Million years ago, abbreviated asMya,Myr (megayear), orMa (megaannum), is aunit of time equal to1,000,000 years (i.e.1×106 years), or approximately 31.6teraseconds.
Myr is in common use in fields such asEarth science andcosmology. 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 ayear numbering system that istime before the present.
Myr is deprecated ingeology, but inastronomyMyr 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).
In geology, a debate remains open concerning the use ofMyr (duration) plusMya (million years ago) versus using only the termMa.[1][2] In either case, the termMa is used in geology literature conforming toISO 31-1 (nowISO 80000-3) and NIST 811 recommended practices. Traditional style geology literature is written:
The Cretaceous started 145 Ma and ended 66 Ma, lasting for 79 Myr.
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 havingmyr for a duration andMya for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply amilliyear (a thousandth of a year, or 8 hours). On this side of the debate, one avoidsmyr and simply addsago explicitly (or addsBP), as in:
The Cretaceous started 145 Ma ago and ended 66 Ma ago, lasting for 79 Ma.
In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".
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:
Since it is often clear from the context that the time must be in the past,mya is often simply replaced byMa.[3] 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 abbreviationmya 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 asmya.
