Splat! How do Raindrops Affect Rocks?

Problem

How do raindrops affect rocks?

Materials

• 1 teaspoon (5 ml) flour
• Sheet of black construction paper
• Eyedropper
• Tap water
• Yardstick (meterstick)

Procedure

1. Place the flour in a mound in the center of the paper.
2. Fill the eyedropper with water.
3. Hold the dropper about 12 inches (30 cm) above the center of the flour.
4. Squeeze 2 drops of water onto the mound of flour.
5. Observe the paper.

Results

You see a burst of tiny specks of flour on the paper.

Why?

The impact of the falling water forces particles of flour outward. Raindrops hitting the surface of rocks behave similarly to the water drops hitting the flour. Raindrops may fall thousands of yards (thousands of meters) before hitting the ground. The force of the raindrops striking weathered rock can move loosened rock particles.

If rain or other precipitation (rain, snow, or other moisture falling from the sky) does not soak into the ground or evaporate, it may form runoff (water that flows across land). Runoff picks up and carries away the rock particles loosened by weathering. The moving of rock materials by an agent of erosion (something moving, such as water, ice, or wind, that can transport sediment) is called erosion. Over a period of time large amounts of rock particles may be eroded.

Let's Explore

1. Does the distance the water drops fall affect the results? Repeat the experiment twice, first holding the eyedropper about 6 inches (15 cm) above the flour, then placing the paper on the floor and holding the eyedropper about 3 feet (1 m) above the flour.
2. Does particle size affect the results? Repeat the original experiment, replacing the flour with a larger grained material such as cornmeal or sand. Science Fair Hint: Take photographs of the construction paper after each experiment and use them to show the results.

Show Time!

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   1. Gravity is the force that pulls everything toward the center of the earth. Gravity pulls rocks on a sloping surface downhill. Thus, erosion of rocks can be caused by gravity. The movement of earth materials and rock caused by gravity is called mass wasting. When rock fragments fall from cliffs or bounce down gentle slopes, they hit other rocks and cause rock slides (the sudden downhill sliding of rocks). Demonstrate a rock slide by placing a 6-by-12-inch (15-by-30-cm) piece of cardboard in a baking pan slightly larger than the cardboard. Place cup (63 ml) of small stones or aquarium gravel in a mound at one end of the cardboard. Slowly raise the mound end of the cardboard about 6 inches (15 cm) or until a few stones slide downhill. Support the cardboard at this height by pressing a block of clay onto the rim of the baking pan. Hold 2 to 3 stones about 12 inches (30 cm) above the mound on the cardboard. Drop the stones.
   2. Does the height of the falling stones affect the results? Repeat the previous experiment twice, first holding the stones at a height of 6 inches (15 cm) above the mound, then holding the stones at a height of 2 feet (60 cm).
   3. Heavy rain can trigger a rock slide. Demonstrate this by repeating the experiment again, but this time instead of stones, hold a paper cup with water above the mound. While holding the cup, ask an adult to punch two to three holes in the bottom of the cup with a pencil.
2. Another way erosion occurs is by abrasion (a process by which rock is worn away by sediments in an agent of erosion). Sand grains carried by wind, water, and ice can pit and polish the surfaces of rocks. The grains of sand act like a file as they strike and cut away the rocks' surfaces. Simulate abrasion by rubbing a nail file back and forth across a piece of chalk. Prepare a display showing the chalk before and after it was eroded by abrasion.

Check it Out!

1. The most common way for rock materials to be moved is by gravity. Use earth science texts to find out more about erosion by gravity. What is creep? What type of slope is more likely to produce creep? What is talus?
2. As glaciers move, they drag along rocks. These rocks carried by the glacier scrape against the land, cutting into exposed rocks. Find out more about glacial erosion. What is till? What are the different types of moraine formed by deposits of till?

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