I’ve often suspected that the Doctor from Doctor Who is somewhat modeled after geologists. You see, we have a lot in common with the Time Lord. He works in a bow tie and coat…
And so do we (well most of us anyways)…
The Doctor has his trademark tools such as the sonic screwdriver and the eponymous TARDIS and we have our tools of the trade, the rock hammer and our hand lens. Both sets of tools allow us to be time travelers much in the same way that the Doctor uses his TARDIS. Instead of flying around in a blue box though, our travels take place through the history that is set in the earth’s stratigraphy, or the multiple layers of rock whose deposition records the conditions of that location at a point in history.
OK, so maybe there’s some hyperbole and pomp in the previous statement. After all, we’re not the only scientists whose work spans millions and, in some cases, billions of years of time. Astrophysicists and astronomers work on scales that start 14 billion years ago with the Big Bang. However, as my friend Jamie pointed out, what makes geology unique is that we get to physically interact with our 3 billion year old rocks. We can touch them, examine them with our hands lens and chip at them with our hammers. And, if you’re feeling particularly old school, you can taste them. Go ahead and chuckle, my non-geologist friends, but this is how some rock identification is accomplished.
Geologists use different tools for determining the age of geological structures. One set of tools allows us to determine the relative age of rock- that is, the age of rock layers relative to one another. What’s really nice about this technique is that anyone can use it. All you need to practice relative dating is your eyes and maybe a journal to draw what you’re seeing. There are a few basic principles to look for:
- Original Horizontality and Superposition- This is the idea that rock layers are deposited in a horizontal fashion with the oldest rocks being on the bottom and the youngest being on the top. This generally holds true unless you’re dealing with thrust faults that shift and bend layers.
- Cross-cutting relations- If a geologic feature such as a volcanic dike or fault cuts across other geologic layers, then those cut layers are older than the layer doing the cutting. A similar principle involves that of “baked contacts”. If you have something like a volcanic sill or dike that injects magma into the local bedrock, then the border between the dike/sill and bedrock will experience some thermal changes, or metamorphism. This injection of magma is called an intrusion and can be inferred to be younger than the surrounding bed rock.
(adapted from Steven Marshak’s Essentials of Geology, 2009.)
A classic example of these principles is the Grand Canyon:
You can think of these rock layers like pages from a history book. You start at page one (the top layer and the youngest) and as you work your way through the book, you begin to get a sense of when things happened and why. However, there are times when our geological text book is not only missing a page or two, but entire chapters. We call these pages of missing history unconformities and they generally come in three flavors:
- Disconformity- This is where you have two layers of sediment parallel to one another, but were not deposited one after the other. This could occur when a layer of sediment is deposited, is slowly eroded away and another layer of sediment is deposited where the old one used to be.
- Nonconformity- These are present where you see a layer of younger sedimentary rocks on top of older intrusive metamorphic or magmatic rocks. This ties directly into the principle of Cross-Cutting Relations and “baked contacts”. If you have a layer of, let’s say river sediments, deposited on top of a dike (magmatic intrusion), then you have a nonconformity.
- Angular unconformity- This occurs if the older bottom layer of rocks has been tilted at an angle and the younger top layer is relatively horizontal. This is also called Hutton’s Unconformity, and a well known example is Siccar Point in Scotland.
People assume that time is a strict progression of cause to effect, but *actually* from a non-linear, non-subjective viewpoint – it’s more like a big ball of wibbly wobbly… time-y wimey… stuff. – from Doctor Who “Blink” (2007)