How does deposition by wind modify landscapes?
On the right is a desert mountain in Arizona. The surface in the foreground is desert pavement. How did wind modify this landscape? On the left is a desert mountain with sand dunes in Death Valley, California. How did wind modify this landscape? Erosion and deposition by wind leave very different landscapes behind.
Like water, when wind slows down it drops the sediment it's carrying. This often happens when the wind has to move over or around an obstacle. A rock or tree may cause wind to slow down. As the wind slows, it deposits the largest particles first. Different types of deposits form depending on the size of the particles deposited.
When the wind deposits sand, it forms small hills. These hills are called sand dunes (Figure below). For sand dunes to form, there must be plenty of sand and wind. Sand dunes are found mainly in deserts and on beaches.
A runner strides across sand dunes. Sand is picked up by her foot as it leaves the dune.
How Sand Dunes Form
What causes a sand dune to form? It starts with an obstacle, such as a rock. The obstacle causes the wind to slow down. The wind then drops some of its sand. As more sand is deposited, the dune gets bigger. The dune becomes the obstacle that slows the wind. This causes more sand to drop. The hill takes on the typical shape of a sand dune (Figure below).
A sand dune has a gentle slope on the side the wind blows from. The opposite side has a steep slope. This side is called the slip face.
Migration of Sand Dunes
Once a sand dune forms, it may slowly migrate over the land. The wind moves grains of sand up the gently sloping side of the dune. This is done by saltation. When the sand grains reach the top of the dune, they slip down the steeper side. The grains are pulled by gravity. The constant movement of sand up and over the dune causes the dune to move along the ground. A dune moves in the same direction that the wind usually blows. Can you explain why?
When the wind drops fine particles of silt and clay, it forms deposits called loess (Figure below). Loess deposits form vertical cliffs. Loess can become a thick, rich soil. That’s why loess deposits are used for farming in many parts of the world.
Loess hills in Missouri are home to the Squaw Creek Wildlife Refuge.
Fine-grained mud in the deep ocean comes from silts and clays brought from the land by wind. The particles are deposited on the sea surface. they slowly settle to the deep ocean floor, forming brown, greenish, or reddish clays. Volcanic ash may also settle on the seafloor.
- The sand is transported until it is deposited in a sand dune.
- Sand is blown up a slope. Gravity pulls it down the other side. This is how dunes migrate.
- Loess is very fine wind-blown deposits.
Use the resource below to answer the questions that follow.
- Wind Erosion and Deposition at http://www.youtube.com/watch?v=9pi0isxZfcg (6:42)
- What size particles can wind move and why can't it move other sizes?
- Why does wind erosion and deposition occur mostly in deserts?
- Why does a stony pavement form in some desert areas?
- What is a ventifact? Why is the ventifact in the video narrower at the base than the top?
- What are sand dunes? Why are they usually made of quartz?
- What is cross bedding?
- Describe how wind deposits sediments as it slows.
- Describe how sand dunes move.
- What is loess?