Toilet Paper Analogy for Geologic Time

• To give students an appreciation for the length of geologic time.
• To relate geologic time to something that is tangible for students.

I use this exercise/demonstration as a short aside in a lecture.

I introduce this concept after we have talked about the construction of the geologic column using fossils and sedimentary rocks. The students have seen the geologic time scale printed in their book (which is almost never to scale) and we have talked about where those numbers on the geologic time scale come from.

To complete this exercise, you will need a roll of toilet paper (preferably a roll with 1000 sheets, but any other will work), about one-half hour before class (to mark up your "geologic time scale") and about 5-10 minutes of class time.

A modifiable Excel table (Excel 25kB Feb8 05) showing the length (squares of toilet paper, inches and centimeters) from the start of the toilet paper roll (present day) to important events in geologic history. This file can be modified to accommodate any number of squares (or inches/centimeters per sheet) by changing the values in the first row.

Before going to class, you'll need about one half hour to prepare the roll of toilet paper for class. I mark up the roll of toilet paper with a colored marker. At the appropriate places on the roll, certain important events in geologic history are marked. This is a somewhat delicate operation, especially if you use Scott (tm) tissue as it is 1-ply, but if the paper rips, it can be taped. A list of "important" events and where they should be marked (in inches or sheets) is included in the teaching materials. By the time I have marked all the important events, I have a large pile of tissue on my desk. I then delicately roll the paper back onto the cardboard roll. You are now ready for the demonstration in class.

After talking about how we put numbers on the geologic time scale (see radioactive decay) and talking about how old geologists think the Earth is. I begin this demonstration by asking them, "How long is 4.6 billion years? Can we compare it to our lifetime? How can we imagine that much time?" Then I introduce the concept of the toilet paper time scale.

I have a volunteer come up to the front and take one end of the toilet paper. I ask her/him to walk slowly toward the back of the auditorium, unrolling the toilet paper as she/he goes. When we get to an important event, the student is asked to stop and I call out what we've just passed. By the time the student gets to the back of the room, we've barely reached the first dinosaurs (barely 5% of geologic time). I point out how much toilet paper is left. The student can continue to walk across the back of the room and continues around and down the other aisle. Still we're barely through 10 or 15%. Students seem to respond to the use of toilet paper (it's something they all can relate to) and the illustration seems to convey the minute amount of time that we and our ancestors have inhabited the Earth. It at least gives students a sense of the vast scale involved in Geologic time.

It is relatively easy to do a quick check on whether students have grasped the concept of deep time. There are several ways that this can be done:

• If you have a student response system, a quick quiz with a question that has students estimate the fraction of total Earth history since the dinosaurs became extinct or represented by the presence of modern humans. An example of this type of question follows:
• Having students work through a short problem (in groups or on their own) that applies these concepts (e.g. calculate the fraction of Earth history that is represented by the Precambrian
• A short written quiz similar to the above question might also be a way to assess comprehension.

• Elizabeth Roettger's page has a template for using a regular (250 sheet) roll of toilet paper to make a Toilet Paper Geologic Time Scale
• University of Pennsylvania has the transcript of a 60-second lecture given by Professor Robert Giegengack on Geologic time compared to a roll of toilet paper