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TITLE: Geologic History of the Moon

The Moon’s special properties hint that it formed in an unusual way. Compared with other moons and their planets, the Moon is unusually large in relation to its primary body, Earth. Also, the Moon is largely devoid of water and elements with low boiling points a clue that high temperatures were involved in its formation. Theories about the Moon’s origins have been plentiful. But lunar rocks brought back by Apollo astronauts finally provided solid evidence of the Moon’s extraordinary history.

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The Birth of the Moon
Modern observations suggest that a massive, Mars-sized object had a glancing collision with a very young Earth. Earth’s core was mostly untouched. But the impact vaporized vast amounts of rock from the mantles of both bodies and sent the vapor into orbit around Earth. It condensed and accumulated into a hot, partly molten Moon.


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Magma Ocean – 4.5-4.4 Billion Years Ago
The infant moon was covered with a deep layer of molten rock, or magma. As this ocean of magma gradually cooled, crystals of low-density feldspar floated to the surface. Denser minerals such as olivine and pyroxene sank. Eventually, giant rafts of feldspar covered much of the Moon's surface, forming the first lunar crust. The lighter areas that we see on the Moon today the lunar highlands are remnants of this early crust.


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The Big Barrage – 4-3.9 Billion Years Ago
Continuous pummeling by rocky bodies from space dominated the Moon’s earliest history. The impacts riddled the lunar surface with craters. Small craters formed inside large ones. Large craters obliterated small ones. Then, about 3.9 billion years ago, a blitz of enormous collisions created today’s gigantic lunar basins a few bigger than Texas. Some of the inner planets also show evidence of this devastating bombardment.


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Age of Volcanism – 4.2-3.1 Billion Years Ago
Just before the era of most intense lunar bombardment, a time of abundant volcanism began. It continued for nearly a billion years. Molten rock welled up from the MoonÕs hot interior and filled the huge basins left by impacts. When the lava cooled, it formed a dark rock called basalt that gives many basins their familiar dark appearance. Galileo and other early astronomers erroneously believed that these low regions were oceans and called them maria, meaning “seas.”


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The Tumult Wanes – 3.1 Billion Years Ago - Now
Eventually, the Moon’s mantle cooled to the point that lava no longer reached the surface. Today large collisions are rare. But a steady stream of small meteoroids still collides with the Moon. Like garden tillers, they have churned up the surface, turning it into a jumble of powder and rock chips 5-20 m (15-65 ft) deep. This lunar "soil" is called the regolith. Heat from later impacts has welded some of this fine debris into new rocks known as regolith breccias. With the exception of this continuous barrage of microimpactors, and an occasional large impact, geologic activity on the Moon has all but ceased.

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits