Purpose of the lesson: To provide students with an easy way to remember the basic characteristics of Franciscan Complex rocks, and to enable students to connect their prior knowledge of common candies with the descriptive science of geology.
Geologic concepts: Observation, description, texture, weathering, “fresh” rock, Franciscan Complex
Background: Our Edible Geology lesson grew out of an experience I had on one of my ranger-led geology programs in the Marin Headlands. As I led my group down the trail to the Point Bonita lighthouse, I pointed to many fine examples of pillow basalt, a dark igneous rock that forms as a result of submarine volcanic eruptions. The rock outcroppings along the Point Bonita trail showcase some of the world’s best examples of pillow basalt.
A young person in my audience had a question for me: “Ranger Roxi” the 10-year old said, “Why do you call these rocks pillow basalts? Pillows are soft, but rocks are hard!”
I asked my keen observer what I should call these rocks, and he energetically exclaimed, “Jelly beans! They look like giant jelly beans!” This was a perfect comparison, for as all geologists know, when pillow lavas erupt underwater, they form with a glassy exterior shell (from rapid cooling), while the lava inside the pillow remains squishy and warm for awhile, since it cools more slowly on its way to forming hard rock.
The following day, I shared my experience with Chris Hey, my education intern. She immediately remembered an activity called “Edible Rocks” from the book, Discover Nature in the Rocks, by Lawton, Lawton, and Panttaja. As Chris reread the activity, she got to work, trying to think of other candies that could represent the three main Franciscan rocks students encounter on their geology field trips to the Marin Headlands. Middle school students offered ideas, and we “field tested” our candy selections. We came up with three candies to represent the three main rocks in the Headlands: The jelly bean pillow basalt, the Jolly Rancher radiolarian chert, and the graywacke gumdrop. Since serpentinite is relatively rare in the Marin Headlands, we chose to leave it out of the lesson, but I have seen Crissy Field Center instructors use green salt water taffy as an excellent representation of the California state rock.