Geoscience Reference
In-Depth Information
began systematically to employ
conservation tillage
, which is a
shallow plowing method that leaves about 30% of the previous
year's crop residue on the ground. This residue increases the sur-
face roughness of the ground, which helps protect the soil from
wind erosion and conserve water in the soil.
Although the Dust Bowl occurred about 80 years ago, its
effects echo to the present day. These echoes are particularly
strong as of this writing because the region is experiencing
a severe drought, one that may ultimately rival the drought
years in the 1930s. Current levels of annual precipitation are
about half the norm, and agriculture has been hard hit. One
major advantage that modern farmers have over their coun-
terparts in the
Dirty '30s
is the soil conservation measures
now employed to protect fields. In addition, many modern
farmers use central pivot irrigation, which, as discussed in
Chapter 15, can provide a steady flow of water to crops,
even during a drought. On the other hand, climate models
discussed in Chapter 9 suggest that the region will likely be-
come even more drought prone in the future as global warm-
ing intensifies. In this context, how will farmers respond if
and when groundwater supplies disappear, as they very well
may this century? Is it possible that another Dust Bowl will
occur in the future? Only time will tell.
KEY CONCEPTS TO REMEMBER ABOUT
DESERTIFICATION OF SEMI-ARID LANDS
AND SAND DUNES AS INDICATORS OF
CLIMATE CHANGE
1.
Large areas of Earth's surface are prone to desertifi-
cation, which is the transformation of formerly stable
(by vegetation) landscapes to desert-like conditions.
2.
The Sahel region of Africa is an ecotone that lies
between the Sahara Desert and the tropical rainforest.
This region has undergone extensive desertification in
the past 30 years due to human impacts and extensive
drought.
3.
The American Dust Bowl is a large area in the central
United States that was extensively desertified during
the 1930s due to a combination of human settlement
patterns and extensive drought.
4.
Soils in semi-arid lands can be protected to some
extent through such measures as windbreaks and
conservation tillage.
S U M M A R Y O F K E Y C O N C E P T S
1.
Arid and semi-arid environments collectively comprise
over 30% of the Earth's land surface. Arid landscapes
are excellent places to study geomorphology because
elements such as rock structure are easily seen.
In places where the structure is horizontal, desert
landscapes often contain a progression of erosional
landforms, including plateaus, mesas, buttes, and
pinnacles.
sand dunes form when windblown sand is deposited
and shaped.
4.
Sand dunes come in many different shapes and sizes.
The shapes of dunes depend largely on the supply of
sand, the amount of vegetation, and the prevailing wind
speed and direction.
5.
Loess is windblown silt carried a great distance by
suspension. Thick deposits of loess occur in many
places around the world, with most accumulating during
glacial cycles.
2.
Eolian processes involve the shaping of the Earth due to
the wind. The wind moves sediment in one of three ways:
(1) fine grains such as silts and clays are carried great
distances in suspension; (2) sand grains bounce along
the ground in the process of saltation; and (3) heavier
particles roll along the ground by creep.
6.
Human-induced desertification is a major environmental
issue facing the world today. Desertification occurs
when the stabilizing vegetation on the landscape is
reduced through human activity. This process exposes
soils to the wind, which causes severe soil erosion and
the transport of eolian sediment.
3.
The wind shapes the landscape through the combined
processes of erosion and deposition. The formation of
yardangs is an example of wind erosion. In contrast,
C H E C K Y O U R U N D E R S T A N D I N G
1.
What are the three large-scale environmental variables
that contribute to the presence of deserts? What are the
processes associated with each of these factors, and
why do they produce arid environments?
2.
Discuss how normal faulting is responsible for the
dominant landforms of the Basin and Range Province.
