<P> The most distinctive aspect of the Moon is the contrast between its bright and dark zones . Lighter surfaces are the lunar highlands, which receive the name of terrae (singular terra, from the Latin for Earth), and the darker plains are called maria (singular mare, from the Latin for sea), after Johannes Kepler who introduced the name in the 17th century . The highlands are anorthositic in composition, whereas the maria are basaltic . The maria often coincide with the "lowlands," but it is important to note that the lowlands (such as within the South Pole - Aitken basin) are not always covered by maria . The highlands are older than the visible maria, and hence are more heavily cratered . </P> <P> The major products of volcanic processes on the Moon are evident to Earth - bound observers in the form of the lunar maria . These are large flows of basaltic lava that correspond to low - albedo surfaces covering nearly a third of the near side . Only a few percent of the farside has been affected by mare volcanism . Even before the Apollo missions confirmed it, most scientists already thought that the maria are lava - filled plains, because they have lava flow patterns and collapses attributed to lava tubes . </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> <Table> <Tr> <Td> Moon--Evidence of young lunar volcanism (October 12, 2014) </Td> </Tr> <Tr> <Td> Volcanic rilles near the crater Prinz </Td> </Tr> <Tr> <Td> Volcanic domes within the Mons Rümker complex </Td> </Tr> <Tr> <Td> Wrinkle ridges within the crater Letronne </Td> </Tr> <Tr> <Td> Rima Ariadaeus is a graben . NASA photo taken during Apollo 10 mission . </Td> </Tr> </Table>

Which rock unit on the surface of the moon is the youngest