Teaching About Time

John Chadwick, Department of Geology and Environmental Geosciences, College of Charleston

We live our lives in terms of hours and days, months and years. If we're lucky, we might live a century. So the concept of deep time stretching on through billions of years is a very difficult concept for students to grasp and for instructors to teach effectively. In my classes that cover geologic time, I first lay the groundwork: a discussion of the scientific method, of relative age dating and the Law of Superposition, and a clear explanation of how radioactive isotopes decay and how they are used to get the ages of rocks. Even though I teach geology classes, I also give an in-depth lecture on biological evolution by natural selection, to provide a framework for the changes in flora and fauna that are observed through geologic time. With this background of knowledge in place, I have found students to be more receptive to the idea of billion-year-old rocks and an evolving Earth.

One of the problems that students face in grasping change on Earth over time is that many of the processes that we talk about take so long. Evolution is not easily observed at human time scales, and mountain ranges take millions of years to grow. I have found it useful to show examples of short-term processes that can be extrapolated to the longer-term. For example, an earthquake in a mountain belt may raise the local topography by a few inches. Therefore, if students accept that such seismicity and faulting is partly responsible for making huge mountains, it's easier to imagine that it must take an enormous amount of time to do so.

I have also found that visual representations of deep time are very useful in the classroom. 100-yard football fields that show the relative time of the various major events (dinosaurs, orogenies, formation of Pangaea, etc.) by placing them at the appropriate location on the field in very instructive. The are also several web-based resources available that clearly display the geologic time scale. The best that I have found is ChronoZoom (http://eps.berkeley.edu/~saekow/chronozoom/).