occurred quickly (in the order of thousands of years), whereas the majority of mare basalts erupted about one billion years later. The reason that the mare basalts are predominantly located on the near-side hemisphere of the Moon is still being debated by the scientific community. Based on data obtained from the Lunar Prospector mission, it appears that a large proportion of the Moon's inventory of heat producing elements (in the form of KREEP) is located within the regions of Oceanus Procellarum and the Imbrium basin, a unique geochemical province now referred to as the Procellarum KREEP Terrane. While the enhancement in heat production within the Procellarum KREEP Terrane is most certainly related to the longevity and intensity of volcanism found there, the mechanism by which KREEP became concentrated within this region is not agreed upon. Chemical composition Using terrestrial classification schemes, all mare basalts are classified as tholeiitic, but specific subclassifications have been invented to further describe the population of lunar basalts. Mare basalts are generally grouped into three series based on their major element chemistry: high-Ti basalts, low-Ti basalts, and very-low-Ti (VLT) basalts. While these groups were once thought to be distinct based on the Apollo samples, global remote sensing data from the Clementine mission now shows that there is a continuum of titanium concentrations between these end members, and that the high-titanium concentrations are the least abundant. TiO2 abundances can reach up to 15 wt.% for mare basalts, whereas most terrestrial basalts have abundances much less than 4 wt.%. A special group of lunar basalts is the KREEP basalts, which are abnormally rich in potassium (K), rare-earth elements (REE), and phosphorus (P). A major difference between terrestrial and lunar basalts is the near-total absence of water in any form in the lunar basalts. Lunar basalts do not contain hydrogen-bearing minerals like the amphiboles and phyllosilicates that are common in terrestrial basalts due to alteration or metamorphism. See also References Further reading Paul D. Spudis, The Once and Future Moon, Smithsonian Institution Press, 1996, . External links Google Moon Lunar and Planetary Institute: Exploring the Moon Lunar and Planetary Institute: Lunar Atlases Ralph Aeschliman Planetary Cartography and Graphics: Lunar Maps Moon articles in Planetary Science Research Discoveries Cryptomare formations Mare