Teaching about Time

Jessica Kapp, Geosciences Department, University of Arizona

The concept of deep time is often a difficult one for students to grasp. In a general education survey course, where students usually do not consider themselves "good at math or science," teaching about time with mathematical or visual tools is commonly met with resistance, and in some cases, complete shut down. I have taught my course in small (150 student) settings and mega (950 students) settings, and regardless of class size teaching about time is one of the biggest challenges in introductory Earth science for non-majors. Students are familiar with thinking in terms of human time scales – 100 years or less. Some of the most widespread misconceptions I am up against with this population of students include, 1) humans and dinosaurs coexisted, 2) humans and the Earth are roughly the same age, 3) the Earth is only several thousand years old, and 4) what we see when we look to the sky is what is currently there. The idea of light taking billions of years to travel to our eyes is one that is almost too abstract to explain. How can we be seeing something that happened 5 million years ago? This concept ties into the idea of scale – understanding the scale of different objects and distances we deal with in science, from the smallest atoms to the vastness of space. Along with time, scale is a concept that many students find confusing. Misconceptions are extremely difficult to break down, as ideas, no matter how incorrect, tend to root themselves in our brains and take hold. As such, I have tried various techniques to help convey the concept of deep geologic time.

One of the earliest methods I employed for teaching about deep geologic time was the demonstration of the geologic time scale as a length of rope that stretches across the entire lecture hall. On the rope are tags marking many of the most significant events in geological history, such as formation of Earth's crust, earliest life on earth, the Cambrian explosion, extinction of the dinosaurs, and appearance of our first human ancestors. Students are asked to volunteer with the demonstration and are given cards with events and dates written on them. As I discuss the formation of Earth and the various events that follow, students with event cards are called up to find their event on the timeline. The visual representation of time on the rope, and the fact that students can see how much time exists between many events, often helps students understand how vast Earth history is. In addition, I ask students to calculate how much of Earth history has passed when an event takes place. For some more recent events, such as the emergence of our human ancestors, I will ask students to calculate how much of Earth history has included humans. Likewise, I often ask them to calculate how much of Earth history included dinosaurs. This comparison usually helps them understand that humans and dinosaurs did not coexist. The main problem with this exercise is the large number of students who get extremely anxious about doing any mathematical calculations, no matter how simple. Many of my students cannot tell me how to calculate a percentage, and have no idea how to even begin this exercise. I do an example with them, but this is still not enough to quiet the math fears of many students.

Another challenging topic relating to time is how we get absolute or numerical ages of rocks and other geological materials. Students are often skeptical of ages quoted in class for things such as age of the Earth, age of the Moon, and ages of rocks and fossils. Calculating a geochronological age is no simple concept, and teaching it to students in this course is a real challenge. One method I have used to try and help them understand radioactive decay is to pass out pennies to all students in the class. Then we do an "experiment" in which I have them all stand up and tell them they are radioactive elements within a rock sample. I choose a "half-life" appropriate for our experiment, usually 5-10 seconds, and tell them every half-life we will flip our pennies – if you flip a head, you remain standing, if you flip a tail, you sit down. If you sit down, it means you decayed into your stable daughter product! We begin, and every half-life I have them flip and either stay standing or sit depending on their result. Once we have been through about four or five half-lives we discuss how the experiment relates to radioactive decay. I often get the question, "doesn't there come a time when all the parent isotope is gone and there can be no more decay?" This is a great question that tells me students understand the concept and are thinking about how it applies to geological dating.

In the many years that I have been teaching about time I have found that these exercises help many students, but often fall short of reaching students who are most anxious about science and scientific reasoning. I am honestly not sure if there are better ways to reach these students, as much of the issue lies with their fear of trying to think critically. Regardless, I am always looking for new, active, interesting ways to reach every student, from the most scientifically inclined to the least. My general feeling is that these techniques are more effective than simply lecturing about time. In a recent class activity we had them think in terms of generations after having them spend time calculating the percentage of Earth history represented by certain geological events. My hope was that the juxtaposition of deep time exercises and human time scale exercises would help bring to light the vastness of time, and that as humans we are involved in such a small piece of geologic history.

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