Geoscience Reference
In-Depth Information
LONGITUDE
60°
120°
180°
120°
60°
80°
80°
60°
60°
40°
40°
Northern subtropical
high pressure zone
20°
20°
Southern subtropical
high pressure zone
20°
20°
40°
40°
Prevailing
winds
Cold ocean
current
Active erg
(sand sea)
Hot deserts
Polar desert
60°
60°
60°
120°
180°
120°
60°
Figure 16.2 The distribution of hot and polar deserts.
Source: Partly after Collinson (1986)
needed to maintain motion but is often transient and
aeolian sediments are characterized by their relentless
progress downwind. Sand and silt are totally segregated
en route.
currents in all environments to produce a series of fluted
landforms, whose general orientation records palaeocur-
rent directions.
Abrasion
AEOLIAN LANDSYSTEMS
Ballistic impacts inevitably bring quartz grains into
sharp contact with softer materials which they abrade, at
the same time removing their own angular edges, either
when matched for hardnesses or when grains are flawed.
Fracture may also occur during saltation, and 'collision
splash' releases fines for further deflation. Abrasion occurs
on larger clasts incapable of deflation and exposed
bedrock surfaces which may undergo general surface
lowering, at rates varying between 1-10 m kyr -1 and 10-50
mm kyr -1 in hot and polar desert respectively. This
difference is probably explained by the protective effect of
snow cover and the almost perennially frozen state of soil
moisture. However, deflated ice fragments become
hardened as temperature falls and may mimic the
hardness of orthoclase feldspar in extreme cold in Siberia
and Antarctica at temperatures below -60
Aeolian denudation occurs through mechanical weath-
ering and rock slopes are cut back, leaving residual buttes
or inselbergs above low-angled pediments . Rapid
evacuation of debris by intense but ephemeral stream flow
develops alluvial fans or bajadas across the pediments and
terminates in mud sheets or playas lining basin interiors
( Figure 16.3 and Plate 16.1 ). Ephemeral lakes add evap-
orites to the supply of deflatable material. Denudation
rates are high when there is syntectonic uplift or basin
subsidence. Fluvial facies die out and aeolian facies
increase in abundance towards basin centres, where
sand seas dominate passive intercratonic basins. Wetter
pluvial phases during the Quaternary charged basins with
fluvial sediments which are now deflating and the
relatively slow development of ergs has not yet obliterated
the palaeofluvial landscape.
C. Rock
structures and differential strength combine with air
 
 
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