Geoscience Reference
In-Depth Information
(a)
(b)
Figure 18.20 Longitudinal dunes and star dunes.
(a) A nicely developed longitudinal dune. Such a dune forms as a result of two
dominant wind directions. (b) Star dunes such as these in the Sahara desert form when winds are multidirectional.
Another kind of dune that forms in barren landscapes is a
longitudinal dune
(Figure 18.20a). These dunes have the same
linear form as transverse ridges, but differ because they develop
along the axis of two prevailing wind directions. In areas of
multidirectional winds with very little vegetation,
star dunes
may form (Figure 18.20b). Star dunes can grow to be very large.
In the Sahara, for example, star dunes may be several hundred
meters in height and many kilometers in diameter. Rather than
migrate, star dunes simply grow in height. A transitional dune
between star dunes and transverse ridges is the
reversing dune
,
which forms where two winds from nearly opposite directions
are balanced with respect to strength and duration. As a result,
a second slip face periodically develops.
In contrast to free dunes, which actively migrate, anchored
dunes are somewhat more stable landforms. The best known an-
chored dunes are
blowout
and
parabolic
dunes (Figure 18.18).
Both of these dune types are essentially deflation features with
slip faces that slope in many directions. A typical blowout is
a semicircular to circular deflation basin with lateral erosional
walls (Figure 18.21). Sand eroded from the deflation basin is
moved immediately downwind to form a depositional lobe. If
the blowout continues to enlarge and elongate downwind, a
parabolic dune will form with U- or V-shaped arms that point
upwind (Figure 18.18). These arms are usually stabilized by
vegetation, which causes the dune to further elongate as it
migrates downwind.
Figure 18.21 A typical blowout.
Blowouts like these in the
Nebraska Sand Hills form when vegetation thins and strong winds
deflate the core of a sandy deposit.
One of the most interesting landforms created by dust de-
position in arid environments is
desert pavement
, which is a
flat surface mantled with coarse pebbles, gravel, and even larger
rock clasts (Figure 18.22). At first glance it defies logic that des-
ert pavement could be the result of eolian deposition because
it is impossible for wind to carry the coarse-textured sediment
at the surface. For some time, it was believed that these sur-
faces develop due to deflation of fine sediments away from host
deposits that also contain coarse particles. In this traditional
model, the fine particles are blown out of the sediment matrix,
Deposition of Windblown Dust
Although sand dunes are the most common feature associated
with eolian processes, deposition of windblown dust can also
occur in ways that shape landscapes in distinctive, yet subtle
ways. Such sediment consists of fine-grained clays and silts that
are deflated from source areas and subsequently transported in
suspension before falling back to Earth's surface. These dust
deposits may travel a short distance before they are deposited,
or they may be carried hundreds of kilometers.
Desert pavement
A resistant, pavement-like surface created
when fine particles blow away and coarse sediment such as
pebbles and gravel is left behind.
