The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria
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".
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, because of 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 because of 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.
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. Read more...