<P> The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting . The oldest radiometric ages are about 4.2 Ga , whereas the youngest ages determined from crater counting are about 1 Ga ( 1 Ga = 1 billion years ) . Volumetrically , most of the mare formed between about 3 and 3.5 Ga before present . The youngest lavas erupted within Oceanus Procellarum , whereas some of the oldest appear to be located on the farside . The maria are clearly younger than the surrounding highlands given their lower density of impact craters . </P>
<P> The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting . The oldest radiometric ages are about 4.2 Ga , whereas the youngest ages determined from crater counting are about 1 Ga ( 1 Ga = 1 billion years ) . Volumetrically , most of the mare formed between about 3 and 3.5 Ga before present . The youngest lavas erupted within Oceanus Procellarum , whereas some of the oldest appear to be located on the farside . The maria are clearly younger than the surrounding highlands given their lower density of impact craters . </P>
which rock unit on the surface of the moon is the youngest
which rock unit on the surface of the moon is the youngest
mare basalts
mare basalts