Universe Cycle - Earth (4)
Pre Lab 

  • Exploring wind erosion and the formation of sand dunes
  • Comparing surface processes on the Earth and Moon.
  • erosion
  • saltation
  • soil
  • wind
  • cross bed
  • worksheet

Students discover how wind transports sediment on the Earth’s surface

Wind-blow sand


Water is a very powerful erosional force on the surface of the Earth. Flowing water, as a liquid or solid (ice), is very effective at eroding and transporting loose material from high places to low places. Mountains are worn down, and valleys, lakes, and part of oceans are filled in.

Wind is also important in shaping the Earth’s surface. Wind is caused by the movement of gas molecules in the Earth’s atmosphere, in response to changing atmospheric pressures. Wind is not as effective or "violent" an agent of erosion as are ice and water, but over time it can transport much material and finely sculpture a landscape. Wind is capable of transporting loose, unconsolidated fragments of sand and dust. Larger particles are too heavy for normal winds to move, except in storm conditions.

Normal winds are barely strong enough to move sand, so this material moves by an intermittent series of jumps or skips along the Earth’s surface. This transport process is called saltation. Dust particles are light enough that they can be picked up and fully transported by the wind. This allows dust to blow long distances, even across wide ocean basins.

Sand dunes are mounds of sand that have been piled up and transported by the wind. They commonly form in dry Earth surface environments. Viewed from one end, sand dunes have a flat back side and a steep front side. The wind blows up the back side of the dune. If the wind blows fast enough, it will pick up and transport sand grains up the back of the dune. When the wind goes over the top of the dune, it slows down. This causes it to drop the sand grains it was carrying. The sand lands on the steep front of the dune. This forms an inclined layer of sand, called a cross-bed. The cross-bed gets flatter at the bottom because some of the sand rolls down to the bottom of the dune and piles up. As more and more sand blows through an area, many cross-bedded layers of sand accumulate. This process causes a sand dune to migrate down wind. Sand is continually eroded from its upwind side, and deposited on its downwind side.

Water and wind are both effective surface processes on the Earth. The Earth is just the "right" distance from the Sun that liquid water is abundant on its surface. The reasons for this are complex. Essentially, the amount of solar radiation that the Earth receives, combined with the concentrations of greenhouse gases in the atmosphere, keep the temperature in the proper zone for water to be liquid. If the Sun were cooler or if there were less greenhouse gases, ice would be more abundant. If there were more of either of these, much liquid water would evaporate. Wind erosion and transport is common on the Earth because it has an atmosphere.

In contrast, neither water nor wind are effective on the Moon. It lacks an atmosphere, so there is little wind to blow on the lunar surface. The Moon also has little if any water on its surface. Any water that may be present is frozen. At present, the main factors that shape the lunar landscape are impacts and the formation of craters, and occasional landslides caused by gravity.

  1. Introduce students to wind erosion and transportation. Make sure the students understand that an atmosphere is necessary for wind to blow. Explain saltation, and the formation of sand dunes. You may wish to have them complete the first three questions on the worksheet as you lecture.
  2. Have the students speculate about the presence or absence of wind erosion on the Moon. Discuss their answers as a group, and have them complete the worksheet.
  3. ANSWERS: 1. and 2. see the drawings above. 3. No, because without movement of the wind, sand would not be piled up into dunes. Water and ice might move the sand from place to place, but would not make sand dunes. 4. No, because the Moon does not have an atmosphere.

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