Page 558 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

522
522
par
t III The earth–atmosphere interface
T
sufficient supply of loose, dry sand or other fine particles unprotected by plant cover, wind erosion and deposition can build dunes (GIA 16.1). Prevailing winds, along with other factors, create dunes of many sizes and shapes (GIA 16.2).
he dramatic, sculptural shapes of dunes occur in a variety of settings: along shorelines, in sandy parts of deserts, and in semiarid regions. Wherever there is a
Sunset in the Sahara Desert [Galyna Andrushko/Shuttershock.]
DUNE PROFILE
Wind erosion and deposition work together to build a dune’s characteristic profile. A dune grows as wind-borne particles accumulate on the gentler, windward slope, then cascade down the steep slipface of the leeward slope.
Effective wind direction
Angle of repose:
The loose particles on the slipface tend to slip and slide downhill until the slope stabilizes at its angle of repose—about 30°–34°—the steepest angle at which the particles are stable.
16.1
Slipface
Previous slipfaces
Cross section:
A cross section of the dune reveals a layered pattern formed by earlier slipfaces as the dune grew.
DUNE FORMS
Leeward slope
Successive slipfaces created as dune migrates
Explain: Why do sand dunes migrate across the land surface?
Direction of dune movement
Dune migration:
Over time, the ongoing process that transfers particles from the windward slope to the leeward slope causes the entire dune to migrate downwind.
16.2
The different types of sand dunes vary in shape and size depending on several factors including: • directional variability and strength (or “effectiveness”) of the wind
• whether the sand supply is limited or abundant
• presence or absence of vegetation
Barchan:
Crescent-shaped dune with horns pointed downwind; found in areas with constant winds and little directional variability, and where limited sand is available.
Transverse:
Long, slightly sinuous dune with an asymmetrical ridge and only one slipface, aligned transverse (or perpendicular) to wind direction; results from relatively ineffective wind and abundant sand supply.
Visit the Study Area in MasteringGeographyTM to explore dunes.
Visualize: Study a geosciences animation of dunes. Assess: Demonstrate understanding of dunes (if assigned by instructor).
e
p
o
l
s
)
s
s
o
t
s
(
d
r
a
w
d
n
i
W
geosystems in action 16 WIND-BLOWN DUNE FORMS