Aeolian accumulations
453
5.1. General characteristics
Desert dust consists of small particles carried away by the wind of arid environments in
suspension and deposited afterwards. Atmospheric dust can come from different
environments than those of arid zones (cosmic dust, volcanic dust, marine salts, etc.)
and the source areas can be distinguished by their composition and size.
In western Europe desert dust is a familiar feature, because almost every 7 years or so
occur the so called "red rains" of aeolian dust originating from the Sahara desert, that can
be observed as a thin film on top of cars. This dust settlement rate is on average
0.25 kg/m2/yr (Goosens and Offer, 1990). Globally it is possible to estimate that dust
carried out from deserts is about 1800 to 2000 million tons per year (D'Almeida, 1989).
A dust storm is recognized when visibility is less than 1000 m. In Egypt some ten storms
are observed every year, about 30 in China, and in the Mexican capital and in Kazakhstan
an average of 60 storms occur every year (Goudie, 1978).
According to grain size, there are two main types of desert dust that reflect the distance
they have travelled. If dust has been transported less than 100 km, sizes are between 0.005
and 0.05 mm; if transport distances are longer, the diameter is smaller than 0.002 mm
(P6w6, 1981). The largest particles are transported by wind whirls and dust storms and
produce extensive loess deposits, whereas the smaller ones move like an aerosol in the
troposphere, and remain suspended until they are entrained by the rain and deposited.
Desert dust is composed of inorganic and organic materials. The most abundant mineral
is quartz, accompanied by feldspars, calcite, dolomite, mica, clay minerals, oxihydroxides, heavy minerals, gypsum, halite, and opal. The organic part is composed of
diatoms, phytoliths, spores, and pollen. As we could expect, composition changes
according to the source area and the distance.
5.2. Source area and generating processes
The main currently active areas providing aeolian dust are the subtropical deserts, which
form a wide belt from western Africa to Central Asia, and the arid and subhumid regions
where dry soils are exposed to strong winds during several periods through the year (P6w6,
1981; Middleton et al., 1986)(Figure 18.26).
The areas covered with loose sediments that contain significant quantities of sand and
silt, but are poor in clay, are the most favourable for generating aeolian dust. One of these
environments is the poorly-sorted and unvegetated sediments of the ouads. They are
usually braided channels with a great lateral migration.
The playas, chotts, and littoral sebkhas, although they vary enormously in their
composition, contain zones characterized by the presence of fine grained material, which
are another source of aeolian dust. Alluvial fans are supply zones, especially their middle
and distal areas, where the percentage of fine particles is important. Ancient dry lakes also
can supply diatom dust, salt, gypsum, and clay minerals. The large regions covered
with loessic sediments are important dust source areas, particularly where vegetation is
nonexistent. An important source area of dust comes from volcanoes, as with the
emissions from Mount St. Helens in USA in 1980, and Pinatubo in the Philippines in 1991,
where large quantities of dust were emitted into the atmosphere, and transported
afterwards (Pye, 1987; Cooke et al., 1993; Livingstone and Warren, 1996). Mechanisms
453
5.1. General characteristics
Desert dust consists of small particles carried away by the wind of arid environments in
suspension and deposited afterwards. Atmospheric dust can come from different
environments than those of arid zones (cosmic dust, volcanic dust, marine salts, etc.)
and the source areas can be distinguished by their composition and size.
In western Europe desert dust is a familiar feature, because almost every 7 years or so
occur the so called "red rains" of aeolian dust originating from the Sahara desert, that can
be observed as a thin film on top of cars. This dust settlement rate is on average
0.25 kg/m2/yr (Goosens and Offer, 1990). Globally it is possible to estimate that dust
carried out from deserts is about 1800 to 2000 million tons per year (D'Almeida, 1989).
A dust storm is recognized when visibility is less than 1000 m. In Egypt some ten storms
are observed every year, about 30 in China, and in the Mexican capital and in Kazakhstan
an average of 60 storms occur every year (Goudie, 1978).
According to grain size, there are two main types of desert dust that reflect the distance
they have travelled. If dust has been transported less than 100 km, sizes are between 0.005
and 0.05 mm; if transport distances are longer, the diameter is smaller than 0.002 mm
(P6w6, 1981). The largest particles are transported by wind whirls and dust storms and
produce extensive loess deposits, whereas the smaller ones move like an aerosol in the
troposphere, and remain suspended until they are entrained by the rain and deposited.
Desert dust is composed of inorganic and organic materials. The most abundant mineral
is quartz, accompanied by feldspars, calcite, dolomite, mica, clay minerals, oxihydroxides, heavy minerals, gypsum, halite, and opal. The organic part is composed of
diatoms, phytoliths, spores, and pollen. As we could expect, composition changes
according to the source area and the distance.
5.2. Source area and generating processes
The main currently active areas providing aeolian dust are the subtropical deserts, which
form a wide belt from western Africa to Central Asia, and the arid and subhumid regions
where dry soils are exposed to strong winds during several periods through the year (P6w6,
1981; Middleton et al., 1986)(Figure 18.26).
The areas covered with loose sediments that contain significant quantities of sand and
silt, but are poor in clay, are the most favourable for generating aeolian dust. One of these
environments is the poorly-sorted and unvegetated sediments of the ouads. They are
usually braided channels with a great lateral migration.
The playas, chotts, and littoral sebkhas, although they vary enormously in their
composition, contain zones characterized by the presence of fine grained material, which
are another source of aeolian dust. Alluvial fans are supply zones, especially their middle
and distal areas, where the percentage of fine particles is important. Ancient dry lakes also
can supply diatom dust, salt, gypsum, and clay minerals. The large regions covered
with loessic sediments are important dust source areas, particularly where vegetation is
nonexistent. An important source area of dust comes from volcanoes, as with the
emissions from Mount St. Helens in USA in 1980, and Pinatubo in the Philippines in 1991,
where large quantities of dust were emitted into the atmosphere, and transported
afterwards (Pye, 1987; Cooke et al., 1993; Livingstone and Warren, 1996). Mechanisms