Weathering processes and resulting forms
243
the intensity of the processes separately by means of cyclic tests, just as actually happens
in the natural situation (Smith, 1994), but did not investigate the processes together or the
interaction between them.
2. Weathering processes
2.1. Insolation weathering
Rocks can rupture as a result of temperature changes that cause expansion and contraction
leading to breakdown. The heat supplied can come either from solar radiation or
occasional fires, relatively frequent in semi-arid environments (Oilier and Ash, 1983). In
the first case, the process of physical disintegration is called insolation weathering. The
rocky surfaces of the deserts are exposed to high temperatures, which can reach values
of 82~ in Sudan (Goudie, 1989a), and to diurnal oscillations of 54~ as in the Atacama
desert (Keller, 1946).
Most rocks are formed of different minerals and these present different thermal
expansions according to their colour and structure. Dark rocks and minerals absorb more
heat than light ones. Moreover, some minerals dilate more easily in one direction than in
other, according to their crystalline structure. The expansion produces compressive failure
inside the rock and contraction tensile failure. Microcracking and granular disintegration
can appear as a result of different expansion and contraction (Birot, 1968a,b). On the other
hand, as the rocks are poorly heat conductive, there are important temperature gradients
from the surface into the rock (Figure 11.3), which give rise to important expansions and
contractions leading to exfoliation (Smith and Warke, 1997). The outcropping stones or
blocks broken in parallel fractures (cleaved boulder) (Fig. 11.4) are interpreted as produced
by insolation weathering (Oilier, 1984), although other investigators estimate that the cause
is salt weathering. The abrupt noises or cracking that the explorers heard in the silence of
desert nights are thought to be due to breakdown when rocks are cooled during the night.
The classical laboratory experiments of Blackwelder (1925) and Griggs (1936) of
heating and cooling cycles over different types of rocks, in the absence of water, show the
inexistence of rock cracking after numerous cycles, equivalent to 244 years of daily
weathering. When cooling the rock with water instead of dry air microfissures were created
after a number of cycles equivalent to two and a half years. It has to be kept in mind that the
presence of humidity is general in all the deserts as fogs, night dews and, sometimes, storms
that cool the rock rapidly and emphasize contractions. Other circumstances to keep in mind
with the early experiments is that rocks go on weakening gradually over long time periods,
or are affected by continuous expansion and contraction cycles that bring the rocks to
fatigue and finally to cracking (Griggs, 1936; Aires-Barros, 1975). Moreover, it is always
necessary to remember that rock breakdown is not only due to a unique mechanism, because
in general two or more processes alternate or superimpose.
2.2. Wetting and drying weathering
Rocks in desert environments are exposed to numerous wetting and drying cycles, which
produces disruptive effects. The disruptive action caused by water is verified by means of
243
the intensity of the processes separately by means of cyclic tests, just as actually happens
in the natural situation (Smith, 1994), but did not investigate the processes together or the
interaction between them.
2. Weathering processes
2.1. Insolation weathering
Rocks can rupture as a result of temperature changes that cause expansion and contraction
leading to breakdown. The heat supplied can come either from solar radiation or
occasional fires, relatively frequent in semi-arid environments (Oilier and Ash, 1983). In
the first case, the process of physical disintegration is called insolation weathering. The
rocky surfaces of the deserts are exposed to high temperatures, which can reach values
of 82~ in Sudan (Goudie, 1989a), and to diurnal oscillations of 54~ as in the Atacama
desert (Keller, 1946).
Most rocks are formed of different minerals and these present different thermal
expansions according to their colour and structure. Dark rocks and minerals absorb more
heat than light ones. Moreover, some minerals dilate more easily in one direction than in
other, according to their crystalline structure. The expansion produces compressive failure
inside the rock and contraction tensile failure. Microcracking and granular disintegration
can appear as a result of different expansion and contraction (Birot, 1968a,b). On the other
hand, as the rocks are poorly heat conductive, there are important temperature gradients
from the surface into the rock (Figure 11.3), which give rise to important expansions and
contractions leading to exfoliation (Smith and Warke, 1997). The outcropping stones or
blocks broken in parallel fractures (cleaved boulder) (Fig. 11.4) are interpreted as produced
by insolation weathering (Oilier, 1984), although other investigators estimate that the cause
is salt weathering. The abrupt noises or cracking that the explorers heard in the silence of
desert nights are thought to be due to breakdown when rocks are cooled during the night.
The classical laboratory experiments of Blackwelder (1925) and Griggs (1936) of
heating and cooling cycles over different types of rocks, in the absence of water, show the
inexistence of rock cracking after numerous cycles, equivalent to 244 years of daily
weathering. When cooling the rock with water instead of dry air microfissures were created
after a number of cycles equivalent to two and a half years. It has to be kept in mind that the
presence of humidity is general in all the deserts as fogs, night dews and, sometimes, storms
that cool the rock rapidly and emphasize contractions. Other circumstances to keep in mind
with the early experiments is that rocks go on weakening gradually over long time periods,
or are affected by continuous expansion and contraction cycles that bring the rocks to
fatigue and finally to cracking (Griggs, 1936; Aires-Barros, 1975). Moreover, it is always
necessary to remember that rock breakdown is not only due to a unique mechanism, because
in general two or more processes alternate or superimpose.
2.2. Wetting and drying weathering
Rocks in desert environments are exposed to numerous wetting and drying cycles, which
produces disruptive effects. The disruptive action caused by water is verified by means of