Rocks and Minerals > Rock Cycle Activities

Metamorphic Rocks: Conglomerate Cookies Become Metamorphic Munchies

by Deborah Hanuscin

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Concept: Metamorphic rock formation

Metamorphic rocks are formed from sedimentary, igneous, or other metamorphic rocks. These rocks are subjected to intense heat and pressure. The heat and pressure are not quite enough to melt the rock, but they do cause the minerals in the rock to change in size or shape, or to combine with other minerals to form new or bigger minerals.

Objective:

Students will be able to explain how metamorphic rocks form.

Materials:

• toothpicks
• paper plates
• cookie recipe:
  • 1 cup butter, softened
  • 1½ cup light brown sugar
  • ½ cup sugar
  • 2 eggs
  • 2 teaspoons vanilla
  • 2¼ cups flour
  • 1 teaspoon baking soda
  • ½ teaspoon baking powder
  • ½ teaspoon salt
  • 1 cup chocolate chips
  • 1 cup peanut butter chips
  • ¾ cup butterscotch chips
  • ½ cup nuts (your choice)
  • Directions: Preheat oven to 350° F. Cream butter and sugars. Beat in eggs and vanilla. Combine flour, baking soda, baking powder, and salt, and gradually add to creamed mixture. Stir in chips and nuts. Place in spoonfuls on greased baking sheets. Bake 10-12 minutes.

Procedure:

1. Have students mix the cookie dough according to the recipe. Explain that the ingredients represent various sediments (small bits of rock) that they are combining to form sedimentary rock. By mixing, they are compacting the sediments. This particular type of sedimentary rock would be considered a conglomerate. It is made up of large sediments like sand and pebbles (the chips and nuts). The sediments are so large that pressure alone cannot hold the rock together, but they are cemented with dissolved minerals (the dough).
2. Ask students what will happen when you add heat to this mixture. Tell students that when sedimentary rocks are exposed to intense heat and pressure, they become metamorphic rocks. The heat and pressure are not quite enough to melt the rock, but they do cause the minerals in the rock to change in size or shape. Some minerals may be flattened, while others combine with surrounding minerals to form new or bigger minerals. While you will not be adding pressure to the cookies, you are adding heat with the oven. Spoon the dough onto the cookie sheets and bake.
3. When the metamorphic "rocks" are done, place them on a plate and ask students to "mine" them with toothpicks.

Questions:

1. What changes do you notice in the "minerals" of the rock?
2. Have some minerals changed and not others?
3. How are metamorphic rocks formed?

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