Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds.
Usage
Myr is in common use in fields such as
Earth science and
cosmology. Myr is also used with Mya or Ma. Together they make a reference system, one to a quantity, the other to a particular point in a
calendar era that is
time before the present.
Myr is deprecated in geology, but in astronomy Myr is standard. Where "myr" is seen in geology, it is usually "Myr" (a unit of mega-years). In astronomy, it is usually "Myr" (Million years).
Debate
In geology, a debate remains open concerning the use of
Myr (duration) plus
Mya (million years ago) versus using only the term
Ma.
In either case, the term
Ma is used in geology literature conforming to ISO 31-1 (now ISO 80000-3) and NIST 811 recommended practices. Traditional style geology literature is written:
The "ago" is implied, so that any such year number "X Ma" between 66 and 145 is "Cretaceous", for good reason. But the counter argument is that having
myr for a duration and
Mya for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply a
milliyear (a thousandth of a year, or 8 hours). On this side of the debate, one avoids
myr and simply adds
ago explicitly (or adds
Before Present), as in:
In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".
See also
-
Billion years ago
-
Kyr
-
Megaannum (Ma)
-
Symbols y and yr
