Chips Off an Old Lava Flow Lava flows with compositions like those of mare basalts began to form before those that occupy the visible maria Now hidden beneath the ejecta from impact basins, these ancient lavas are called cryptomaria Evidence for their existence comes from analysis of samples (Apollo and meteorites) and remote sensing studies

Chips Off an Old Lava Flow Cryptomaria are revealed when craters punch through overlying crater ejecta to make dark-haloed craters Ancient ages of mare basalts in highland breccias also show that mare-like volcanism operated long before 3.8 billion years ago Rock on right is 4.23 billion years old

Chips Off an Old Lava Flow Isotopic measurements by Kentaro Terada et al., using an ion microprobe on phosphate grains in a basalt fragment in Kalahari 009, were used to date the time of crystallization Phosphates contain considerable uranium, so U-Pb dating is feasible Three ways of plotting the data are shown Diagram on right is a projection from the 3-dimensional space containing the other two and gives the best estimate of the age of the clast, 4.35 ± 0.15 billion years

Crystallization ages of the ferroan anorthosite suite of lunar rocks indicate formation in a magma ocean 4.46 billion years ago Formation of the other main rock type in the highlands, the Mg-suite, began after formation of the anorthosite crust and continued until about 3.9 billion years ago Sample studies like that done by Terada et al. show that basalt formation overlapped that of Mg-suite magmatism, beginning at least 4.35 billion years ago Remote sensing observations indicate basalt formation ceased about a billion years ago, but peaked during the billion years time interval Chips Off an Old Lava Flow