Intro to Relative Geologic Time

If you managed to plow through my latest post on Absolute Age Dating (Intro to Geologic Time), you probably muttered something like “This sounds too much like real science to be any fun at all,” and you’d be right.

The good news is that, although we had to suffer through the pain of radioactive decay to set the stage, now we get to gambol through the joys of Relative Age Dating. Since this is the method of sorting events across earthtime used by just about everyone, relative dating will have many more practical applications; not just related to earth history, but in all aspects of our lives.

Relative dating is just what the name implies. I know I’m older and taller than my 2-year-old granddaughter just by looking (as can she), but by how much (and does it matter)? Such is the difference between relative and absolute time. As humans we do this all the time with each other, and the good news is that — if we understand the clues and can interpret them correctly — we can do it with the earth too.

The better news is that the clues we need are really simple, and based — for the most part — on obvious things we can observe with even the most cursory of inspections.

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The Clues: Since there are no official “Laws” in geology (save Strickler’s Laws of GeoFantasy), we’re stuck with several guiding principles that can help keep us sane. It’s important to note that these are NOT laws that rule our lives at all times and in every case! They are simply a group of guidelines that work most of the time, and can be used by earth scientists to try and unravel the history of our planet.

The Principal of Uniformity: Uniformity is a fundamental assumption (I can hear Obie again) which tells us that “the present is the key to the past.” Uniformity is a powerful concept. Regrettably — but as is the case with many powerful concepts — it often sounds more complicated than it actually is.

The short version is simple: over the immense span of geologic time, the fundamental processes shaping our planet have remained constant and have—again, for the most part—always worked as they do now.

We’re talking about some pretty basic stuff here, including such favorites as gravity, and how it attracts matter and pulls it downhill; energy, as it restlessly moves through the environment, completing all manner of useful tasks; the marvelous changes that affect substances as they vacillate between the phases of solid, liquid, and gas; easygoing and obliging carbon’s meandering journey through nearly everything; the miracle of density.

Yep, some pretty basic stuff. But these are the recognized and accepted Rules of Reality, and — in what are surely nontrivial ways — determine how the earth works (and therefore how we work). Without the support of Uniformity to keep The Rules honest, any attempt to decipher the planet’s past would be, for all practical purposes, impossible.

Very few earth scientists doubt the veracity of Uniformity — to say we need Uniformity would be the grossest of understatement. The big question may be, “does Uniformity also tell us that the present is the key to the future?” I vote yes, but suggest we table this poser for the time being…

The Principal of Original Horizontality: Sedimentary layers are initially deposited in horizontal layers. The forces of reality beat up on the earth and break it into small pieces, which then wash down rivers to the ocean and settle out on the seafloor. In horizontal layers. Lots of exceptions here, but it generally holds true.

The Principal of Superposition: This clue tells us that the layer on the bottom is the oldest, and they get younger as they pile up. This makes a lot of sense if you think about it, especially if the sediments are settling down from above. But what if they aren’t? Definitely a few exceptions to this one.

The Principal of Cross-Cutting Relationships: This one tells us that the thing being cut is older than the thing doing the cutting. Of all the guidelines, this is my personal favorite, mostly because it usually works. This one is absurdly simple, and obvious: you can’t cut something if it isn’t there. Works for things like faults and dikes and veins. And in life: if you don’t want to feel the pain, stay out of the way!

The Principal of Inclusions: This clue tells us that the thing inside has to be older than the thing it is found inside of. In the above image from the Merlin Batholith, the metamorphic gneiss had to have already been in existence before it broke off and was included in the granitic magma before it crystallized into solid rock. What with faulting and fracturing and related forms of crustal abuse, there are definitely exceptions to this one but, again, it usually works.

The Principal of Faunal Succession: This one assumes, in general terms, that life has evolved from more primitive organisms to more complex — from Cambrian trilobites to you, for example. While this trend is often complicated by mass extinction events, which tend to wipe out the complex organisms and reset the biosphere with more simple life forms, faunal succession can be a valuable clue in establishing the relative order of events through earthtime.

In the image above, pretend our hero is mapping a sequence of fossiliferous sedimentary strata. The layer on the bottom contains trilobite fossils from the early Paleozoic, and the layer on the top has a partial skeleton of T-rex from the upper Cretaceous. Since we all know that dinosaurs came well after trilobites went extinct, we can assume — supported by the Principals of Original Horizontality and Superposition — that the strata are right side up.

But what if you find the same layers with T-rex on the bottom? The Principal of Faunal Succession tells us that the entire stratigraphic section must have been overturned after the deposition of the sediments.

Weathering and Erosion: Evidence of weathering and erosion can only happen at (or near) the surface of the earth. Therefore, the uplift and surface process must be younger than the rocks that were folded, spindled, and/or mutilated. Not many exceptions to this one!

Put it all together, and one of the things that relative dating techniques lead to is the Relative Geologic Time Scale — the bible when it comes to defining the temporal history of our planet, evolution, and how we ended up with a biosphere that could support us. No small matter, indeed!

The main divisions are almost entirely due to changes in the biosphere, and most of these involve extinction events of a lesser or greater extent. We’ll be devoting posts touching on relative age dating, paleontology, evolution, and global extinction events real soon, as I prepare to publish Marker Bed, which is intimately tied to life on earth — past, present, and most especially the future…