Rocks and Minerals > Rock Cycle Activities

Igneous Rocks: Cooling Crystals

by Deborah Hanuscin

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Concept: Igneous rock formation

Continued rise in temperature can eventually melt any rock until it is molten (called a magma). When the molten rock cools it forms an igneous rock. The size of the grains (crystals) in an igneous rock will depend on the rate of cooling. The faster it cools, the smaller the crystals.

Objective:

Students will be able to explain the relationship between rate of cooling and crystal size.

Materials:

• goggles
• hot plate or burner
• heat-resistant glass beaker
• oven mitts or beaker holder
• water
• Epsom salts (or alum)
• spoon
• two baby food jars
• pipe cleaners
• magnifying lens

Procedure:

1. Heat beaker of water until near boiling.
2. Stir in Epsom salts until no more will dissolve.
3. Pour the solution into two baby food jars.
4. Wrap two pipe cleaners around a pencil to form them into a shape that looks like a stretched spring. Remove them from the pencil and place one inside each jar.
5. Place one jar in the refrigerator to cool, and the other in a place in the room where it will not be disturbed.
6. Allow the crystals to form overnight, then remove the pipe cleaners and compare.

Questions:

1. How do igneous rocks form?
2. What do you notice about the crystals from the two solutions you made?
3. Why do you think they are different? Which one cooled more slowly?
4. How might the crystals or grains in an igneous rock differ because of how quickly the molten rock cooled?
5. Examine several igneous rocks and compare the grains. Which do you think were cooled slowly? Why?

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