<P> The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria (Latin for "seas"; singular mare), as they were once believed to be filled with water; they are now known to be vast solidified pools of ancient basaltic lava . Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water . 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". </P> <P> Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side, compared with 2% of the far side . 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 . 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 . Until recently, the youngest eruptions, dated by crater counting, appeared to have been only 1.2 billion years ago . In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust - free features that, due to the lack of erosion by infalling debris, appeared to be only 2 million years old . Moonquakes and releases of gas also indicate some continued lunar activity . In 2014 NASA announced "widespread evidence of young lunar volcanism" at 70 irregular mare patches identified by the Lunar Reconnaissance Orbiter, some less than 50 million years old . This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer due to the greater concentration of radioactive elements . Just prior to this, evidence has been presented for 2--10 million years younger basaltic volcanism inside Lowell crater, Orientale basin, located in the transition zone between the near and far sides of the Moon . An initially hotter mantle and / or local enrichment of heat - producing elements in the mantle could be responsible for prolonged activities also on the far side in the Orientale basin . </P> <P> The lighter - coloured regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria . They have been radiometrically dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean . In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events . </P> <P> The concentration of maria on the Near Side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow - velocity impact of a second moon of Earth a few tens of millions of years after their formation . </P>

What are the light areas on the moon called