- Pre-Nectarian
The Pre-Nectarian Period of the
lunar geologic timescale runs from 4533 million years ago (the time of the initial formation of theMoon ) to 3920 million years ago, when theNectaris Basin was formed by a large impact. It is followed by theNectarian period. Pre-Nectarian rocks are rare in the lunar sample suite; they are mostly composed of lunar highlands material which have been heavily churned, brecciated, and thermally affected by subsequent impacts, particularly during theLate Heavy Bombardment that marks the approximate beginning of theNectarian Period. The primary Pre-Nectarian lunar highland material is dominated by the rock typeanorthosite , which suggests that the early stage of lunar crustal formation occurred via mineral crystallization of a globalmagma ocean.This geologic period has been informally subdivided into the Cryptic Era (4.533 - 4.172 Ga ago) and Basin Groups 1-9 (4.172 - 3.92 Ga ago),cite book | title = [http://ser.sese.asu.edu/GHM/ Geologic History of the Moon] | last = Don Wilhelms | publisher = U.S. Geological Survey Professional Paper 1348 | year = 1987] but these divisions are not used on any geologic maps.
Relationship to Earth's geologic time scale
Since little or no geological evidence on
Earth exists from the time spanned by the Pre-Nectarian period of the Moon, the Pre-Nectarian has been used as a guide by at least one notable scientific work [cite book | title = A Geologic time scale 1989 | publisher = Cambridge University Press | year = 1990 | last = W. Harland, R. Armstrong, A. Cox, L. Craig, A. Smith, D. Smith] to subdivide the unofficial terrestrialHadean eon. In particular, it is sometimes found that the Hadean eon is subdivided into theCryptic era ,Basin Groups 1-9,Nectarian andLower Imbrian , though the first two of these lunar divisions are informal and collectively make up the Pre-Nectarian.References
External links
*http://www.palaeos.org/Hadean
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