Although the terrae are highly modified, they are composed of rock material that formed very early in the Moon's history by the process of magmatic differentiation. By this process, minerals formed within an igneous melt become segregated according to differences in their physical properties. Lighter materials rise to the top of the magma body by virtue of their lower specific gravity, and, after solidification, form low density rocks. Among the returned lunar samples thought to represent terra materials not completely altered by subsequent events, varieties of gabbroic anorthosite are the most common. This type of rock is composed largely of plagioclase with varying amounts of olivine and pyroxene.
Plagioclase is a common mineral on Earth and one of rather low specific gravity.
The preponderance of anorthositic rocks in the lunar highlands is supported by data from Apollo remote sensing instruments. Some of the chemical differences between anorthositic and basaltic rocks could be determined by the X ray fluorescence and gamma ray experiments of Apollos 15 and 16. The X ray fluorescence results show a higher ratio of aluminum to silicon in the terrae than in the maria, corresponding to the known chemical difference between anorthositic and basaltic rocks. This also is consistent with the chemical compositions of anorthositic versus basaltic rocks. The lower specific gravity of anorthositic rocks compared to basalts is another characteristic that was measured directly or indirectly by orbital experiments.