Myr
Encyclopedia
The symbol myr was formerly used in English-language geology
, and remains as the standard usage in astronomy
, as a unit of one million years.
It is an abbreviation for 'million years' and lower case is used in geology, while upper case is used in astronomy.
In English-language technical literature in geology, the term 'Ma' is preferred, as this conforms to ISO 31-1
and NIST 811 recommended practices. "M" denotes mega- (106), and "a" denotes years (from the Latin annum. Note that "ma" would technically imply a milliannum (also called a milliyear in English usage), a period of a thousandth of a year or about 8 hours.
The correct ISO 31-1 usage is megaannum or Ma which unambiguously denotes a duration of 106 years. To denote a date one would add ago or bp to denote before present
.
In non-SI usage, Ma was used to denote a specific number of millions of years ago, but it was not properly used to describe a duration, so: the Cretaceous started 145 Ma and ended 65 Ma, but it lasted for 80 myr (or 80 My). More often, the term "mya" (million years ago) is used in these contexts in geology.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, and remains as the standard usage in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, as a unit of one million years.
It is an abbreviation for 'million years' and lower case is used in geology, while upper case is used in astronomy.
In English-language technical literature in geology, the term 'Ma' is preferred, as this conforms to ISO 31-1
ISO 31-1
ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to space and time.Its definitions include:Annex A of ISO 31-1 lists units of space and time based on the foot, pound and second...
and NIST 811 recommended practices. "M" denotes mega- (106), and "a" denotes years (from the Latin annum. Note that "ma" would technically imply a milliannum (also called a milliyear in English usage), a period of a thousandth of a year or about 8 hours.
The correct ISO 31-1 usage is megaannum or Ma which unambiguously denotes a duration of 106 years. To denote a date one would add ago or bp to denote before present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
.
In non-SI usage, Ma was used to denote a specific number of millions of years ago, but it was not properly used to describe a duration, so: the Cretaceous started 145 Ma and ended 65 Ma, but it lasted for 80 myr (or 80 My). More often, the term "mya" (million years ago) is used in these contexts in geology.