Geology Reference
In-Depth Information
climates (such as Antarctica), along coastlines (such as the Atacama Desert of South
America's west coast), and in mountainous continental interiors (such as the Himalayas
of Asia and the Rocky Mountains of North America).
When water is present in arid regions, the moving water in streams creates geologic fea-
tures, but wind is still by far the most common mover of earth materials.
Transporting Particles by Air
Wind carries particles of rock, or sediment, in much the same way that moving water
carries sediment. (I describe movement of sediments by water in Chapter 12.) Both wind
and water flow in currents and pick up larger particles as their flow increases. The
biggest difference between the two is the size of the particles that can be carried.
Air is much less dense than water, which means there is more space between the molec-
ules in a given volume of air than between molecules in the same volume of water. (See
Chapter 10 for more details on density.) The result of this extra space (and therefore
lower density) is that wind must move much faster to pick up particles the same size
that water picks up at lower speeds.
Like water, wind flows in two ways:
Laminar flow: Its lines of flow, or streamlines, are unmixed and proceed in straight
lines.
Turbulent flow: The streamlines are mixed.
I show an illustration of laminar and turbulent flows in Chapter 12.
The methods of particle movement, or transport, by wind are the same as some of the
methods described for particle movement by water in Chapter 12. I define them here
briefly.
Skipping right along: Bed load and saltation
Particles transported by wind that move along very close to the ground are called
wind's bed load. Most sediments transported by wind move along as bed load, close to
and sometime in contact with the surface. This fact is due to the low density of air,
which limits its ability to lift large particles, even at high speeds.
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