Sweet Geology

This is a delicious way to use chocolate to explore the three different kinds of rocks, seeing the difference between igneous, sedimentary and metamorphic rocks.

You will need:

  • fudge
  • brownies
  • ice cream
  • nuts
  • a clear, glass bowl
  • a spoon

There are three basic kinds of rocks. Igneous rocks are formed from molten rock that cools. Metamorphic rocks are rocks that have been changed by heat and/or pressure. Sedimentary rocks are made up of bits of other rocks that have been transported by water, wind, ice or gravity. How would you illustrate these categories with chocolate?

To simulate an igneous rock, make some fudge. You heat and stir the ingredients until they are completely melted. When it cools, it hardens into solid fudge, just as lava and magma cool into solid rocks.

For a metamorphic rock, bake some brownies. The heat of cooking causes changes in the ingredients. What comes out is very different from what went in. That is very much like what happens to metamorphic rocks. Marble is not much like the limestone that it came from.

I had to think a bit about sedimentary rocks. Since sedimentary rocks are made up of bits of other rocks, my first thought was to crumble the brownies. Instead of using water, wind or ice to transport the fragments, I decided to use gravity. Sprinkle the crumbled brownie into a clear, glass bowl. Sedimentary rocks usually form layers, so make a nice layer of brownie fragments. One layer by itself is no fun, so lets make another one. Ice cream would represent an igneous rock, since it is cooled to make it a solid. Use a spoon to dig out chunks of the ice cream and let gravity deposit them on top of the fragments of metamorphic brownie. Keep adding fragments until you have a second layer.

Now we have two layers of sedimentary rocks. How about a third layer? Chop some nuts very fine, to represent sand and gravel. Sprinkle a thin layer of the nuts to make the third layer. Now we have a very nice sedimentary example.

You can even "read" the history. Looking at the layers, which one was put down first? The bottom one, right? The same is true in geology. It is called the Law of Superposition. As long as the layers have not been turned over or disturbed, the bottom layer is the oldest and they get younger as you get higher.

Ok, so the bottom layer was put down first. But, we know that something happened even before that. The bottom layer is made up of metamorphic fragments. With a geologic site, that would tell us that those fragments came from a layer of rock that had been changed by heat and pressure. We could also tell what the rock was before it was changed. For example, slate starts as shale, marble begins as limestone, and quartzite starts as sandstone.

The next layer up was ice cream, which we said was igneous. But again, it was fragments of the "rock", so again we know that some older, igneous rock has been eroded to form this layer.

The top layer is made up of plant material, the nuts. Not all rocks are made of minerals. Peat and coal are both examples of rocks that are made up of plant material.

If you want, you can get even more involved. You could slice the layers and then offset the two pieces, to simulate a fault. Then you could "erode" the top to even it out. An easy way to do this is with a spoon. Just eat any part that sticks up. Then you could top it off by simulating a volcanic eruption with a lava flow of hot fudge. With a little thought, you can use chocolate to explore many different rocks and structures. What a fantastic way to study geology!


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