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Rock Sandwiches: Layering of Earth Particles

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Author: Janice VanCleave

Sedimentary rocks are formed from fragments of weathered rocks. The fragments from these rocks that are carried from one place to another by wind, water, ice, and gravity are called sediments. Over millions of years, thick layers of sediments have collected, and their weight has contributed to the compression of the sediments into sedimentary rocks.

In this project, you will demonstrate the formation of sediment layers in sedimentary rock. Lithification, which involves processes that change sediments into clastic rock, are studied and modeled. You will use models to distinguish between the rock fragments making up conglomerate and breccia. You will also demonstrate the formation of the evaporite halite, a chemical sedimentary rock.

Getting Started

Purpose:   To demonstrate how horizontal layering can occur in sedimentary rocks.

Materials

  • Scissors
  • Ruler
  • 2-liter plastic soda bottle
  • 2 cups (500 ml) of tap water
  • 1-quart (l-liter) jar with lid
  • 1/2 cup (125 ml) each of flour, dry rice, and dry red beans (pinto beans work well)

Procedure

Rock Sandwiches: Layering of Regolith Particles

  1. Cut away the top 4 inches (10 cm) of the soda bottle. Keep the bottom section and discard the top.
  2. Pour 1 cup (250 ml) of water into the bottom of the bottle.
  3. Put the remaining 1 cup (250 ml) of water into the jar, then add the flour, rice, and beans. Secure the lid and shake to thoroughly mix the materials.
  4. Pour the contents of the jar into the bottle.
  5. Observe the initial appearance of the contents of the bottle, then make observations every 20 minutes for 1 hour and again in 24 hours (see Figure 15.1). Note: Keep the bottle for the following experiment.

Results

Initially the contents appear cloudy, with beans and rice mixed together in a layer on the bottom. With time, the flour settles, filling the spaces between the beans and rice in the bottom layer and forms a separate top layer.

Why?

The lithosphere is composed mainly of rock and regolith (the loose, uncemented rock particles, including soil, that cover the Earth.) Regolith that has been transported by agents of erosion and deposited in another place is called sediment.

The stirring and pouring of the water, flour, rice, and beans mixture into the bottle of water represents the transport of different-size sediments by fast-moving water into a stationary body of water. Sediments, like the food particles in this experiment, settle in stationary water. The settling of the food particles represents the formation of two distinct horizontal sediment layers. The time it takes a sediment to settle out of its transporting agent is called its settling rate. The settling rate of the rice and beans is faster than that of the flour. Thus, a lower layer of rice and beans forms first, then the flour fills in the spaces between the rice and beans, and forms a layer above the rice and beans layer.

Try New Approaches

What effect would the addition of more sediments have? Repeat the experiment adding a second mixture of water, flour, rice, and beans after 24 hours. Science Fair Hint: Take photographs of the bottle in both experiments and display them to represent the results.

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