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Megayear and Gigayear: Two Units of Geological Time


FOR measuring geological time and for indicating the age of minerals and rocks before the Quaternary, geologists are at present using two time units, namely, one million years (106 years) and one “billion” years (109 years). The abbreviations of these units are indeed variable. A check of a number of recently published books, monographs, and symposium volumes1 produced the following expressions and abbreviations: million years, millions of years, years × 106, m.y., m.yr., m.yrs., Myr, 106 a, 106 y, 106 yr, 106 years; and billion years, b.y., 109 a, 109 y, 109 yr, 109 yrs, and 109 years. Such abbreviations are also current in meteorite chronology. In German publications2, Mio Jahre, and in French works3, MA, M.A., m.a., and mA for a million years are customarily used.


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RANKAMA, K. Megayear and Gigayear: Two Units of Geological Time. Nature 214, 634 (1967).

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