Lunar mare
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The Lunar maria (singular: mare, pronounced MAH-ray) are large, dark, basaltic plains on Earth's Moon, formed by ancient basaltic flood eruptions caused by extremely large meteoroid impacts. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the 'highlands' or mountains which are older and have had a soil or 'Regolith' created by the impact of micro-meteoroids over hundreds of millions of years. The regolith is more reflective than the basalt of the maria. The maria cover 16% of the lunar surface, mostly on the near-side visible from Earth. The few maria on the far-side are much smaller, being mostly very large craters where a small amount of flooding occurred.
This bias of distribution is thought to have assisted in the 'phase-locking' of the Moon's rotation to its orbit. (The phenomenon whereby the Moon spins on its axis in the same timespan as it takes to orbit the Earth) This results in only one side of the Moon being visible from the Earth (excepting some small leeway from libration). The reason the maria have assisted in phase-locking is that they are denser than much of the rest of the surface and are therefore more strongly attracted towards the Earth by gravity. Over millennia, the Moon's rotation has slowed so that the heaviest side of the Moon with the maria on it is constantly towards the Earth.
Most of the volcanic eruptions that produced the maria occurred at many points along fissures around the edges of the impact basins. The basaltic lava usually flowed out in several distinct episodes over the course of many thousands of years, well after the initial formation of the basins themselves. Although these are relatively flat surfaces, during their contraction phase they can produce low, blister-like formations known as wrinkle-ridges.
The traditional nomenclature for the Moon also includes one "oceanus" (ocean), as well as features with the names "lacus" (lake), "palus" (marsh) and "sinus" (bay). The latter three are smaller than maria, but have the same nature and characteristics.