Page 128 - Sedimentology and Stratigraphy

Aeolian Transport 115

Fig. 8.1 The distribution of high- and
low-pressure belts at different latitudes
creates wind patterns that are deflected by the
Coriolis force.

altitudes towards the poles and a complementary density of the medium. Air has a density of 1.3 kg
movement of cold air back to the Equator closer to m 3 , which is three orders of magnitude less than
ground level. This simple pattern is, however, compli- that of water (1000 kg m 3 ) so, whereas water flows
cated by two other factors. First, the circulation pattern of only a few tens of centimetres a second can cause
breaks up into smaller cells, three in each hemisphere. movement of sand grains, much higher velocities are
Second, the Coriolis force (6.3) deflects the pathway of required for the wind to move the same grains. Winds
the air mass from simple north–south directions. The of 55 m s 1 or more are recorded during hurricanes,
result is the pattern of winds shown in Fig. 8.1, but strong winds over land areas are typically around
although these patterns are modified and influenced 30 m s 1 , and at these velocities the upper limit on
by local topographic effects. Air masses blowing over the size of quartz grains moved by the wind is around
mountain ranges are forced upwards and are cooled, a half a millimetre in diameter, that is, medium sand
and similarly the air is chilled when winds blow over size storms (Pye 1987; Nickling 1994). This provides
ice caps: this results in katabatic winds, which are an important criterion for the recognition of aeolian
strong, cold air masses moving down mountain slopes deposits in the stratigraphic record: deposits consist-
or off the edges of ice masses. ing of grains coarser than coarse sand are unlikely to
be aeolian deposits.
At high wind velocities silt- and clay-sized particles
8.1.2 Aeolian transport processes are carried as suspended load. This aeolian dust can
become entrained in the wind in large quantities
A flow of air over a loose grain of sand exerts a lift force in dry areas to create dust storms that can carry
on the particle (4.2.3) and with increasing velocity the airborne sediment large distances away from its ori-
force may increase to the point where the grain rolls gin. The dust will remain in suspension until the wind
or saltates (4.2.2). The strength of the lift force is speed drops and the fine sediment starts to fall to
proportional to both the velocity of the flow and the the ground or onto a water surface. Significant