Sand dunes form as a result of wind and water movement and often form because the sand begins to rest against a tree or some other object. Some dunes shift as much as 100 feet a year if the winds are strong enough. You can use simple experiments to teach your chld how dunes are formed and what causes them to shift.
Explore why dry sand is carried so easily by the wind. Use a cup of dry sand, a shallow pan, a straw and a spray bottle filled with tap water. Shake the sand in a pan so the sand is even. Hold the straw on an angle about two inches above the sand's surface, and gently blow out. Next, spritz the sand with water from the spray bottle and blow the sand through the straw again. Wet sand doesn't move since it's heavier and tends to stick together.
When dunes are buried under large amounts of sand for a long time, the sand is compressed and changes into sandstone. The color of sandstone varies depending on the materials, leading to different colors for different layers. To create a jar of sandstone, your child needs salt, four small bowls, food coloring and a jar. Pour four tablespoons of salt into each bowl. You can to use more salt for a bigger jar. Add food coloring, using about ten drops per bowl, and mix. She should stir the salt once an hour until it's completely dry. Next layer the salt in the jar, alternating the colors. It's important not to tip or shake the jar since the layers will mix. Now she has her own bottle of sandstone-like material, showing her how the layers build over time.
Sand Dune Erosion
This experiment deals with erosion and its impact on dunes. Use red and blue food coloring, a liter of sand, the top from a box or a shallow pan and wind from a blow dryer or fan. Dye and dry the sand in advance: blue in one cup of sand and red in the other. Place the box on a flat surface and create a 2-centimeter high red sand dune in a straight line, about 8 centimeters wide. Pour the blue sand beside it, and then use the uncolored sand for the next two rows. Place the fan on low next to the red sand. If your child is using a blow dryer, turn the heat off. Try different speeds and see how the results change. She can also spray the sand with water to see how that impacts the erosion.
Teach your child about slopes and how gravity and the interaction of particles impacts the angle of rest (the angle of repose). This is the force that controls the angles on sand dunes. She'll need a glass jar with a lid and sand. Fill the jar until it's about half full. Put the lid on, lay it on its side and roll it along the floor. As the sand builds up, it forms the angle of initial yield. Then the sand avalanches down the slope and creates the angle of repose.