Learn How to Talk About Geologic Time in 'Ma' or 'Myr'

davida3/Flickr/CC BY-ND 2.0

Geologists have a bit of awkwardness in their language in talking about the deep past: distinguishing dates in the past from durations or ages. Ordinary people don't have a problem with the weirdness of historical time—in 2017, we can easily say that an event in 200 BCE happened 2216 years ago and that an object made back then is 2216 years old today. (Remember, there was no year 0.)

But geologists have a need to separate out the two types of time with different abbreviations or symbols, and there is a debate about establishing a standard way of expressing it. A widespread practice has arisen in the last few decades that gives dates (not ages) in the format "X Ma" (x million years ago); for example, rocks that formed 5 million years ago are said to date from 5 Ma. "5 Ma" is a point in time that is 5 million years from the present.

And instead of saying that a rock is "5 Ma old," geologists use a different abbreviation, such as m.y., mya, myr, or Myr (all of which stand for millions of years, in reference to age or duration). This is a little awkward, but the context makes things clear.

Agreeing on a Definition for Ma

Some scientists see no need for two different symbols or abbreviations, as something formed 5 million years before the present would indeed be 5 million years old. They are in favor of one system or set of symbols for all sciences, from geology and chemistry to astrophysics and nuclear physics. They wish to use Ma for both, which has caused some concern from geologists, who want to make the distinction and view it as unnecessarily confusing going forward to have Ma apply to both.

Recently the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Geological Sciences (IUGS) convened a task force to decide on an official definition of the year to go into the Système International or SI, the "metric system." The exact definition isn't important here, but the symbol they chose, "a," (for the Latin annus, which translates to "year") would override geological custom by requiring everyone to use "Ma" for millions of years ago, "ka" for thousands of years ago, and Ga for billions of years ago, etc. everywhere. That would make writing geology papers somewhat harder, but we could adjust.

But Nicholas Christie-Blick of Columbia University has looked more deeply at the proposal and cried foul in GSA Today. He raised an important question: How can SI accommodate the year as a "derived unit" when SI rules require that these must be simple powers of base units? The metric system is for physical quantities and measurable distances, not time: "points in time are not units." There's no room in the rules for a derived unit called the year, which would be defined as 31,556,925.445 s. Derived units are things like the gram (10 -3 kg). If this were a legal dispute, Christie-Blick would be arguing that the year has no standing.

"Start over," he says, and get buy-in from geologists."