What event led to the formation of the Mare Imbrium/Imbrium Basin on the Moon?

The formation of the Mare Imbrium, or Imbrium Basin, is attributed to a large impact event in which a planetesimal collided with the Moon. This collision was powerful enough to create a vast multi-ringed basin, which is characteristic of large impact craters. The impact excavated material and allowed for the subsequent processes that shaped the basin's features.

The significance of this impact lies not only in forming the basin itself but also in the geological history that followed. After the initial impact, lava flowed into the basin, contributing to its smooth, dark appearance, which characterizes the lunar maria. This type of structure is different from volcanic activity filling an existing crater or a series of smaller meteorite impacts reshaping the surface, as those processes do not specifically account for the extensive geological features and the size of the Mare Imbrium basin. Additionally, the idea that the Moon formed from debris left from the Earth pertains to a different period in the Moon's history, rather than the events that created Mare Imbrium.