the chronology.

Modern geological mapping

LRO imagery and topography, spectral datasets and USGS mapping combine morphology with composition and structure. Digital mapping lets researchers trace units across large regions and compare them with landing sites.

A good lunar map is therefore an interpretation built from multiple datasets, not just a labelled photograph.

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