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Teaching the Rock Cycle With Marshmallow Fudge
Dr. Sandra L. Hunt

Students' favorite science experiments, not surprisingly, are the ones they can eat! Although it took some planning, my class successfully explored the rock cycle using, of all things, jumbo marshmallows, chocolate chips, and a few other ingredients needed to make fudge. Oddly enough, marshmallows seem to lend themselves well to teaching the rock cycle because they can be manipulated to change in a way similar to how to rocks form — luckily, however, a lot faster and under a lot less pressure and heat.

To set up my "lab," I gathered the following equipment: a hot plate, large spoon, saucepan, can opener, three small rectangular baking pans, and plastic bags for every table group. Ingredients consisted of two bags of jumbo marshmallows (one for the experiment and one for eating), a bag of chocolate chips, sugar, butter, and canned milk. Lastly, to illustrate the rock cycle and incorporate the types of rocks we had been studying as part of our unit, I modified a story I found about a rock.

The day before the "great rock experiment," I introduced the names and samples of the three types of rocks (igneous, sedimentary, and metamorphic) to the class. On the day of the experiment, I laid the samples out again, this time in the form of a triangle with arrows between them to demonstrate that they were all part of a cycle.

Once students settled into their seats, I began with the idea that new rocks are made from old rocks and informed them that we were about to learn how that happens, starting with the following story.

Sally Quartz was piece of quartz rock located in a layer of granite deep inside the earth. Millions of years went by, and rain washed away the layers of dirt and rock above her. With the weight of crust removed, Sally gradually moved closer and closer to the surface of the earth. One day a river wore through the final layer of rock, and Sally was finally out in the sunlight. Thousands of more years went by. The sun was hot. Hard rain and cold snow came and went.

I then asked what they predicted would happen next.

Finally, a raindrop hit a little crack in Sally. She broke off the rock and became a grain of sand. Sally Sand then floated down the river, landing on a nice, soft beach.

Next, I pulled out a marshmallow and added it to a baking pan packed with other marshmallows already assembled. I briefly explained that it represented a grain of sand, checking to see if the students were able to make the connection of where grains of sand come from (small bits of rock).

Thousands of more years went by and the river dried up. The sun baked the sand like an oven. Minerals began to form between the grains of sand and Sally turned into sandstone.

I laid a piece of wax paper on top the pan of marshmallows.

Another million years went by, and the big wide ocean rose up to cover and press down on Sally Sandstone. Year after year new blankets of sand and layers of shells settled on top of her and the surrounding rock. Millions of more years went by and eventually Sally lay deep beneath the ocean floor. The pressure from the weight on top slowly changed Sally Sandstone into a stronger, harder rock called quartzite.

I then asked the students to come up, two at a time, and press down on the marshmallows in the pan. I asked them to predict what they thought would happen as they pressed the marshmallows. If you're a trusting soul, you can also pass out baggies with marshmallows inside to each table group and allow students to take turns pressing down on the marshmallows for 15 seconds each. After adding one more layer of plastic, I allowed each group of students to stomp on their baggies of marshmallows for 30 more seconds.

Then we looked at the results of our experiment and discussed ways the marshmallows were the same as before and ways they were now different. We further explored the idea of pressure, heat, and time by pressing our palms together and feeling the warmth that is generated. Then it was back to our ever-changing friend, Sally.

But wait! Sally Quartzite wasn't sitting still. She was a part of the ocean floor that was slowly moving, inch-by-inch, year-by-year, back under the crust of the earth.

I then slid the pressed marshmallows from their baking pan into the saucepan, added the butter, and turned the heat on low. You can also add red food coloring if you desire to simulate lava. Again I asked the students what they thought would happen next.

Finally, Sally melted and joined back up with the molten rock deep under the ground. There she stayed waiting to emerge from the earth as a new rock. Next time, however, she might continue the rock cycle as Sally Lava and burst out of a thundering volcano…only time would tell.

While the students took a few moments to write in their science journals about what happens to rocks, I called up the groups, one at a time, to watch the marshmallows melt for a few minutes. During their observation, we discussed where they thought the heat came from that created molten rock (the pressure of the gravity exerted by the earth). As the marshmallows melted, I added the remaining fudge ingredients from the recipe on the marshmallow bag (all except for the chocolate chips). After the students had all been up, I added the chocolate chips, and transferred the mix to the greased baking pan to cool. I then wrapped up by reviewing the main elements of the story — mainly through sharing what students had written in their journals.

Sally started out as a piece of quartz inside a layer of granite, which is called igneous rock. Then, due to erosion, Sally broke off and turned into sand or sediment. When the sand did not move for a long time, the beach turned into a sedimentary rock called sandstone. The ocean then covered the sandstone, pressing down on it for a long time, which turned the sandstone into a metamorphic rock called quartzite. Finally, the movement of the crust of the earth pushed the quartzite back under the crust, turning it into molten rock that, in time, will be used to form a new igneous rock. The tray of marshmallows was like sedimentary rock. The smashed marshmallow was like metamorphic rock. And, the cooked marshmallows would eventually cool and be like igneous rock.

The entire lesson took 45 minutes, although set-up and clean-up added extra time. After class, I poured the leftover marshmallows and chocolate chips into the second pan, and scraped the "metamorphosed" marshmallows from the students' bags into a third pan. The fudge took a little longer to cool, but by the following day, I had three great (if not edible) examples of the three types of rocks to review in class. Before the end of our experiment, however, there was still one more procedure to conduct. Students simply could not go home without eating their marshmallow sediment — somehow sand never tasted so good!

Dr. Sandra L. Hunt earned her Ph.D. from the Department of Speech Communication at the University of Washington in 1996. She began her education career in the mid 1980s, training teachers in a State Department-funded refugee training program in Thailand. Currently, she is student teaching in a third grade classroom with the Seattle School District. She will earn her Master's Degree in Teaching in June, 2001.

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