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Erosion: The Breakdown and Movement of Crustal Material

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Author: Janice VanCleave

The physical nature of the Earth's crust is in a constant state of change. The changes are the result of erosion, the process by which the crustal material is broken down and carried away by wind, water, ice, and gravity acting against crustal material.

In this project, you will demonstrate erosion and weathering, the part of erosion that involves only material breakdown. The effects of surface area, composition of materials, humidity, oxidation, and temperature on weathering will be determined. You will also learn about the effect of agents of erosion, such as gravity and water.

Getting Started

Purpose:   To demonstrate erosion by water.

Materials

  • 1/2 cup (125 ml) of soil or coarse sand
  • four 12-ounce (360-ml) Styrofoam cups
  • tap water
  • spoon
  • cookie sheet
  • pencil
  • ruler
  • masking tape

Procedure

  1. Pour the soil into one of the cups, dampen with enough water to make thick mud, and stir.
  2. With your hands, mold the mud into eight equal-sized balls.
  3. Place the mud balls on the cookie sheet and either allow them to air-dry (which may take three or more days) or, with adult permission, bake them in an oven at 275°F (135°C) for 1 hour or until they are dry.
  4. Erosion: The Breakdown and Movement of Crustal Material

  5. Prepare the remaining three cups, to be called cups A, B, and C, as follows:
    • Use the pencil to make 8 to 10 holes around the bottom edge of cup A.
    • Use the pencil to make 12 holes in the bottom of cup B.
    • Fill cup C with tap water.
  6. Place the dry mud balls in cup A. Observe the shape of the mud balls in the cup.
  7. Set cup A in the center of the cookie sheet.
  8. Stand the ruler against the side of cup A and secure it to the cup with tape.
  9. Hold cup B 4 inches (10 cm) above cup A.
  10. Pour the water from cup C into cup B (see Figure 14.1).
  11. After the water has drained out of cup B, observe (1) the shape of the mud balls in the cup, and (2) the contents of the cookie sheet.

Results

The mud balls change shape. Parts of the balls are dissolved in the water, and parts are broken off. The dissolved and broken parts are washed out through the holes in the bottom of cup A, where the soil collects on the cookie sheet.

Why?

The mud balls are said to have eroded. Erosion is the process by which rock and other materials of the Earth's crust are broken down and carried away by natural agents, such as water, wind, ice, and gravity. The part of erosion that involves only the breakdown of crustal materials is called weathering. The crustal materials are broken into pieces by two weathering processes: chemical weathering, which affects the chemical properties of substances making up crustal materials, and mechanical weathering, which breaks the crustal materials by physical means. Chemical weathering is a chemical change, meaning the change produces one or more kinds of matter that are different from those present before the change. Mechanical weathering is a physical change, meaning the appearance of matter changes but its properties and makeup remain the same.

This experiment demonstrates one of the main causes of chemical weathering, the action of water. This part of the erosion process is the dissolving of some of the substances forming a solution. Another part is the mixing of the substances with the water forming a suspension (a mixture made of parts that separate upon standing). A suspension forms when substances mix with the water but do not dissolve. The water erodes the mud ball by first dissolving and mixing with the substances in the ball, then carrying the substances away.

The agents of erosion in this experiment are water and gravity. Gravity pulls the water down, and the water carries the dissolved materials and mixed substances down with it as it flows out of the holes in the cup. When the water stops moving, gravity pulls the undissolved materials in the water down, where they collect on the pan. Gravity pulls weathered particles to low places. Particles carried away by erosion are called sediments. The buildup of sediments is called deposition. Erosion is a wearing-down process, and deposition is a building-up process.

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