Unit of time equal to 1,000,000 years
Million years ago
, abbreviated as
Mya
,
Myr
(megayear) or
Ma
(megaannum), is a
unit of time
equal to
1,000,000 years
(i.e.
1
×
10
6
years), or approximately 31.6
teraseconds
.
Usage
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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 place 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
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In geology, a debate remains open concerning the use of
Myr
(duration) plus
Ma
(million years ago) versus using only the term
Ma
.
[1]
[2]
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 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 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
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".
See also
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References
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