Volcanic features Main article: Lunar mare The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called maria (Latin for "seas"; singular mare), since they were believed by ancient astronomers to be filled with water. They are now known to be vast solidified pools of ancient basaltic lava. While similar to terrestrial basalts, the mare basalts have much higher abundances of iron and are completely lacking in minerals altered by water.[38][39] The majority of these lavas erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side maria.[40] Maria are found almost exclusively on the near side of the Moon, covering 31% of the surface on the near side,[41] compared with a few scattered patches on the far side covering only 2%.[42] This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.[26][43][44] Most of the Moon's mare basalts erupted during the Imbrian period, 3.0–3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years,[45] and the youngest eruptions, dated by crater counting, appear to have been only 1.2 billion years ago.[46] Απανταει σε μερικά από τα ερωτήματα για τη μορφολογία.