The lunar maria, those large dark patches visible on the Moon’s surface, formed through a two-stage process: massive asteroid impacts punched enormous basins into the crust, and then volcanic lava welled up from deep inside the Moon to fill them. Most of this volcanic flooding happened between 3.1 and 3.9 billion years ago, though some eruptions continued until roughly 2 billion years ago. The result is the distinctive face of the Moon we see today.
Why They’re Called “Seas”
Early astronomers looking through telescopes saw these dark, smooth regions and assumed they were bodies of water. The name stuck. In 1651, Italian astronomer Giovanni Battista Riccioli published a lunar map that gave individual maria the Latin names still used today: Mare Tranquillitatis (Sea of Tranquility), Oceanus Procellarum (Ocean of Storms), and dozens more. Riccioli reportedly joked that he had “flung the heliocentrists into the Sea of Storms.” The Apollo astronauts who walked on the Moon in 1969 landed on Mare Tranquillitatis, a name coined over three centuries earlier.
Giant Impacts Created the Basins
The story starts with collisions. During the Moon’s first billion years, enormous meteorites slammed into the surface and excavated wide, deep depressions called impact basins. These basins, some hundreds of miles across, formed the “containers” that lava would later fill. Without these impacts, there would be no maria. The basins themselves are older than the dark rock that fills them, sometimes by hundreds of millions of years.
Volcanic Lava Filled Them In
Deep beneath the lunar surface, heat from radioactive decay melted rock in the Moon’s mantle. That molten rock, or magma, rose through cracks in the crust and flooded the impact basins. Recent research published in Science Advances traced the source of this magma to depths of roughly 60 to 120 kilometers below the surface. The lava that poured out was basalt, a dense, iron-rich volcanic rock similar to what forms ocean floors on Earth and erupts from Hawaiian volcanoes.
This wasn’t a single catastrophic event. Lava flowed in repeated episodes over hundreds of millions of years, building up layers of basalt. The oldest known mare basalt from a visible maria is an Apollo sample from Mare Tranquillitatis dated to 3.86 billion years ago. The youngest Apollo sample is 3.08 billion years old. But the story doesn’t end there. China’s Chang’e-5 mission returned samples in 2020 that were radiometrically dated to just 1.96 billion years ago, proving that volcanic activity on the Moon persisted far longer than the Apollo-era samples suggested.
What Mare Basalt Is Made Of
Mare basalt is notably rich in iron and titanium compared to the lighter-colored highland rocks that make up most of the Moon’s surface. This is why the maria appear dark. The titanium content varies significantly from one mare to another. High-titanium basalts, found especially in Mare Tranquillitatis, Mare Imbrium, and Oceanus Procellarum, contain titanium oxide concentrations above 8%. In some spots, the titanium-bearing mineral ilmenite makes up as much as 20% of the rock by weight. Other maria have much lower titanium levels, below 3%. Ilmenite itself is roughly one-third titanium and one-third iron by weight, making these deposits a potential resource for future lunar mining.
Why Maria Cover Only One Side
One of the Moon’s biggest puzzles is that the maria are almost entirely on the near side, the face permanently turned toward Earth. About 15% of the near side is covered by mare basalt, compared to just 1% of the far side. The main explanation involves the Moon’s lopsided crust: the far side’s crust is significantly thicker, and that thick layer of low-density rock acted as a barrier, preventing dense basaltic magma from reaching the surface.
But this explanation has a complication. The far side hosts the South Pole-Aitken basin, the largest and deepest impact basin on the Moon, where the crust is extremely thin. If thin crust were the only factor, this basin should be full of mare basalt. It isn’t. A 2024 study in Geophysical Research Letters proposed a solution: the same giant impact that created the South Pole-Aitken basin also stripped away the insulating crustal layer above it. Without that insulation, the underlying rock cooled and solidified faster than elsewhere on the Moon. This rapidly thickening layer of cold, rigid rock blocked magma from rising to the surface, shutting down volcanism in the basin long before the near side’s eruptions ended.
Surface Features Within the Maria
The maria aren’t perfectly flat. As the lava cooled and the Moon slowly shrank over billions of years, the surface buckled and cracked, producing distinctive landforms. Wrinkle ridges are the most prominent: curved hills that snake across the mare surface for miles, pushed up by compressive forces as the ground contracted. The longest wrinkle ridges stretch about 250 miles and rise as high as 1,000 feet. NASA’s Lunar Reconnaissance Orbiter has also revealed lobate scarps (small curved cliffs) and graben (shallow trenches) created by the same tectonic forces. Some of these features appear geologically recent, suggesting the Moon is still contracting and shifting today.
Sinuous rilles, which look like dried river channels, wind across parts of the maria as well. These were carved by flowing lava rather than water, marking the paths of ancient volcanic rivers or collapsed lava tubes. Together, these features tell the story of a surface shaped not just by impacts and flooding, but by billions of years of slow cooling and contraction.