Geology 101: Deep Time, or Geologists as Time Lords

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…

He works in a bowtie... (image from doctorwho.tumblr.com)

And so do we (well most of us anyways)…

West Virginia Geologists from 1897 (Image from USGS)

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:

This cross section of the Grand Canyon illustrates the principles of Original Horizontality, Superposition, and "baked contacts" (Image from Wikipedia)

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.
  

  An artists rendition of a nonconformity. The area below the black line represents the older intrusive rock and the rocks above are younger sedimentary rocks (Image from Wikipedia)

• 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.
  

  The top layers of sandstone are horizontal compared to the nearly vertical rocks underneath it. (Image from Wikipedia)

These concepts give geologists a relative age of the rocks they’re working with. It’s not meant to give a conclusive age, but rather constrain the time line that we’re working with.

If we want to obtain an actual age we turn to radiometric dating.

This form of dating gives us an absolute age value that allows us to assign a numerical value to the rock layers. While decidedly more sophisticated than relative dating, this is a technique that is most familiar to the public and also the least understood. Radiometric dating involves the measurement of isotopes. An isotope is an atom that has an unequal ratio of neutrons to protons (Edit: In my efforts to simplify the chemistry behind radiometric dating, I used a definition of isotope that is not entirely true. An isotope, such as carbon-12 which has 6 protons and 6 neutrons, can have an equal ratio of protons to neutrons. A couple readers pointed this mistake out. I posted those comments in the comment section for clarification). This makes the atom pretty unstable and causes it to undergo radioactive decay whereby it becomes the atom of a different element. For example, the atom that allowed us to determine the age of the earth, uranium 238, decays into lead 206. The original atom is called the parent isotope and the new atom is called the daughter isotope.

The amount of time it takes for a parent isotope to decay into a daughter isotope is called the half-life. This decay occurs with regularity and allows us to obtain an accurate age of the rocks we’re studying. To clarify this example, and pick up with our Doctor Who theme, let’s imagine we have 32 Daleks. In one hour 16 of those Daleks decay into Cybermen. Another hour passes and 8 of the 16 remaining Daleks become Cybermen. After those two hours we have a total of 8 Daleks and 24 Cybermen. At the start of the third hour 4 of those poor Daleks have gone over to the Cybermen side. For an explanation of what happens when there is less than one atom present, click here.

Now, this is a highly simplified explanation of a rather difficult process. Truth be told, there are some beastly equations that deal with this decay process, but the idea is relatively straightforward.

However, isotopes do have their limits. For example, carbon dating has a limit of about 60,000 years and as such is excellent for dating human artifacts and fossils, but not so great for volcanic rock. Beyond that 60,000 year mark it becomes wildly inaccurate and we need to use atoms such as Uranium 238. This is the atom that has allowed us to determine that the earth is 4.5 billion years old and not 6,000 years.

Well, I don’t know about you folks, but my brain is completely fried and as such I have no fitting conclusion to end this, the most epic post I have ever taken on. So, instead I will leave you with a quote from the good Doctor that somewhat applies to everything we’ve discussed-

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)

*I wanna give a big thank you to Dana at En Tequila Es Verdad and Suzanne at TwoTonGreenBlog for helping me put this post together!*