Geoscience Reference
In-Depth Information
5.3.5.4 Inorganic (soil or rainbeat) crusts
5.4
NATURAL IMPACTS ON PROCESSES
Inorganic crusts may develop on soil surfaces
during or shortly after a rainstorm event. These
relate to physico-chemical changes relating to
water and clay content and rainfall characteristics.
These can be grouped into (i) structural seals
or (ii) depositional seals (Maulem et al. 1990).
Structural seals develop as a result of the soil
structure as a function of aggregate breakdown
by rainsplash and slaking; washing of fines into
adjacent, larger pores; particle segregation and
compaction. Depositional seals are a function
of the deposition of fine suspended sediments in
a thin, comparatively impermeable film on the
soil surface. Sealing intensity has been demon-
strated to decrease with increasing slope angle
as a function of higher erosion on higher slopes
(Poesen 1986). Poesen also found that embedded
rock fragments in sandy soils can generate seals,
whereas those with rock fragments sitting on the
top do not. This was attributed to the fact that
non-embedded stone particles typically have
'overhangs' that protect the area immediately
adjacent to the rock fragment from raindrop im-
pact, thus a complete soil seal cannot be created.
Once developed, surface seals may prevent the
escape of air as a wetting front penetrates the soil,
generating associated vesicular layers 1-30 mm
below the surface (Chatres et al. 1985; Ringrose-
Voase et al. 1989). These vesicles further reduce
hydraulic conductivity of the soils.
Arid environments, as described previously,
are dynamic. Their spatial extent and degree of
aridity can change over a range of temporal and
spatial scales (Fig. 5.4). Although some of these
changes can be accounted for by intrinsic con-
trols, extrinsic controls can play an important
role. These consist of external forcing mechan-
isms that cause the arid system to change. What
is considered to be the dominant external con-
trol will change depending on the scale of the
landscape element examined and the time-scale
involved. For example, long term (million year)
time-scales are typically dominated by tectonic
controls due to the large spatial and temporal
scale on which they cumulatively operate. Climate
controls tend to dominate the medium scale
(million to thousand year) and short term time-
scales (annual) are dominated by factors such as
storminess.
5.4.1 Long term time-scale controls: tectonics
Active tectonics can have an impact on arid envir-
onment sediment systems directly or indirectly.
Direct effects may have an impact on localized
areas of an arid landscape, for example they
may generate fault scarps as a function of an
earthquake event, which can then divert local
drainages, or rejuvenate local erosion. These types
of features, however, are typically ephemeral, and
are removed in a matter of years, depending on
the rates of erosion. Even within hyper-arid areas
where water availability is low, in situ weather-
ing, aeolian deposition and gravitational col-
lapse can conceal the effects of faulting relatively
quickly. Within alluvial fan systems tectonics can
directly rejuvenate the catchment areas that pro-
duce and deliver the sediment to the alluvial fan,
for example by increasing slope angles through
uplift. Within the sediment accumulation area
of the alluvial fan itself tectonics may locally
directly modify the fan surface. The morphology
and internal sedimentology of alluvial fans can
be seen at its simplest as a balance between the
discharge of sediment exiting the fan source area
and the available accommodation space. Thus
5.3.5.5 Duricrusts
These are indurated surface/near-surface crusts.
They may be exhumed relict deposits (e.g.
ancient lacustrine deposits) or may relate to
contemporaneous hydrological or pedological
processes in the desert environment. Some crusts
are ephemeral, for example salcretes (Table 5.5),
others may persist for many millions of years
(such as the gypcretes reported by Hartley &
May 1998). Table 5.5 describes the duricrusts
most commonly found in arid environments.
Note that there are many more subtypes, such
as the nitrate deposits in the Atacama Desert of
northern Chile.
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