The Work of Wind and Deserts - The Changing Earth: Exploring Geology and Evolution - page 396

vegetation. Most chemical weath-
ering takes place during the winter
months when there is more preci-
pitation, particularly in the mid-
latitude deserts.
An interesting feature seen in
many deserts is a thin red, brown, or
black shiny coating on the surface
of many rocks (see Geo-inSight on
pages 394 and 395). This coating,
called rock varnish, is composed
of iron and manganese oxides
( Figure 15.20). Because many of

the varnished rocks contain little or
no iron and manganese oxides, the
varnish is thought to result either
from wind-blown iron and manga-
nese dust that settles on the ground
or from the precipitated waste of
microorganisms.
Desert soils, if developed, are
usually thin and patchy because the
limited rainfall and the resultant
scarcity of vegetation reduce the ef-
fi ciency of chemical weathering and
hence soil formation. Furthermore, the sparseness of the veg-
etative cover enhances wind and water erosion of what little
soil actually forms.

◗

◗ Figure 15.19 Desert Vegetation Desert vegetation is typically sparse, widely spaced,
and characterized by slow growth rates. The vegetation shown here in Organ Pipe National
Monument, Arizona, includes saguaro and cholla cacti, paloverde trees, and jojoba bushes and
is characteristic of the vegetation found in the Sonoran Desert of North America.

that falls each year is unpredictable and unreliable. It is not un-
common for an area to receive more than an entire year's aver-
age rainfall in one cloudburst and then to receive little rain for
several years. Thus, yearly rainfall averages can be misleading.
Deserts display a wide variety of vegetation ( Figure 15.19).

Although the driest deserts, or those with large areas of shift-
ing sand, are almost devoid of vegetation, most deserts sup-
port at least sparse plant cover. Compared to the vegetation
in humid areas, desert vegetation may appear monotonous.
A closer examination, however, reveals an amazing diversity
of plants that have evolved the ability to live in the near ab-
sence of water.
Desert plants are widely spaced, typically small, and grow
slowly. Their stems and leaves are usually hard and waxy to
minimize water loss by evaporation and to protect the plant
from sand erosion. Most plants have a widespread shallow
root system to absorb the dew that forms each morning in
all but the driest deserts and to help anchor the plant in what
little soil there may be. In extreme cases, many plants lie dor-
mant during particularly dry years and spring to life after the
fi rst rain shower with a beautiful profusion of fl owers.

◗

Mass Wasting, Streams, and Groundwater
When traveling through a desert, most people are impressed
by such wind-formed features as moving sand, sand dunes,
and sand and dust storms. They may also notice the dry
washes and dry streambeds. Because of the lack of run-
ning water, most people would conclude that wind is the
most important erosional agent in deserts. They would be
wrong! Running water, even though it occurs infrequently,

Weathering and Soils
Mechanical weathering is dominant in desert regions.
Daily temperature fl uctuations and frost wedging are the
primary forms of mechanical weathering (see Chapter 6).
The breakdown of rocks by roots and from salt crystal
growth is of minor importance. Some chemical weather-
ing does occur, but its rate is greatly reduced by aridity
and the scarcity of organic acids produced by the sparse

◗

Figure 15.20 Rock Varnish The shiny black coating on this rock,
exposed at Castle Valley, Utah, is rock varnish. It is composed of
iron and manganese oxides.