I've noticed that geology and related fields use mega-annum (Ma) instead of mega-year (Myr) as the unit for 1 million years. Why is that? Is an "annum" different from a "year" in geology?
Both mega-annum(Ma) and mega-year(Myr) designate one million years. The distinction is that Ma is used to designate the time before present or geohistoric dates and Myr to designate a duration or span of time. So if you want to say a rock is dated to 10 million years before the present you would use 10 Ma, and if you want to say that a rock is 10 million years old you would use 10 Myr.
There has been debate among scientific disciplines to simply adopt Ma as the standard unit. However, the distinction that geologists have been using to distinguish between dates and durations for decades can best be illustrated as such:
No one objects to the storming of the Bastille on 14 July 1789 (a date) or to the construction of Stonehenge from 2600–1600 BC (an interval specified by two dates). In the case of the latter, we say that the job took 1000 years, not 1000 BC. The distinction between geohistorical dates and spans of geological time is conceptually analogous. There is no internal inconsistency, and the International System of Units (SI) rules don’t apply to dates in either case because points in time are not units, even if they are specified in years (Aubry et al., 2009). The year, moreover, is not a part of the SI. It cannot be a “derived unit of time,” the designation proposed by the task group, because under SI conventions “derived units are products of powers of base units” (BIPM, 2006). The base unit for time is the second. The task group is thus intent on fixing a problem that doesn’t exist and in a manner that is at odds with their stated goal of “adherence to SI rules.”
The above quote is from the Geological Time Conventions and Symbols manuscript from 2011. More detailed discussion can be found at this source. This Science Blogs article from 2009 has some interesting discussions regarding this issue.
annum (ka, Ma, Ga)
Time before present. A point of time in the geological past, measured backwards from now. Often, “now” is defined as the year 1950.
year (kyr, Myr, Gyr)
A measure of duration, not necessarily age and not necessarily measured from the present time.
- The fossil is dated to 110±2 Ma.
- According to zircon dating, the granite crystallised at ~30 Ma.
- The volcano was erupting intermittently for 10 kyr.
- The Jurassic period lasted for 56 Myr, starting at 201 Ma and 145 Ma.
- The Jurassic period lasted for 56 Myr, starting 201 Myr ago and ending 145 Myr ago.
Annum and year are not the exact same thing. The exact length of a year depends on what exactly you are measuring, whether distance between perihelions, distance between solstices, etc. It also varies because of the gravitation of other planets. These effects are minor, but enough that measuring units of years is tricky. So an annum is defined as 31,557,600 seconds exactly.