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Splitters: How Can Seeds Break Rocks Apart?

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Author: Janice VanCleave

Problem

How can seeds break rocks apart?

Materials

  • 4 tablespoons (60 ml) plaster of paris
  • two 3-ounce (90-ml) paper cups
  • tap water
  • craft stick
  • 4 pinto beans
  • marking pen
  • masking tape
  • 2 paper towels

Procedure

    NOTE: Mix the plaster in a throwaway container. Do not wash the container or the craft stick in the sink, because the plaster can clog the drain.

  1. Place 2 tablespoons (30 ml) of plaster in each cup.
  2. Add 1 tablespoon (15 ml) of water to each cup and stir with the craft stick. Discard the stick.
  3. In one of the cups, stand the 4 beans as far apart as possible on the surface of the wet plaster. Push the beans into the plaster so that about three-fourths of each bean is below the surface of the plaster.
  4. Splitters

  5. Use the marking pen and tape to label the cup with the beans Test and the cup without beans Control.
  6. Record the appearance of the surface of the plaster in each cup.
  7. Fold each paper towel in half twice. Wet the folded towels with water so that they are moist but not dripping wet.
  8. Splitters

Results

The plaster in the Test cup cracks.

Why?

In this experiment, the plaster in the Control cup does not crack, showing you that it is the growth of the beans that causes the plaster in the Test cup to crack. As the beans grow inside the plaster, they expand, which applies pressure to the plaster. This pressure causes the plaster to crack. The same process can occur when a seed falls into a crack in a rock. The growing seed and its roots push against the rock, forcing the crack to widen and deepen. Eventually the rock can break apart. The breaking down of rock into smaller pieces by natural processes is called weathering. If the rock weathers but there is no change in the chemical composition of the rock, the process is called physical weathering.

Let's Explore

  1. Does the size of the mock rock affect the results? Repeat the experiment twice, first using half as much plaster and water, then using twice as much plaster and water. Science Fair Hint: Draw diagrams daily of the surface of the plaster. Use the diagrams as part of a project display.
  2. Does the amount of water on the surface of the mock rock affect the results? Repeat the original experiment twice. The first time, use dry paper towels instead of wet ones and do not add water. The second time, cover the surface of the plaster with about 1 inch (2.5 cm) of water instead of towels.
  3. Do different seeds affect the results? Repeat the original experiment, using different seeds such as those of lima bean, squash, zinnia, and mustard. Test each type of seed separately. For small seeds, push the seeds into the plaster so their surface is level with that of the plaster.
  4. Demonstrate the effect of plant roots that move into cracks in rocks by repeating the original experiment, but this time do not press the beans into the surface of the plaster. Science Fair Hint: Take photographs each day of the plaster's surface and display them along with textbook photographs of plants growing from split rocks.
  5. How does the composition of the mock rock affect the results? Repeat the original experiment, replacing the plaster and water with modeling clay.

Show Time!

Water that seeps into cracks in rocks and freezes can also cause the rocks to split. This occurs because water expands as it freezes. The expanded ice acts as a wedge, widening the crack in the rock. The weathering of rock as a result of repeated freezing and thawing of water is called frost action. In time, frost action breaks rocks into smaller and smaller pieces. Demonstrate the expansion and force of water as it freezes by completely filling a drinking straw with water. Plug both ends of the straw with small pieces of modeling clay. Neither plug should extend past the ends of the straw. The water inside the straw should make contact with both clay plugs. Place the straw in a freezer. Observe the position of the clay plugs after 24 hours. Display a diagram showing the position of the clay plugs before and after freezing.

Splitters

Check it Out!

Physical weathering can also be caused by extreme temperature changes. Use an earth science text to find out more about physical weathering. How can burrowing animals such as earthworms, ants, and moles cause rocks to weather? What is exfoliation?

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