Anorthosites, rocks composed almost entirely of plagioclase feldspar, are the oldest rocks on the Moon. They constitute the light-coloured areas of the Moon's surface. They appear to have formed when feldspar crystallised and floated to the top of a global magma ocean that surrounded the Moon soon after it formed.

Lunar anorthosiites are important for our understanding of the early history of the Moon because these rocks crystallized directly from the Moon's magma ocean, so they can be used to date the origin of the moon, about 4.4 billion years ago.