DESERTS AND WIND ACTION

The photo shows a group of active star dunes in the desert--blowing sand can etch windows and shifting sand dunes can interfere with buildings and airports in desert regions

I. Desert regions and wind erosion

A. Desert and adjacent regions

1. Deserts

• are those regions on Earth which are categorized as having arid climates

2. Steppes

• are those regions adjacent to deserts categorized as having semiarid climates--steppes separate arid and humid climates
• click here to see the Earth climate zones

B. Wind erosion processes

1. Deflation

• is the removal of rock waste from the land by the wind
• a blowout or deflation hollow is a depression excavated by the wind in easily eroded materials such as sand or silt deposits with little or no moisture or vegetation content--vegetation present in materials at the surface impedes deflation
• click here to see a blowout

2. Abrasion

• is the grinding and scraping of a rock surface by the friction and impact of rock particles carried by the wind--it results in a "sandblasting" effect of the rock causing the rock to be polished and smooth

C. Wind erosion loads

1. Suspended load

• suspended materials in the wind consists primarily of silt or clay size particles and if highly concentration can result in dust storms--the deposition of the suspended load can form large deposits which lack layering called loess

2. Bed load

• refers to rock particles which are transported by a bouncing effect and consist primarily of sand size particles--the accumulation of this type of load forms the desert sand dunes (active dunes)

II. Active sand dunes

A. Nature of active (unstable) dunes

1. Dune profile

• the dune slopes upward in the direction of the wind and is called the windward slope--the slope then breaks downward sharply to a much steeper slope called the leeward slope or slip face
• click here to see the dune profile

2. Shifting dunes

• active dunes can readily shift positions down wind and bury roads, airport runways, homes, etc.

B. Categories of active dunes

• based on the shape of the dune in the map or plan view--click to see page 320(342) after discussion of active dune types

1. Barchan

• is a crescent shaped dune with the convex portion facing the wind direction and formed when the wind direction is constant and the sand supply is limited--see page 320-A(342A)
• click here to see photo of barchans

2. Transverse

• is a dune elongated perpendicular to the direction of the wind and formed when the wind is constant and there is a large supply of sand--see page 320-B(342B)

3. Longitudinal or Seif

• is a dune elongated in the general direction of the wind and formed from slightly different wind directions with a limited supply of sand--see page 320-D(342D)
• click here to see photo of longdunes

4. Star or complex

• is a dune formed when the direction of the wind is variable--see page 320-F(342F)
• click here to see photo of star dune

• click here to see and review the sand dune types

III. Inactive or stable

• the parabolic dune or fore-dune is the only type of stable dune--typically this type of dune is formed along the coastlines of oceans or large lakes--strong winds cause blowouts in the sand deposits along the coastline resulting in a crescent shaped dune whose concave side faces the direction of the prevailing wind
• return to see this type of dune

IV. Evolution of a desert landscape

A. Alluvial fans

• are fan shaped sediment deposits formed at the foot of mountains in the desert valley or basin--if there is a coalescence of alluvial fans an apron of sediment called a bajada is formed

B. Playa lake and playa

• a (shallow) playa lake may form in the central basin of a desert from abundant rainfall on rare occasions-- because of evaporation and infiltration the water in these lakes are present for only a few days or weeks--the dry flat lake bed that remains is called a playa

click here to see a photo of a bajada and playa lake

C. Inselberg

• is an isolated erosional remnant formed after long term erosion of the mountain areas which are reduced to these bedrock knobs elevated above the surrounding sediment-filled basin

click here to seethe stages of landscape evolution in mountainous desert regions (A=early stage, B=middle stage, C=late stage)

click here for more information on deserts

page 307 click to return

page 314 click to return

page 318 click to return

page 320 click to return

barchan dunes click to return

longdunes click to return

stardunes click to return

bajada and playa lake click to return

page 322 click to return