Intro to Geologic Time

One thing about time is that it keeps moving—always into the future and always in polar opposite, yet mutually supportive rhythms: relative and absolute. Relative time is agreeably flexible and adapts itself to the needs of the moment. But absolute time keeps to a rigidly defined schedule and brooks no variation or dispute—argue and whine and negotiate all you want: absolute time, like water, always wins.

Ordering events across earthtime makes use of both of these temporal rhythms, although they vary greatly in several fundamental ways. Let’s spend a few minutes with a bit of overview and a closer look at absolute time — certainly the tougher of the two for most of us to wrap our heads around. We’ll circle back for the details of the easier relative time in a later post.

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Absolute Time is just what the name implies: it gives specific dates and times for events through earth’s history. When we say that the planet is 4.6 billion years old, that is an absolute date. Same with the extinction of T-rex 66 million years ago.

Absolute ages are based upon the assumption (I can hear Obie reminding all of us what ‘assume’ means) that there are several elements that occur in the minerals that make up the planet that are unstable: they will ‘decay’ over time, and change from the ‘parent’ element into the ‘daughter’ product. Two familiar examples are how Uranium-238 will decay and turn into Lead-206, or — upon the death of an organism — the Carbon-14 it has been producing while alive will begin to lose neutrons and decay into Carbon-12.

(Don’t sweat the -238s and -14s and such. These are simply the ‘atomic mass’ values, and largely reflect how many protons and neutrons exist in the nucleus of the atom. It’s easy: if you wanna call it carbon, for example, it must have six protons, but the number of neutrons can vary (and obviously will — this is how carbon dating works, after all). In the case of U-238 (with 92 protons and 146 neutrons) decaying into Pb-206 (82 protons, 124 neutrons), there is a loss of protons as well as neutrons, affecting not only the atomic mass, but also the atomic number. This loss of protons is mostly what changes it from uranium to lead.)

Anyway, the assumption is that the rate of decay is fixed, and is described by the term half-life. The concept here is that, in a specific span of years, half of the parent will change into the daughter.

So, it should be simple: if you start with a gram of uranium, in a mere 4,500,000,000 years, you’ll be left with half a gram of U-238, and half a gram of Pb-206. Wait another half-life cycle, and you’re down to a quarter gram of uranium, with three-quarters of a gram lead. (Of course, we’re not talking about grams here, but much smaller amounts of material — your bathroom scale probably wouldn’t be sensitive enough to give you an accurate reading.)

Note that in the above image (Hi, Abby — we all miss you), the half-lives are not the same. This variation directly impacts what specific elements are best suited to estimate an absolute age for a specific sample. For example, with a half-life of only 5730 years, using C-14 to C-12 wouldn’t work for calculating the age of the earth — all the C-14 would have decayed to C-12 long before we got to 4.6 billion years. Conversely, trying to use the decay of U-238 into Pb-206 to date the age of some charcoal found in an archeological dig probably wouldn’t work very well, either.

So yeah, absolute dates are great. Unfortunately, along with needing an accurate scale, there are several additional problems that make them essentially useless for most of us normal mortals — I am forced to admit that, in my fifty-plus years of exploration and teaching, I have only ever attempted to obtain an absolute date once.

Calculating an absolute age date for a rock or geological event requires exact measurements of minuscule amounts of a substance that is usually hidden inside solid rock. As you can imagine, these measurements require some pretty sophisticated equipment. I don’t have any of this stuff in my workshop, and doubt you do, either.

Due to the need for such fancy gear, the cost of setting up the necessary lab is ridiculously high, as is the cost of actually conducting the analysis. Put it all together — the sophistication of the equipment, along with the associated costs — and it’s no wonder these specialized labs don’t just crop up everywhere. We certainly didn’t have the necessary goodies at any of the colleges I’ve been associated with, and surely not at the high school!

Then it should be no surprise to any of us that all this expensive, sophisticated equipment needs some really smart people to run it. It’s a sure bet that I wouldn’t qualify without some serious training (at this point my brain is flooded with images of a dog with a greying muzzle trying to learn a new trick).

And even if you can afford the lab and have access to the brains to run it, there is also the accuracy issue to contend with. I used to do a week-long Astronomy activity at the high school, where the class would model the reality of a Giant Rock From Space slamming into the earth. One of the groups was tasked with designing a viable plan to deflect the GRFS before it even got here, thus saving us from the impact altogether.

The kids came up with some mighty fanciful mission parameters (remember, these were mid-teens), but — class after class — one thing was agreed upon by all: the further away it was when the effort was made, the better the chance that a relatively minor tweak would result in enough of a change in the asteroid’s trajectory, by the time it got here, so that we could all just stand in the commons and wave as it hurtled past.

Absolute age dating runs into the same issue. Remember Abby and the half-lives? Along with measuring incredibly small amounts of materials, we’re usually trying to project the age calculations back in time millions, to hundreds of millions, to even billions of years. Pushed out that far, ANY small mis-measurement or calculation glitch can result in a potentially significant bust in the calculated age.

In these PC days of modern times, absolute age dates generally include a “plus or minus” range, often defined as a percentage of wiggle room in the result. “So what if it’s +/- 5%” I hear you cry. But consider: if using the U-238 to Pb-206 method to calculate the age of the earth, plus or minus 5% results in a bust of 230,000,000 years. Sounds like enough to me!

But hey — having a specific age date can be incredibly helpful. It’s one thing to be able to say with confidence that the earth is older than me, but how much older may also be important. Sadly, few of us can afford to get an absolute age date whenever we want one.

Which brings us to the other rhythm of earthtime…

Relative Time is also what the name implies, and something we all do all the time. Run into anyone on the street and one of the first questions/comparisons each of us will make is: who is older, me or them? Our ability to do this is based upon simple clues: hair color, wrinkles, posture, dress, and other subtle observations that we don’t even think about.

Relative age dating differs from absolute in several important ways. At the top of the list is probably that it’s free — all relative dating requires are two things you already have: senses to observe with, and a brain to analyze the data.

On the con side, relative age dating doesn’t give us much in the way of hard data, and is based upon interpretation as much as anything else. When discussing the value of relative time, we’re stuck with assessing which is more important: getting an ‘objective’ date or a ‘subjective’ date. Things that are objective are based upon hard empirical data (like absolute time gives us), while those that are subjective are generally based more upon our personal opinions and what we want reality to be.

Yeah, yeah, yeah, I agree: subjective sounds like a lot more fun, SO much easier, and how I’d like to be able to organize and define my reality. Sadly, the lack of concrete definition can, and often does, result in confusion regarding the things that impact us, and can lead to some truly unfortunate blunders, especially with regards to long-term planning (consider again at the dudes tied to the posts, above).

Oh well, since we can rarely afford an objective absolute date, and subjective relative dating is something most of us already do all the time anyway — and is the basis of the Relative Geologic Time Scale all earth historians rely upon — I vote we table it for now (this post is objectively overlong and my brain is subjectively fried), and circle back to it for the details real soon. The good news is that they will absolutely be much easier to understand, relatively speaking.