NASA :

The Moon presently has very slight seismic activity and little heat flow, indications that most internal activity ceased long ago. It is now known that billions of years ago the Moon was subject to violent heating - resulting in a differentiated crust - followed later by volcanic outpourings of lava.

Once the huge mare lava eruptions had diminished, apparently the Moon’s heat source had run down. The last few billion years of its history have been calm and essentially geologically inactive except for the continuing rain of impacts. Scientists now believe that the Moon formed as a result of a collision between the early Earth and a former, smaller planet about 4.6 billion years ago. The giant impact sprayed vaporized material into a disk that orbited the Earth: This vapor later cooled into droplets that coagulated into the Moon.

About 4 billion years ago, a series of major impacts occurred, forming huge craters. These craters are now the sites of basins called maria (e.g., Mare Imbrium, Mare Serenitatis). Between 4 and 2.5 billion years ago, volcanic activity filled these basins with dark-colored lavas, called basalts. After this time of volcanism, the Moon cooled down, and has since been relatively inactive, except for the occasional “hits” of meteorites and comets.