The outer several hundred kilometres of the lunar mantle became molten yielding a Lunar Magma Ocean (LMO) that began to cool and differentiate like a thick intrusion of mafic magma. Dense ferromagnesian minerals, such as olivine and pyroxene, crystallised and sank to the deeper mantle. Less dense calcium plagioclase feldspar minerals accumulated into anorthosite masses (along with minor amounts of ferromagnesian minerals) that rose and concentrated near the surface. These masses formed the early lunar crust of which the highland regions are made.