Introducing Students to Geologic Time

A couple weeks back, before I had figured out how to record Google+ Hangouts, one of our Monday afternoon Geology Office Hours discussions took an unexpected turn toward a discussion of how to teach students about geologic time in the most effective way, given a range of secular and religious backgrounds. This discussion, though not recorded, was faithfully reflected in a blog post by Dana Hunter, and that in turn inspired another thoughtful blog post by Tim Sherry. Since I just discovered Tim’s blog Up Section this week and added it to my Geoblogosphere list, I had the opportunity to come across his post on the subject for the first time today.

Toward the end of Tim’s post he asks:

What evidence can we present to show how we know rocks are really old? Show pictures of zircons? Well, then you’re talking about radioactive decay and isotopes, way too advanced for elementary school. Maybe show a sedimentary rock and talk about how it formed into it’s present form? Any elementary teachers out there with experiences/insight they’d like to share?

I answered in Tim’s comments, but I wanted to record that answer here and elaborate on it a bit.

The way I approached introducing geologic time with my Intro Geology classes was to mirror James Hutton‘s likely line of thought. Begin with Steno‘s Laws (Superposition, Original Horizontality, Lateral Continuity). These all make intuitive sense, thus don’t raise any hackles. Then apply them in a view of the strata of the Grand Canyon (or some local exposure of relatively flat-lying sedimentary rocks); no discussion of absolute ages yet, just make sure they get the principles of relative age dating.

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Horizontal Strata, Goosenecks of the San Juan River, southern Utah

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Next, introduce them to Hutton’s angular unconformity at Siccar Point. Work them through the types of rocks (sandstone, shale) and discuss geologic environments and processes by which sand grains are first weathered and broken down from larger rocks, then eroded, transported, and deposited. Then there’s burial and later, the process of lithification. Finally some tectonic event causes these layers to be tilted. But that’s not all; you have to erode all the overlying rock so that they get back to the surface as tilted rocks. And then, in order to form the rocks above the unconformity surface you have to do all that over again!

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Hutton’s Angular Unconformity at Siccar Point, Scotland
Photo by Meg Stewart, used under a CC BY-SA License

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The coup de grĂ¢ce is explaining Hutton’s understanding of Hadrian’s Wall. It was built by the Romans in the second century AD, and Hutton would have known this from written histories. By the time Hutton sees the wall in the 1780s its age is somewhere between a quarter and a third of the entire earth’s history by a Creation = 4004BC perspective. And yet it still stands. Yes, it’s overgrown and many boulders have fallen out of the wall, but they’ve hardly begun their process of weathering down to sand.

Hadrians Wall
Hadrian’s Wall, Scotland
Photo by Walt Jabsco, used under a CC BY-NC-ND License

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Now think back to all the geologic events that had to occur for the Siccar Point unconformity to have formed, and with this single insight into the rates of geologic processes, it should immediately be clear how Hutton was able to apply naturalistic explanations to break free of the Biblical literalist interpretations of his time, and not at all shocking that he would make the even greater leap to “… no vestige of a beginning, no prospect of an end…”

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