Myr
- This article is about the unit of time. For the Magic: the Gathering creatures, see Creatures of Magic: The Gathering.
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The symbol myr was formerly used in English-language geology, and remains as the standard usage in astronomy, as a unit of one million years. It is an abbreviation for 'million years' and lower case is used in geology, while upper case is used in astronomy.
In English-language technical literature in geology, the term 'Ma' is preferred, as this conforms to ISO 31-1 and NIST 811 recommended practices. "M" denotes mega- (106), and "a" denotes years (from the Latin annum. There is currently some debate on geological literature on the use of 'myr' versus 'Ma' to denote time before present.[1] The argument that having one abbreviation for a duration ('myr') and another for time before present ('Ma') has not been resolved. Note that "ma" would technically imply a milliannum (also called a milliyear in English usage), a period of a thousandth of a year or about 8 hours.
The correct ISO 31-1 usage is megaannum or Ma which unambiguously denotes a duration of 106 years. To denote a date one would add ago or bp to denote before present.
In non-SI usage, Ma was used to denote a specific number of millions of years ago, but it was not properly used to describe a duration, so: the Cretaceous started 145 Ma and ended 65 Ma, but it lasted for 80 myr (or 80 My). More often, the term "mya" (million years ago) is used in these contexts in geology.
[edit] See also
[edit] References
- ^ Mozley, Peter. "Discussion of GSA Time Unit Conventions". web page. Geological Society of America. http://www.geosociety.org/TimeUnits/. Retrieved 2012-01-07.
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