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Weathering of Materials and Formation of Sand Dunes

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Author: Janice VanCleave

Erosion is the process by which materials of Earth's surface are broken down and carried away by natural agents, such as wind and water. The part of erosion that involves only the breakdown of materials into smaller parts is called weathering. In some deserts, winds are strong enough to lift and suspend hundreds of pounds (kg) of sand for days at a time. The sand grains hit and grind against all surfaces they touch. The soft parts of structures are weathered first, and these parts fall or are blown away by the wind. Grinding, sand-filled winds create holes in rocks that often become homes for desert animals. Desert landforms created by blowing wind include arches and caves.

While deserts in different locations have different ground coverings, both hot and cold deserts have ground coverings that include sand, coarse or sandy soil, and pebbles. Erosion occurs in both hot and cold deserts. Where sand is present and winds are strong and blow from the same direction, hills of loose sand called sand dunes are formed. Areas with many sand dunes are called dune fields, which exist in hot deserts such as the Kalahari in southwestern Africa, the Arabian desert in the Arabian Peninsula, the Sahara in northern Africa, and cold deserts such as the Atacama along the coasts of Peru and Chile and the Takla Makan in western China. Sand dunes can change size as well as migrate (move to another location). Dunes on the hot Sahara desert can be over 800 feet (240 m) high. Some dunes migrate as much as 100 feet (30 m) per year, depending on the speed and duration of the wind. Faster winds and consistent winds cause higher dunes and farther migrations.

Purpose

To determine why dry sand is easily carried by wind.

Materials

  • 1 cup of dry sand
  • shallow baking pan
  • drinking straw
  • spray bottle
  • tap water

Procedure

  1. Pour the sand into the pan.
  2. Shake the pan so that the sand is as smooth and even in the pan as possible.
  3. Hold one end of the straw at an angle to, and about 2 inches (5 cm) above, the surface of the sand.
  4. Gently blow through the straw. You want to direct your exhaled breath toward the sand. Caution: Remove the straw from your mouth before inhaling.
  5. Fill the spray bottle with water. Then wet the surface of the sand by spraying it with water.
  6. Repeat step 4.

Sandy Deserts

Results

The dry sand moves, forming small mounds, but the wet sand does not move.

Why?

Wet sand is heavier than dry sand. The lighter, dry sand grains are more easily picked up and carried by wind, forming sand dunes when dropped. In addition to the extra weight, wet sand sticks together, so it's harder for the wind to move it.

More Fun With Sand!

Some ancient sand dunes that were buried under more sand have changed into sandstone. Sandstone is a type of rock made from a buildup of layers of sand. Over time, as more sand is deposited, the layers are compressed (pushed together) and cemented, forming sandstone. Sandstone comes in a variety of colors, depending on the materials the sand is made of. The painted desert in Arizona is made of different colors of sand that have over time formed mountains of sandstone. You can make a model of layers of colored sand forming sandstone by putting layers of colored salt in a small jar. First, make four colors of salt. Measure 4 tablespoons (45 ml) of salt into four small bowls. Add 10 drops of one color of food coloring to one bowl of salt. Repeat with three more colors. Stir the salt every hour, or as often as possible until it dries. Spoon some of the salt into a small jar. Then add a layer of another color. Keep adding layers of colored salt until the jar is full. You can add designs to the colored layer by opening a paper clip and pushing its end into the salt right alongside the glass. Do not stir! Add more salt if needed to fill the jar, then put the lid on tight. Keep the jar upright and do not shake.

Sandy Deserts

 

Book List

  • Bernard, Robin. Deserts. New York: Scholastic Professional Books, 1995. Information and hands-on activities about desert plants and other organisms and topics.
  • NatureScope: Discovering Deserts. Washington, D.C.: National Wildlife Federation, 1989. Information and activities about deserts.
  • VanCleave, Janice. Earth Science for Every Kid. New York: Wiley, 1991. Fun, simple earth science experiments, including information about erosion and sand dunes.
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