“Whoa! Ms. Parker, is that uranium?” one of my students shouts at me as he enters the room after lunch, eyeing the beakers of yellow liquid on the front table.
Students don’t seem to struggle with relative dating- stratigraphy, at least in the simplistic form that we use in class, makes sense. The laws of geology are intuitive in relative dating- layers are laid down in horizontal lines (the law of original horizontality), the processes that erode or change rock formations happen gradually, slowly, over huge amounts of time (law of uniformitarianism) rather than in abrupt events; older layers are underneath younger layers (law of superposition); that layers need to exist before magma can ooze up into cracks and form intrusions (law of cross cutting). Relative dating can be modeled in the classroom fairly easily, too, with paper models and with field sketches.
Relative dating doesn’t get at specific ages. Relative dating tells you how old something is in comparison to something else- rock layer A is older than rock layer B which is older than rock layer C. I use the example that it is like using obvious signs to determine that I am older than a particular student, but we don’t yet know exactly how old I am.
Relative dating and the laws are fun to teach, I like having the kids create their own construction-paper stratigraphy columns for their science notebooks. They compare rock formations from around Wisconsin and use index fossils and other features to correlate their rock layers from site to site. Everyone, teacher and student, gets to feel some success with the concepts of relative dating.
And then comes absolute dating. The principles of radioactive decay and half lives are much more complicated, and a lot more difficult to model effectively in lab. Many of my students have not taken any chemistry, and so the idea of isotopes isn’t within their working vocabulary or knowledge base.
The labs we do around the radioactive decay process might include popping popcorn- the kernels pop, but we can’t predict which individual kernel will pop at any particular moment…and once they’ve popped, they don’t ever return to “kernel” form. We also try shaking a set number of pennies- each “shake” of the box represents a half life and they remove the flipped pennies until none remain. They graph their results and we look at what happens through time.
The “uranium” in question was, indeed, colored water in beakers. This is our third day working with the concepts of absolute time. The kids poured out half of the water into a dry beaker with some drink mix powder in the bottom causing the yellow parent isotope in the original beaker to “decay into” a red daughter isotope in the second beaker. They had a “half life” for their isotopes, graphed the results, and answered some analysis questions about their decay process.
The goal is to get students to understand that radioactive decay happens naturally, spontaneously, and at constant rates that aren’t changed by the processes of the rock cycle (heat, time, pressure); and that half lives for different isotopes can be used to determine the ages of different materials. But sometimes I think that the kids see these models – popcorn, pennies, Koolaid- as being completely separate from the geological concepts we’re aiming for. How does food coloring and water help us determine how long ago a massive extinction event occurred?
It turns out that the kids, of course, understand the concepts on a variety of levels. Some of them are clear on what we modeled, and can explain both the strengths and weaknesses of the models. They can draw parallels between rocks and popcorn. Many of them at least understand that the graphs of the decay process are always similar, even if they don’t completely understand isotopes and exponential equations. If their understanding of why we graphed pennies in a box is a little shaky, they are able to explain that decay happens in a measured way and that somehow scientists use that to attach dates to events and organism.
Much like geologists choosing different isotopes depending on which rock/fossil/time period that they are interested in, my students take different depths of understanding with them depending on their background knowledge and the time and effort that they put into the course.
Turns out that both radioactive decay and high school students can be considered spontaneous and unpredictable. I just hope all of them are clear that we didn’t, in fact, really get to use uranium in class last week…