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The dark, relatively lightly cratered maria cover about 16% of the lunar surface and is concentrated on the
nearside of the Moon, mostly within impact basins. This concentration may be explained by the fact that the
Moon's center of mass is offset from its geometric center by about 2 kilometers (1.2 miles) in the direction
of Earth, probably because the crust is thicker on the farside. It is possible, therefore, that basalt magmas
rising from the interior reached the surface easily on the nearside, but encountered difficulty on the farside.
Mare rocks are basalt and most date from 3.8 to 3.1 billion years. Some fragments in highland breccias date to
4.3 billion years and high resolution photographs suggest some mare flows actually embay young craters and may
thus be as young as 1 billion years. The maria average only a few hundred meters in thickness but are so massive
they frequently deformed the crust underneath them which created fault-like depressions and raised ridges.
The relatively bright, heavily cratered highlands are called terrae. The craters and basins in the highlands are
formed by meteorite impact and are thus older than the maria, having accumulated more craters. The dominant rock
type in this region contain high contents of plagioclase feldspar (a mineral rich in calcium and aluminum) and are
a mixture of crustal fragments brecciated by meteorite impacts. Most terrae breccias are composed of still older
breccia fragments. Other terrae samples are fine-grained crystalline rocks formed by shock melting due to the high
pressures of an impact event. Nearly all of the highland breccias and impact melts formed about 4.0 to 3.8 billion
years ago. The intense bombardment began 4.6 billion years ago, which is the estimated time of the Moon's origin.
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