Teaching Earth Science with Food - Rock Cycle Shortbread

This month we've been working on Earth Science, and last week we talked about the three main types of rocks and the rock cycle. What better way to bring our week's discussion to a close than with food? I saw a couple examples online of edible rock cycles, but they generally involved making three separate kinds of things for the three different kinds of rocks. The Big One was understanding the types of rocks, but was a little sketchy on the cycle of one type of rock becoming another, so I wanted to come up with something to put it all together into one cycle. What I ended up with was a science baking demonstration I'm calling Rock Cycle Shortbread.

Ingredients 
1/3 cup chocolate chips
1/2 cup (1 stick) butter
1/8 cup granulated sugar
1/8 cup brown sugar
1 cup flour

Step One - Igneous rocks

To demonstrate the creation of igneous rocks, we used chocolate. I started by melting the chocolate chips in a double boiler. The melted chocolate served as our lava, which we then poured onto a piece of greased aluminum foil, and then let cool into a "rock".

Step Two - Sedimentary rocks
 
Before starting this step, I set our "deep Earth" (oven) to preheat to 300 degrees Fahrenheit.

To demonstrate the creation of a sedimentary rock, we added various kinds of "sediment" to a large bowl. First, we subjected our igneous rock and our stick of butter to erosion via a grater, making some nice chocolate and butter sediments. We then measured out our granulated sugar sediment, brown sugar sediment, and flour sediment. I let the kids get nice and messy, using their hands to simulate the movement and compaction of sediment by rivers and streams. After rolling the mix all around, we came up with some flaky sedimentary rocks. 

Step Three - Metamorphic rocks

Finally, to get a metamorphic rock, we needed some heat and pressure. I explained that while the real rocks would be heated and pressurized at the same time (we'll save the classes of metaphoric rocks and different formation mechanisms for a later lesson), we would have to do it in parts for our demonstration. We started with pressure, simulating the pressure of being deep in the Earth by pressing our sedimentary rocks hard into an ungreased 8"x8" baking pan. We then put the pan in our preheated "deep Earth" and allowed the pressurized rock to bake for 30-35 minutes.

I scored the rock into squares while it was still hot, then once it was cool we broke it apart to see if we could see any layers or textures in it. 

In the end, while they were no culinary masterpiece, these shortbreads were pretty darn tasty. Cutting the 8"x8" pan of shortbread into 16 pieces makes each final metamorphic rock equal 108 calories, which is not overly terrible, either. Best of all, of course, is that The Big One was able to explain the demonstration accurately to a number of family members, so I think he really got it this time.