5.1.2 The Genesis Rock – a key find
The Genesis Rock was the first sample of early crustal material to be found on the Moon. It is an anorthosite, a rock composed almost entirely of the mineral anorthite, a type of plagioclase feldspar. As you saw in the previous step, anorthosite was predicted to have collected at the top of the magma ocean (low density), so its discovery was very significant in supporting the magma ocean hypothesis; it represents early crustal material that floated to the surface. We now know that the lunar highlands are mostly anorthositic in composition.