An older theory said the moon split from the Pacific Ocean area; thus leaving thinner crust there (and hence, the Ring of Fire, plus lots of volcanic action forming the Pacific islands).
Also, the side of the moon facing us is reflective stone (granite?) which is also allegedly missing from the crust area beneath the Pacific (sorry if I got the nature of the stone wrong). The back side of the moon, which never faces us, is not the same reflective material.
I believe the moon is mostly basalt.