Page 390 - Laboratory Manual in Physical Geology

not true in drylands, where degradation may cause the land their productive cropland and grazeland to useless desert.
to become true desert with no agricultural value. This type of For this reason, the Third World Academy of Sciences
land degradation is called desertification (the process of land declared the 1990s as the “Decade of the Desert,” and
degradation toward drier, true desert conditions). the United Nations General Assembly declared 2006 the
UNEP estimates that 70% of all existing drylands International Year of Deserts and Desertification. In 2012,
(about 25% of all land on Earth) are now experiencing a United Nations Convention to Combat Desertification
the hazard of desertification from factors related to human affirmed a goal of zero net land degradation.
population growth, climate change, poor groundwater
use policies, overgrazing, and other poor land management Eolian Processes, Dryland
practices. More than 100 nations now risk degradation of
Landforms, and Desertification
ACTIVITY Many drylands have specific landforms that result primar-
ily from processes associated with degradation, erosion
14.1 Dryland Inquiry by streams and flash floods developed after infrequent
rains (fluvial processes), or erosion and deposition associ-
THINK | What are some characteristic processes, ated with wind (eolian processes) ( FIGURES 14.1 , 14.2 ,

About It landforms, and hazards of drylands? 14.3 ). Fluvial and eolian processes erode dryland land-
scapes, transport Earth materials, and deposit sediments.
OBJECTIVE Analyze satellite images and Rocky surfaces, sparsely vegetated surfaces, sand dunes,
photographs of American drylands and infer processes and arroyos (steep-walled canyons with gravel floors) are
and hazards that occur there. typical of dryland landscapes. Therefore, humans living
in drylands must adapt to these landforms, conditions,
PROCEDURES
processes that have created the landforms, and the prospect
1. Before you begin , do not look up defi nitions and of land degradation or even desertification.
information. Use your current knowledge, and
complete the worksheet with your current level of Eolian Processes and Landforms
ability. Also, this is what you will need to do the
Water and ice are capable of moving large particles of
activity:
sediment. The wind can move only smaller particles
____ Activity 14.1 Worksheet (p. 367 ) and pencil

( FIGURES 14.4 and 14.5 ). For this reason, the eolian (wind-

2. Complete the worksheet in a way that makes related) landforms may be subtle or even invisible on a
sense to you. topographic map. (However, they may be more evident
3. After you complete the worksheet , be prepared on aerial photographs that have a higher resolution.) They
to discuss your observations and classifi cation with may be superimposed on fluvial (stream-related) or glacial
other geologists. (ice- or glacier-related) features, particularly where recently
exposed and unvegetated sediment occurs.
A lack of a dense vegetation cover is a prerequisite for
ACTIVITY significant wind erosion. This lack of vegetation can occur:

■ On recently deposited sediment, such as floodplains
14.2 Mojave Desert, Death and beaches.
Valley, California
■ In areas where vegetation has been destroyed by fire,

THINK | What can we learn from topographic overgrazed, or removed by humans, or

About It maps and satellite images about ■ In true deserts, where the lack of water precludes
dryland processes and landforms? substantial growth of vegetation.
When examining a topographic map, keep in mind

OBJECTIVE Identify dryland landforms of Death
that the green overprint represents only trees and shrubs.
Valley, California, and infer how the valley is forming.
There could be an important soil-protecting grass cover
PROCEDURES present that is not indicated on the map. Your evaluation
of the present climate of a topographic map area should
1. Before you begin , read the Introduction and Eolian
consider surface water features, groundwater features, and
Processes, Dryland Landforms, and Desertifi cation
the geographic location of the area.
below. Also, this is what you will need :
____ colored pencils Blowouts
____ Activity 14.2 Worksheet (p. 369 ) and pencil
The most common wind-eroded landform visible on a
2. Then follow your instructor’s directions for topographic map is usually a blowout —a shallow depres-
completing the worksheets.
sion developed where wind has eroded and blown out the
soil and fragmented rock ( FIGURE 14.6C ). Blowouts may

358 ■ L ABOR ATORY 14