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Andrew Alden

Ma or Myr? How We Talk About Geologic Time

By February 15, 2012

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Geologists have a bit of awkwardness in their language in talking about the deep past: distinguishing dates from durations. None of us has a problem with the weirdness of historical time—we can easily say that an event in 200 BCE happened 2211 years ago, and that an object made back then is 2211 years old today. (Remember, there was no year 0.)

Among geologists, a widespread practice has arisen in the last few decades that gives dates (not ages) in the format "X Ma"; 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. Instead of saying that the rock is "5 Ma old," geologists use a different abbreviation like m.y., mya, myr, Myr or whatever. This is a little awkward, but context makes things clear.

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 in the Système International or SI, the "metric system." The exact definition isn't important here, but the symbol they chose, "a," would happen to override geological custom by requiring everyone to use "Ma" (and ka and Ga, etc.) everywhere. That would make writing geology papers somewhat harder, but I suppose we could adjust.

But Nicholas Christie-Blick of Columbia University has looked more deeply at the proposal and cries foul in the current issue of GSA Today. He raises a question I'm sympathetic to: How can SI accommodate the year as a "derived unit" when SI rules require that these must be simple powers of base 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 kilogram). 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. I agree.

Steve Drury of Earth-pages doesn't understand the fuss

Mapping geologic time
Measuring geologic time
The geologic time scale


February 15, 2012 at 8:43 pm
(1) Lab Lemming says:

Who are these IUPAC nitwits who insist of screwing up perfectly good conventions. If they keep it up, I will retaliate by reporting zircon ages in petaseconds!

( a petasecond, or 10^15 seconds, is about 32 million years)

February 16, 2012 at 1:46 am
(2) Karen says:

I was taught that Ma, ka, etc. refers to N years AGO, while My, ky, etc. refers to N years. So the Little Walker Caldera in the Eastern Sierra erupted between 8 and 9 Ma, and has been quiescent for at least 8 My. Is that not as much of a standard as I’ve been taught?

February 16, 2012 at 7:59 am
(3) David says:

At university in the UK during the 1980s I was taught to use Ma in all instances. This is also the house-style of the British Geological Survey. In fact, in all these decades of reading geological texts, the existence of this convention has never sunk in which suggests that, to me at least, the distinction between the concepts is generally adequately elucidated by the context. At the risk of being mocked for my ignorance, I’ve asked some professional colleagues, including a senior geological editor, and so far haven’t found anyone else who was aware of the convention – or the controversy. I guess the UK must be more of an academic backwater than I like to admit!

February 16, 2012 at 9:23 pm
(4) Howard says:

I’m with Drury on this one. I’ve never detected any convention (i.e. consistency) in the use of these terms in the geological literature, so it’s much ado about nothing: the meaning is almost always clear from the context. In my mind it makes sense to use Ma (“mega years”) for spans of time, and mya (“million years ago”) for dates. I think my own policy will be to follow Christie-Blick’s second recommendation: “Professional societies and journals should maintain whatever conventions they currently use, as they see fit.” If it ever becomes a practical issue, it would be easy enough to simply define the abbreviations you’re using at the beginning of your paper/book/article.

March 14, 2012 at 3:07 pm
(5) Andrew Okulitch says:

Conventions and standards are created to clarify communication, nothing more. Application of arbitrary rules, no matter how well they clarify communication in some branches of science, is bad practice if it reduces clarity. The long-standing dual use of m.y. and Ma in geology should therefore be retained. No harm is done to the S.I. system in doing so.

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