Daily Dose of Destruction
Like all erosive processes, wind erosion is very powerful and given enough time, very destructive. When wind blows across loose sediment, it removes small particles like clay, silt, and sand, leaving the courser heavier sediment behind. The ground actually get lower over time and those heavier sediments become quite firm in the ground. This process of removing lighter sediment is called deflation. The harder ground left behind can be so firm that roads can be built on it and in fact is called desert pavement. As wind blows sediments around, the sediments strike other sediment chipping off pieces of larger rocks. This process of abrasion can smooth rock surfaces and actually polish some of them. It tends to take on a varnished appearance and is called desert varnish.
Some sediment that is blown rolls along the ground in a process called creep. If particle get blown into the air, but are too heavy to be suspended there, they fall back to the ground in another process called saltation. Some particles of sediment are so light that they fly in the wind and we call that suspension. Wind can carry clay and silt sized sediment miles away and into the atmosphere. As particles creep along the ground and jump into the air, they knock other sediment loose and it too begins to move in the direction of the wind. These process can be so persistent and effective that they can create a sand storm called a haboob. Haboobs are very common in Arizona during the monsoon season.
In order to combat wind erosion in agricultural areas, farmers and ranchers will plant windbreaks around their fields to try and slow the wind down. To keep beaches from eroding in the wind, they too use windbreak fences and plant grasses that hold the sand in place slowing the erosion down.
When wind loses its energy the sediment stops moving and gets deposited on the ground. Sometimes over many years, fine grained windblown sediment like silt and clay have piled up in huge layers that are called loess. Many naturally fertile fields are fertile because of this loess that is made up of topsoil from another area.
Wind builds sand dunes. Sand dunes actually migrate over time, changing their locations in a process called dune migration. There are many prehistoric rock formations that are actually ancient sand dunes. Sand dunes start by sediment piling up against something that is blocking sediment transport, like a shrub, rock, or side of a hill. These sand dunes can grow very large because the higher they grow the more sediment can become trapped by them. You can tell the general wind direction, even ancient wind direction by studying and understanding sand dunes. The side facing the wind has the gentle slope. Dunes move away from the direction of the wind. As dune gets higher, its angle of repose will be reaches, it will collapse, and then grow again. We call the steeper face of the sand dune, the slip face.
There are several types of sand dunes.
- Barchan dunes: Crescent-shaped. Has "horns" that point the direction the wind is blowing. The slip face points away from the wind. Can get as tall as 9-30 meters high. Can migrate up to about 100 meters each year. They can be seen on Mars as well.
- Parabolic dunes: Also crescent-shaped but in the opposite direction. Wind is blowing into the crescent and the slip face is on the outside of the "horns." Vegetation holds the "horns" in place. There is one parabolic dune that has a 12 km "horn." Don't grow very tall. Maybe 10 to 20 meters.
- Star dunes: Has three or more slip faces. Has three ore more arms that move out from the center of the dune. Can grow very tall. Some have reached 500 meters.
- Transverse dunes: Very long. Sit at right angles to the direction of the prevailing wind. Formed in lines. Slip face is on the leeward side. Most beach dunes are transverse.
- Longitudinal Dune: Sits parallel to the prevailing wind. Tens of meters high. Some have been know to grow up to 100km long.
- Dome dunes: Looks like a dome with a lot of crests. No distinctive slip face.
One of the coolest places with sand dunes to go visit is the Great Sand Dunes National Park in Southern Colorado. Below is the Great Sand Dunes National park website. Check out the animation and read about how the national park was formed. There is an awesome geologic story to the park.
In the following lab you are going to take your new knowledge of sand dunes and model their shapes and demonstrate the wind direction. You know that you miss playing in the sand so here is your chance to relive your childhood.