Most of the Moon's craters were formed by volcanic activity.

Through a telescope, lunar craters could plausibly be read as volcanic features. Earth has circular volcanic calderas, and the Moon’s dark maria clearly involved ancient lava flows. For much of the nineteenth and early twentieth centuries, volcanic and impact explanations competed.

Impact cratering was difficult to evaluate because geologists had few well-understood terrestrial examples and little experimental experience with hypervelocity collisions. As impact physics developed, crater shapes, ejecta patterns, and the enormous energy available from high-speed projectiles increasingly favored an external origin.

Apollo provided physical samples. Lunar rocks contained shock effects produced by pressures far beyond ordinary volcanic processes, giving direct evidence that violent impacts had excavated the cratered surface.

The Moon is not a world without volcanism. Vast basaltic eruptions filled enormous ancient impact basins and created the maria, and some smaller depressions are volcanic.

The overturned claim is about the dominant crater population. Most ordinary lunar craters are scars left by asteroids and comets, while volcanism primarily modified other parts of the landscape.