Experiment 12.1: A Simulation of Plastic Rock

Student Information

Purpose

To explore the properties of plastic rock by producing a mixture that simulates such behavior.

Background Information

Most theories that describe Earth's mantle have plastic rock moving about in huge currents in the asthenosphere. The asthenosphere is a layer of the Earth's mantle that behaves like warm taffy candy, flowing around in the mantle. It's important to note that these rocks in the mantle are not liquid, just as taffy is not liquid. When subjected to an abrupt force, the flowing rocks can harden into a firm solid. After the force passes, the rock returns to its flowing state.

Below the asthenosphere is the mesosphere. The rocks of the mesosphere are stiffer than those of the asthenosphere. Despite being stiffer, these rocks can still flow only much more slowly. Scientists call these rocks "stiff" plastic rock.

This experiment will help you understand how plastic rock behaves by creating a cornstarch and water mixture that exhibits similar properties.

Materials

Safety

IMPORTANT: Always wear eye protection during this experiment. While cornstarch is not particularly dangerous, it's good practice to protect your eyes during any science experiment.

Be careful with cleanup - the mixture can clog drains if not properly diluted. Follow the cleanup instructions carefully.

Procedure

  1. Put one cup of cornstarch into the pan (237mL or 8oz).
  2. Add 2/3 cup of water (158mL or 5.3oz).
  3. Stir the water and cornstarch together. You should find it very hard to stir at first, but as you continue to stir, you should get to the point where you have an evenly mixed, white liquid in the pan.
  4. Add an additional 1/4 cup of cornstarch to the mixture.
  5. Stir the newly added cornstarch into the mixture. Now you should start seeing an interesting behavior: As you stir, the mixture should solidify, making it hard to stir. However, when you stop stirring and let the mixture sit, it should liquefy again. If you don't see that behavior, your mixture is not correct. If the mixture stays liquid, you have too much water. To fix this, add cornstarch a little at a time until you get the desired behavior. If the mixture never turns to liquid, you have too much cornstarch. Add water a little at a time until you get the desired behavior.
  6. Explore this interesting behavior for a while. Describe your observations in the data table.
  7. Pick up some of the mixture with your hand. As you pick it up, it should become solid.
  8. Squeeze the solid into a ball, then relax your hand and watch what happens to the ball.
  9. Repeat this a few times. Describe your observations in the data table.
  10. To clean up everything, you will need to use hot water from the tap. As you add hot water to the pan, the mixture will become a liquid. Continue to add water until it is very fluid and you can safely pour it down the drain. Make sure to leave the hot water running for several minutes after the mixture has been cleaned up, as you need to flush it down the drain. Failure to do so could clog your drain.

Data Collection

Observation Point Description of Behavior
Initial mixing behavior
Behavior when stirring
Behavior when still
Behavior when picking up
Behavior when squeezing into a ball
Behavior when relaxing hand

Analysis Questions

1. How does the cornstarch mixture's behavior relate to plastic rock in the Earth's mantle?

2. What happens to the mixture when force is applied? How is this similar to what happens in the asthenosphere?

3. How does the behavior of the mixture help explain the difference between the asthenosphere and mesosphere?

4. Why do you think scientists use models like this to understand Earth's interior?

Conclusion

Did this experiment help you understand the properties of plastic rock? Explain your observations and make connections to the text.

Extension Activities

Try these additional activities to further explore the properties of non-Newtonian fluids like your cornstarch mixture:

  1. Try placing a small speaker playing bass-heavy music under a thin layer of the mixture. Observe how the vibrations affect the fluid.
  2. Try different ratios of cornstarch to water and record how the behavior changes.
  3. Research other examples of non-Newtonian fluids and how they are used in real-world applications.