The Later Development of the Moon

The newborn moon was at first covered by magma (molten rock). This feature, which geologists call the magma ocean, was at least a few hundred kilometers deep. The magma ocean was created by heat from the many large final impacts of moonlets on the by-then largely formed moon. Liquid iron sank to the moon's center, and electrical currents in the molten core generated a magnetic field. As the magma ocean cooled, about 4 billion years ago, it solidified. Heavier minerals sank, while lighter ones rose to form a crust.
Even as the lunar surface hardened, it was being peppered by meteoroids, asteroids, and comets. The largest of these objects produced huge basins up to 2,500 kilometers (1,600 miles) across. Later, heat released by radioactivity deep inside the moon caused magma to well up from the interior, partially filling and leveling many of the basins.

Bodies large enough to carve out basins stopped striking the moon by about 3.2 billion years ago, but smaller objects continued to hit the surface, forming many craters. At the centers of some craters the rebound of surface material after the impact created a mountain. The effects of impacts also produced mountain chains at the boundaries of many basins and craters. No lunar mountains were formed by the folding and upthrust of surface layers, as occurs to form many mountains on Earth. Nor did any large volcanic mountains develop on the moon.

The countless impacts of meteoroids slowly fragmented the lunar surface. This created a regolith (surface layer) of broken rocks and soil particles as deep as 15 meters (50 feet) in some areas.