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Section 7.2
7.2 Deserts
1 FOCUS
Section Objectives
Key Concepts
How does running water
affect deserts?
What roles do mechanical
and chemical weathering
play in forming deserts?
Reading Strategy
Summarizing Write each blue
heading in the section on a sheet
of paper. Write a brief summary
of the text for each heading.
Vocabulary
◆
◆
alluvial fan
playa lake
Weathering
7.5
?
?
7.6
The Role of Water
?
Describe how running water
affects deserts.
Explain the roles mechanical
and chemical weathering play
in the formation of deserts.
?
Reading Focus
Build Vocabulary
D
L2
esert landscapes reveal the effects of both running water and
wind. As you will see, these combine in different ways in different
places to result in a variety of desert landscapes.
Paraphrase Have students write the
definition of each vocabulary term in
their own words.
Geologic Processes in Arid Climates
Reading Strategy
If you live in a humid region, visiting a desert might at first seem like
encountering an alien planet. Rounded hills and curving slopes are typical of humid regions. By contrast, deserts have angular rocks, sheer
canyon walls, and surfaces covered in pebbles or sand, shown in Figure 14.
Despite their differences, the same geologic processes operate in both
humid regions and deserts.
Figure 14 Desert landscapes vary
a great deal. This landscape is in
California’s Death Valley.
Weathering In humid regions, well-developed
soils support an almost continuous cover of vegetation. In these regions, the slopes and rock edges are
rounded and the landscape reflects the strong influence of chemical weathering .
By contrast, much
of the weathered debris in deserts has resulted from
mechanical weathering. That debris consists of rock
whose minerals remain unchanged. In dry lands,
rock weathering of any type is greatly reduced
because of the lack of moisture and scarcity of
organic acids from decaying plants.
Chemical
weathering, however, is not completely absent in
deserts. Over long time spans, clays and thin soils
do form. Many iron-bearing silicate minerals oxidize, producing the rust-colored stain found tinting
some desert landscapes.
L2
Weathering Sample answer:
Mechanical weathering is dominant in
the desert. Chemical weathering does
occur, but the process is very slow.
The Role of Water Sample answer:
Although it doesn’t rain often in the
desert, the erosional effects of rain are
significant.
2 INSTRUCT
Geologic Processes
in Arid Climates
Build Science Skills
Glaciers, Deserts, and Wind
199
L2
Observing Have students look closely
at Figure 14. Ask: If someone showed
you this photograph, what are three
features that would lead you to
conclude that this was a desert
climate? (Sample answers: sparse
vegetation; only small, shrub-like
vegetation present; lots of exposed
soil and gravel)
Visual, Logical
Glaciers, Deserts, and Wind
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Section 7.2 (continued)
Build Reading Literacy
L1
Refer to p. 586D in Chapter 21, which
provides the guidelines for SQ3R
(Survey, Question, Read, Recite, Review).
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Figure 15 A Most of the time
stream channels in deserts remain
dry. B This is the same stream
shortly after a heavy shower.
Ephemeral streams can cause a
large amount of erosion in a
short time.
Predicting How long will the
water flow in this stream?
SQ3R Teach this independent-study
skill as a whole-class exercise. Direct
students to survey the section and write
headings such as Geologic Processes in
Arid Climates. As they survey, ask students
to write one question for each heading,
such as “What type of weathering
occurs in a desert climate?” Then, have
students write answers to the questions
as they read the section. After students
finish reading, demonstrate how to
recite the questions and answers,
explaining that vocalizing in your own
words helps you retain what you have
learned. Finally, have students review
their notes the next day.
Verbal
Use Visuals
A
L1
Figure 16 Have students look carefully
at Figure 16. Ask: When rain falls at the
top of these barren mountains, what
will the water look like when it reaches
the bottom? (The water will be dirty
because it will contain a lot of sediment
that it has picked up as it flowed down the
mountainside.) What happens to the
sediment when the water reaches the
gentle slopes in the foreground of this
picture? (The rain water loses velocity and
dumps its load of sediment on the gentle
slopes.)
Verbal
B
Why do deserts experience less chemical
weathering than humid regions?
The Role of Water Permanent streams are normally found in
humid regions. However, in the desert, you’ll find bridges with no
water beneath them and dips in the road where empty stream channels
cross.
In the desert, most streams are ephemeral—they only
carry water after it rains. A typical ephemeral stream might flow for
only a few days or just a few hours during a year. In some years, the
channels may not carry any water. In the western states people call
these dry creeks washes or arroyos.
200 Chapter 7
Customize for Inclusion Students
Learning Disabled For students with
difficulty reading and writing, customize the
Writing in Science feature on p. 202 to allow
200
Chapter 7
students to make a multimedia presentation
instead of a written report.
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Figure 16 Alluvial Fans Over
the years, alluvial fans enlarge
and merge with fans from
adjacent canyons to produce an
apron of sediment along the
mountain front.
Basin and Range:
A Desert Landscape
Desert Water Erosion
Purpose Students will observe how
water erodes a barren landscape and
how alluvial fans form.
San
Francisco
NV
CA
Pacific
Ocean
L2
Death
Valley
Materials sharpened pencil, paper
cup, scissors, 1/2 of a drinking straw,
modeling clay, cookie sheet, ruler, large
beaker, water, soil, two 2" ⫻ 4" boards
about 15 cm long
Procedure Use the sharpened pencil to
make a hole in the side of the paper cup
near the bottom. Insert one end of the
straw into the hole in the cup. Seal the
hole around the straw with modeling
clay. Cover the cookie sheet with a thin
layer of soil. Elevate the cookie sheet
about 10 cm with a board. Set the cup
at the top of the cookie sheet. Hold your
finger over the end of the straw to keep
the water from flowing. Use the beaker
to fill the cup with water. Remove your
finger and let the water flow. Observe
what happens to the soil. Observe how
far the soil flowed past the end of the
cookie sheet. Elevate the cookie sheet
5 cm and repeat the experiment.
Observe what happens to the soil and
how far the soil flowed past the end of
the cookie sheet. Note any differences
in the two elevations.
Ephemeral streams are known for dangerous flash flooding after
heavy rains. During heavy showers, so much rain falls that the soil
cannot absorb it. The lack of vegetation allows water to quickly run
off the land, as shown in Figure 15. The floods end as quickly as they
start. Because there are fewer plants in deserts to anchor the soil, the
amount of erosion caused during a single-short lived rain event is
impressive. Floods in humid regions are different. A flood on a river
like the Mississippi can take days to reach its crest and days to subside.
Basin and Range: A Desert Landscape
Because arid regions typically lack permanent streams, they have interior drainage. This means that they have intermittent streams that do
not flow out of the desert to the ocean. In the United States, the dry
Basin and Range provides an excellent example. The region includes
southern Oregon, all of Nevada, western Utah, southeastern California,
southern Arizona, and southern New Mexico. The name Basin and
Range is an apt description for this region, because it contains more
than 200 relatively small mountain ranges that rise 900 to 1500 meters
above the basins that separate them.
When the occasional torrents of water produced by sporadic rains
move down the mountain canyons, they are heavily loaded with sediment. Emerging from the confines of the canyon, the runoff spreads
over the gentler slopes at the base of the mountains and quickly loses
velocity. Consequently, most of its load is dumped within a short distance. The result is a cone of debris known as an alluvial fan at the
mouth of a canyon, as shown in Figure 16.
Expected Outcomes Students will
observe the soil flow off the cookie
sheet just as the soil flows off a barren
landscape. Students also should observe
that soil will not flow as far when the
slope is less steep. Alluvial fans form
when the slope is not steep.
Visual
Glaciers, Deserts, and Wind
201
Facts and Figures
Many of the world’s deserts are located in two
belts. One belt is located along the Tropic of
Cancer in the Northern Hemisphere. The
deserts located in this belt are the Gobi in
China, the deserts in southwestern North
America, the Sahara in North Africa, and the
Arabian and Iranian deserts in the Middle East.
The second belt is located along the Tropic of
Capricorn in the Southern Hemisphere. These
deserts include the Patagonia in Argentina,
the Kalahari in southern Africa, and the Great
Victoria and Great Sandy deserts of Australia.
These belts are formed when hot, moist air at
the equator rises, cools, and loses its moisture.
Then, the air descends, picking up moisture
and drying out the land, creating these desert
regions along the tropics.
Answer to . . .
Figure 15 for a few hours to a
few days
Water is necessary for
chemical weathering, so
arid climates experience less chemical
weathering than humid regions. Also,
fewer plants exist to decay and
contribute organic acids.
Glaciers, Deserts, and Wind
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Section 7.2 (continued)
3 ASSESS
Evaluate
Understanding
Q I heard that deserts are
L2
Have students write three review
questions for the section. Invite students
to take turns asking questions to the
class.
Reteach
L1
Use Figures 14, 15, and 16 to review the
main ideas in this section.
First the ephemeral stream will be dry.
Then, a sudden rush of water will occur
that builds both in volume and velocity
for several hours. The flood will then
subside as quickly as it started.
expanding. Is that true?
A Yes. The problem is called
desertification, and it refers to
the alteration of land to desertlike conditions as the result of
human activities. It commonly
takes place on the margins of
deserts and results mostly from
inappropriate land use. It is triggered when the modest natural
vegetation in marginal areas is
removed by plowing or grazing.
When drought occurs, as it
often does in these regions, and
the vegetative cover has been
destroyed beyond the minimum
to hold the soil against erosion,
the destruction becomes irreversible. Desertification is
occurring in many places but
is particularly serious in the
region south of the Sahara
Desert known as the Sahel.
On the rare occasions of abundant rainfall, or snowmelt in the
mountains, streams may flow across the alluvial fans to the center of
the basin, converting the basin floor into a shallow playa lake. Playa
lakes last only a few days or weeks, before evaporation and infiltration
remove the water. The dry, flat lake bed that remains is called a playa.
Humid regions have complex systems of rivers and streams that
drain the land. Streams in dry regions lack this extensive drainage
system.
Most desert streams dry up long before they ever reach
the ocean. The streams are quickly depleted by evaporation and soil
infiltration.
Some permanent streams do manage to cross arid regions. The
Colorado and Nile Rivers begin in well-watered mountains with huge
water supplies. The rivers are full enough at the beginning to survive
their desert crossings. The Nile River, for example, leaves the lakes and
mountains of central Africa and covers almost 3000 kilometers of the
Sahara without a single tributary adding to its flow. In humid regions,
however, rivers generally gain water from both incoming tributaries and
groundwater.
The point to remember about running water in the desert is this:
although it is infrequent, it is an important geological force.
Most
desert erosion results from running water. Although wind erosion is
more significant in deserts than elsewhere, water does most of the
erosional work in deserts. Wind plays a different primary role in the
desert. It transports and deposits the sediments to create dunes.
Section 7.2 Assessment
Reviewing Concepts
1.
How are ephemeral streams different form
streams in humid locations?
2.
How do weathering processes affect
deserts?
3. Why is erosion by running water important in
deserts?
4. How does a river survive crossing an arid
region?
Critical Thinking
5. Comparing and Contrasting Compare
and contrast the Nile River with the Mississippi
River. Which factor is most responsible for their
differences?
6. Applying Concepts Explain how evaporation
affects drainage systems in desert areas.
Suppose you are standing on a bridge
over an ephemeral stream in the desert.
Write a paragraph describing what you
might see following a sudden downpour.
202 Chapter 7
Section 7.2 Assessment
1. Ephemeral streams are not permanent but
have a greater propensity to produce flash
floods, which cause substantial erosion.
2. Water and wind cause mechanical weathering and produce angular rocks, sheer canyon
walls, and pebble-covered surfaces.
3. Because there are fewer plants in deserts to
anchor the soil, there can be a great amount
of erosion caused during a single short-lived
rain event.
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Chapter 7
4. It must be full enough at the beginning to
survive the soil infiltration and evaporation
that occur in the desert.
5. Both carry water. The Nile has few tributaries. The Mississippi drainage system is
highly branched. The Mississippi takes longer
to crest and subside. Climate is the factor
most responsible for the rivers’ differences.
6. Streams in desert areas lack extensively
branched drainage systems. They do not flow
out of the desert to oceans, and instead have
interior drainage, helping evaporation to dry
up ephemeral streams.