Desert Landforms - Sand Dunes

Sand Dune - any hill of loose sand shaped by the wind
Constantly changing shape - reworked by wind

Conditions for Dune Formation

There must be a source of sand
- sandstone deposit
- erosional material supplied by streams
- beach deposits
Area must be vegetation free or nearly free of vegetation
Vegetation will cause a dune to become inactive or modifiy its shape

Sand

Quartz and feldspar are the most common minerals found in sand
Color of the sand indicates composition
Common sand - mostly quartz, some feldspar
Black sand - basalt fragments, olivine, pyroxene, plagioclase
White sand - pure quartz or gypsum

Sand Movement- Saltation

1. Wind rolls sand grain along until it bounces or is aerodynamically lifted into the air
2. Wind drives airborne grain forward until it falls back to the ground
- bounces into air again
- kicks a second grain into the air

Dune Formation

The stronger the wind the more saltation
Saltation moves grains along the ground
Irregularities in surface form areas where the wind is blocked - wind breaks
Moving sand will settle into these breaks

Dune Anatomy

Backslope - slope of a dune which faces into the wind
Crest - sharp ridge at the top of a dune which separates the backslope from the slipface
Slipface - leeward side of the dune
Horn- the pointed end of a dune

Dune Migration

Wind moves sand grains up the backslope
Grains are deposited at the crest or fall over the crest and deposited on the slipface
Crest becomes unstable - mass wasting moves sand down the slipface
Sand accumulates on the slipface
Over time the entire dune moves as grains are moved from the backslope to the slipface
Dunes are moved by the wind one grain at a time.

Ripplemarks

Small dune forms
Parallel ridges a few centimeters high
Generally form perpendicular to wind direction

Dunes

Nine major dune forms
Barchan
Transverse
Star
Parabolic
Longitudinal (Linear)
Coastal Blowout
Barchanoid
Dome
Reversing

Barchan Dunes

Crescent shaped
Horns point in the direction of wind movement
Develop on flat, barren surfaces

Transverse Dunes

Wave-like, straight crested dunes
Wind direction is perpendicular to crest

Star Dunes

Multipointed star forms
Wind direction is variable
Tend to remain stationary for long periods of time
Common in Subtropical deserts

Coastal Blowout Dunes

Horseshoe shaped
Wind blows into open end of the horseshoe
Occur on coastal areas with strong onshore winds
Vegetation anchors the horns of the dune

Parabolic dunes

Parabolic curve shape that is similar in shape to Coastal Blowouts
Found in semiarid plains with sparse vegetation
Vegetation anchors the horns
Wind direction is into the open end of the parabola

Longitudinal dunes

Long, narrow, parallel ridges
Wind direction is parallel or subparallel to the crest

Sand Seas

Large areas covered by loose, blowing sand
Erg - Arabic term for a sand sea

Loess Deposits

Wind transported silt deposits
Generally the silt is formed due to glacial erosion
Provide rich soil - but is easily eroded by wind and water if vegetation cover is removed

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