Million years ago

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 year numbering system 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 BP), as in: In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".

Conflicts with the International System of Units and possible solutions

The abbreviation mya does not comply with the International System of Units (SI) in three respects, each of which can be attributed to one of its letters:

1. (million): Neither t (in tya) nor b (in bya) are internationally recognized prefixes for units of measurement. The lowercase m (in mya) stands for milli, i.e., one thousandth, in the International System of Units (SI) and is therefore also not SI-compliant. It is therefore an attempt to approximate the International System of Units by using kya (kilo years ago) instead of tya and Gya (giga years ago) instead of bya.
2. (year): y is not a coherent SI unit. - Instead of y or yr (for year), the derived unit a (lat. annus, year) can be used, which is a multiple of the SI base unit second. This leads to the following formulations:
3. 1 tya = “1,000 years ago” = “1 ka ago”
4. 1 mya = “1,000,000 years ago” = “1 Ma ago”; alternatively, bma for “before million annos,” or 'bMA' for “before Mega annos”
5. 1 bya = "1.000.000.000 years ago" = "1 ga ago"
6. Unlike units of time, this scale does not refer to a period of time, but to a point in time in the past. Since it is often clear from the context that the time must be in the past, mya is often simply replaced by Ma. Physically, this means that a point in time is replaced by a period of time, but this does not necessarily pose a problem if a definition (such as the ISO 8601 standard for date notation) is agreed upon. This makes it clear why the abbreviation mya is still so popular: it stands for the phrase “so many years ago,” which makes it clear even without a definition that it must refer to a point in time so many years ago in the past. If it is not clear from the context that it refers to the past, when replacing mya with Ma, a bp (for "before present") is added to the latter. This also makes it clear that it refers to a point in time and not a period of time. However, Ma bp does not necessarily mean exactly the same as mya.

References

References

1. Mozley, Peter. "Discussion of GSA Time Unit Conventions". [[Geological Society of America]].
2. Biever, Celeste. (27 April 2011). "Push to define year sparks time war". [[New Scientist]].
3. {{Internet source. link