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Ma vs. M.Y. (was 1st Inland sea incursion)

On Sun, 3 Jan 1999, Patrick Norton wrote:

> I ask because I do not know: isn't the abbreviation Ma (for Mega annum) used
> as an absolute temporal reference, as in "20 million years ago"--rather than
> a relative reference, as in "it took 20 million years to accomplish?"

My understanding is also that the abbreviation "Ma" (and "ka" and "Ga")
refers to a given radioisotopic date on the geochronologic scale, whereas
M.Y. (often spelled "My") corresponds to a duration of time. Hence, for
example, one should not say "the Cretaceous lasted 80 Ma" but rather
"the Cretaceous lasted 80 M.Y.".
The glossary in Woodburne's Cenozoic mammals of North America (1987) gives
the following definitions for each abbreviation:

        Ma: Megannum; one million years in the radioisotopic time scale.
            Thus, for example, 10 Ma refers to the ten-million-year level
            of that time scale.

        M.Y. (or m.y.): A segment of geologic time one million years in
                        duration, or something that happened ten million
                        years ago (without reference to a given point or
                        set of points on the radioisotopic time scale).

Someone trained in the earth sciences might want to confirm (or infirm)
the above.

Hope this is of help,

Michel Chartier