The Moon’s face is pockmarked with dark patches we’ve called maria—’seas’—since Galileo first squinted through his telescope in 1609. Turns out they’re not oceans at all but vast frozen lakes of basaltic lava, the remnants of volcanic eruptions that peaked around 3.5 billion years ago.
When Dead Worlds Turn Out to Have Had Fiery Adolescences
Here’s the thing: we used to think the Moon was geologically dead on arrival, a cold rock that never had much going on. Then Apollo 11 landed in 1969, and the samples told a completely different story. The basalts brought back were volcanic, plain and simple. Some dated to 3.9 billion years ago, others to a relatively recent 3.1 billion years—which in lunar terms is practically yesterday.
The Moon had fire.
Between roughly 4 billion and 1 billion years ago, vast quantities of magma welled up through the lunar crust, flooding impact basins with flows that stretched for hundreds of kilometers. Mare Imbrium alone covers about 750,000 square kilometers. That’s bigger than Texas, if Texas were made of solidified lava on an airless rock hurtling through space.
The Mystery of Why a Small World Got So Angry
Small bodies cool fast. That’s planetary science 101. The Moon’s radius is only 1,737 kilometers—roughly a quarter of Earth’s—so it should have frozen solid within a few hundred million years of formation. Yet volcanic activity persisted for bilions of years, maybe even as recently as 100 million years ago based on some crater counts from Chang’e-5 samples returned in 2020.
Wait—maybe the Moon had help staying warm. Radioactive decay of elements like uranium, thorium, and potassium generates heat. The KREEP terrane—named for its enrichment in potassium (K), rare earth elements (REE), and phosphorus (P)—might have acted like a geologic heating blanket, keeping parts of the interior molten long after they should have solidified.
Or tidal heating from Earth played a role when the Moon orbited much closer than its current 384,400 kilometers.
Volcanic Ghosts That Haunt the Surface Today
The maria aren’t the only evidence. In 2014, researchers identified over 70 irregular mare patches—areas that looked surprisingly young, maybe only tens of millions of years old. These features don’t fit the standard timeline. They’re too fresh, too unmarked by impacts. Something weird happend there.
Then there are the sinuous rilles, snaking channels carved by flowing lava. Vallis Schröteri in Oceanus Procellarum stretches 168 kilometers long and reaches widths of 10 kilometers. That’s a river of molten rock that would dwarf any terrestrial lava channel. Imagine standing on the edge watching incandescent stone flow past at maybe 10 meters per second, glowing orange against the black sky.
Volcanic domes and cones dot the landscape too—shield volcanoes built by repeated eruptions. Mons Rümker rises about 1,100 meters above the surrounding plains, a testament to localized volcanic construction that took place over extended periods.
What This Means for Lunar Geology and Maybe Future Settlers
The volcanic history matters for more than academic curiosity. Those ancient lava flows created relatively smooth surfaces—ideal for landing spacecraft or building lunar bases. The maria also contain titanium-rich minerals, potentially valuable resources if humans ever decide to actually stay on the Moon rather than just visit for photo ops.
But more fundamentally, the Moon’s volcanic past reveals how planetary bodies evolve. Small doesn’t necessarily mean boring. Even after losing most of its primordial heat, the Moon found ways to stay geologically active for billions of years through a combination of compositional quirks and maybe gravitational massaging from Earth.
We’ve sent orbiters, landers, rovers, and humans to investigate these frozen lava fields. Each mission adds another piece to the puzzle. China’s Chang’e missions have focused on younger volcanic regions, trying to pin down exactly when the fires finally went out. NASA’s Lunar Reconnaissance Orbiter maps the surface in exquisite detail, revealing features that hint at volcanic activity far more complex than we initially imagined.
The Moon isn’t dead—it’s just resting after a very dramatic youth spent spewing molten rock across its surface while Earth watched from nearby.
