456
Climatic Geomorphology
atmosphere, and often is deposited when it encounters obstacles perpendicular to the wind
flow. Nevertheless the finest particles that travel in the higher parts of the atmosphere are
not affected by the roughness of the terrain, and can remain suspended for long periods of
time (Coudr-Gaussen and Rognon, 1988; Coudr-Gaussen, 1991).
In hyperarid zones only small quantities of dust occur, because precipitation is rare and,
generally, obstacles are not common. On the margin of deserts, however, corresponding to
semiarid zones, there are great quantities of aeolian dust deposited. These particles are easily
trapped by the disperse vegetation. In the north of Nigeria, up to 3 m of desert dust have been
deposited during the last 40,000 years (McTainsh, 1987) and, at present, sedimentation rates
vary between 0.1 and 0.7 mm/yr in different zones of Israel and the Sahel.
5.4. Geomorphological implications
Desert dust has a considerable importance to the better understanding of certain
geomorphic processes that have strong implications for human and economic activities.
Some superficial formations appear because dust deposited in the vicinity contributes to
their genesis in semi-arid and arid areas. So, the supply of calcium carbonate to the soils
produces caliches in areas without calcareous rocks, as we can interpret from the studies
on calcretes in Namibia and southeast of Spain (Bltimel, 1982). Also dust rich in silica can
be important in the formation of silcretes in deserts (Summerfield, 1983). Likewise,
sedimentation of gypsiferous dust is the main cause of the formation of gypsum crusts in
southern Tunisia (Coque, 1955). Desert varnish, that forms a thin film rich in manganese
and iron, covers rocks in arid zones. As was pointed out in a previous chapter, particles that
form the varnish are provided by the sedimentation of aeolian dust. Laminations that
constitute the varnish are analysed carefully and paleoclimatical interpretations are carried
out, according to the fluctuations in manganese and iron contents (Oberlander, 1994).
Desert dust can also affect the weathering developed on buildings. In dune zones,
sedimentation of dust and salts can form a crust that is enough to stabilise the dunes,
allowing the establishment of biocrusts and plants (Pye and Tsoar, 1987). Aeolian dust is
also collected in playas, sebkhas, alluvial fans, and ouads causing modifications of their
sediment composition. These sediments are quickly mobilized by water within their
depositional environments. Many soils from desertic and peridesertic regions contain
significant quantities of aeolian dust, and some soils are virtually formed by these
allochtonous supplies. According to Rapp (1984), the terra rossa from Spain, Italy, and
other Mediterranean countries are formed essentially by reworked dust coming from the
north of Africa. This desert dust fertilizes many soils, affecting even some of those
developed in the Caribbean Islands.
Desert dust deposited in oceans and seas constitutes a significant percentage of oceanic
sediments. The study of these accumulations provides very useful information on
modifications of wind systems, and about environmental changes that have taken place on
the continents during the Quaternary period (Middleton, 1997). Therefore, this research
provides information on the arid phases and their intensity in terrestrial environments. Ice
accumulations, especially in the polar ice caps, preserve a record of aeolian dust that is
known because test drillings have been done. In Greenland, during the Last Glacial
Maximum (18,000 years ago), dust concentration was 40 times greater than now. Phases
Climatic Geomorphology
atmosphere, and often is deposited when it encounters obstacles perpendicular to the wind
flow. Nevertheless the finest particles that travel in the higher parts of the atmosphere are
not affected by the roughness of the terrain, and can remain suspended for long periods of
time (Coudr-Gaussen and Rognon, 1988; Coudr-Gaussen, 1991).
In hyperarid zones only small quantities of dust occur, because precipitation is rare and,
generally, obstacles are not common. On the margin of deserts, however, corresponding to
semiarid zones, there are great quantities of aeolian dust deposited. These particles are easily
trapped by the disperse vegetation. In the north of Nigeria, up to 3 m of desert dust have been
deposited during the last 40,000 years (McTainsh, 1987) and, at present, sedimentation rates
vary between 0.1 and 0.7 mm/yr in different zones of Israel and the Sahel.
5.4. Geomorphological implications
Desert dust has a considerable importance to the better understanding of certain
geomorphic processes that have strong implications for human and economic activities.
Some superficial formations appear because dust deposited in the vicinity contributes to
their genesis in semi-arid and arid areas. So, the supply of calcium carbonate to the soils
produces caliches in areas without calcareous rocks, as we can interpret from the studies
on calcretes in Namibia and southeast of Spain (Bltimel, 1982). Also dust rich in silica can
be important in the formation of silcretes in deserts (Summerfield, 1983). Likewise,
sedimentation of gypsiferous dust is the main cause of the formation of gypsum crusts in
southern Tunisia (Coque, 1955). Desert varnish, that forms a thin film rich in manganese
and iron, covers rocks in arid zones. As was pointed out in a previous chapter, particles that
form the varnish are provided by the sedimentation of aeolian dust. Laminations that
constitute the varnish are analysed carefully and paleoclimatical interpretations are carried
out, according to the fluctuations in manganese and iron contents (Oberlander, 1994).
Desert dust can also affect the weathering developed on buildings. In dune zones,
sedimentation of dust and salts can form a crust that is enough to stabilise the dunes,
allowing the establishment of biocrusts and plants (Pye and Tsoar, 1987). Aeolian dust is
also collected in playas, sebkhas, alluvial fans, and ouads causing modifications of their
sediment composition. These sediments are quickly mobilized by water within their
depositional environments. Many soils from desertic and peridesertic regions contain
significant quantities of aeolian dust, and some soils are virtually formed by these
allochtonous supplies. According to Rapp (1984), the terra rossa from Spain, Italy, and
other Mediterranean countries are formed essentially by reworked dust coming from the
north of Africa. This desert dust fertilizes many soils, affecting even some of those
developed in the Caribbean Islands.
Desert dust deposited in oceans and seas constitutes a significant percentage of oceanic
sediments. The study of these accumulations provides very useful information on
modifications of wind systems, and about environmental changes that have taken place on
the continents during the Quaternary period (Middleton, 1997). Therefore, this research
provides information on the arid phases and their intensity in terrestrial environments. Ice
accumulations, especially in the polar ice caps, preserve a record of aeolian dust that is
known because test drillings have been done. In Greenland, during the Last Glacial
Maximum (18,000 years ago), dust concentration was 40 times greater than now. Phases