The bright parts of the Moon are made of a rock that floats. Early on the whole surface was molten, and the light crystals rose to the top and set there.

Apollo 11 came back with white anorthosite fragments that had no business being in a basalt plain, and the explanation was a magma ocean hundreds of kilometres deep. Feldspar crystallising out of it was buoyant enough to float; the denser minerals sank. The Moon's whole crust is that scum, four billion years cold.