We started class with “A Brief History of Geologic Time” by PBS Eons and :Great Minds: Mary Anning, The Greatest Fossilist in the World” by SciShow.
I also brought out some fossils of horn coral, heliophyllum halli, that we have and explained how they have growth ridges, kind of like tree rings. Scientists count the growth ridges and find 400 ridges per year (roughly a centimeter of growth) and since they produce one growth ridge per day that tells us that a year on earth used to consist of roughly 400 days, not the 365 days we have now. This is because the earth’s rotation is slowing down so the Earth goes through fewer rotations in a one trip around the sun.
We then talked about the different ways to determine the age of a fossil, relative dating, index fossils, and radioactive dating. The students did an activity from Real Science Odyssey Biology 2 where they cut out strips of paper that had different animals/plants on them and put them in order the way they would expect to find the fossils in the layers of earth – oldest fossils (simplest organisms) at the bottom and animals resembling what we see living on earth now, at the top. They glued these into their notebooks after confirming they were correct.
The second activity was about radioactive decay and carbon-14 dating. Students were each given a cup of chocolate candies, M&Ms, and dumped them on a paper plate. All the candies that landed with the M facing up were ‘decayed’ and disposed of (eaten or placed in a separate bowl). Students counted the remaining candies and recorded it in their data table. They put the candies back in their cup and tossed them again and again until they were out of candies. We put everyone’s data on the white board and added all the results together and made a bar graph of the results. The graphing part of this activity was the hardest part. Just about everyone had trouble making the axes of the graphs, not sure how many squares to use, not being consistent in the number of squares, ie just randomly putting numbers on the side of the paper. Its really good to make the students graph by hand and learn how to do this and to check their axes before they start plotting the data. You can find lots of different versions of this activity on the web, I used the one here on the AAAS website, Radioactive Decay: A Sweet Simulation of Half-Life.
Here’s a video explaining carbon dating.