Transcript GEOG 123B Lec. #10

Chapter 15
Eolian Processes and Arid
Landscapes
Geosystems 5e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen
Final Exam
The final exam is going to take place on
April 11 at 2pm. It will last about 2 hours.
It is going to take place at the lecture room.
It is non-cumulative.
Bring #2 pencil.
Same format as the midterm.
Key Learning Concepts:
1.
2.
3.
4.
5.
6.
After reading the chapter you should be able to:
Characterize the unique work accomplished by wind and eolian
(caused by wind; refers to erosion, transportation, and deposition of
materials; spelled “aeolian” in some countries) processes.
Describe eolian erosion, including deflation, abrasion, and the
resultant landforms.
Describe eolian transportation and explain saltation, and surface
creep.
Identify the major classes of sand dunes and present examples
within each class.
Define loess deposits, their origins, locations, and landforms.
Portray desert landscapes and locate these regions on a world map.
1. Who was Ralph Bagnold? What was his
contribution to eolian studies?
A British major, Ralph Bagnold was stationed in Egypt in 1925.
Bagnold was an engineering officer who spent much of his time in
the deserts west of the Nile, where he measured, sketched, and
developed hypotheses about the wind and desert forms. His oftencited work, The Physics of Blown Sand and Desert Dunes, was
published in 1941 following the completion of wind-tunnel
simulations in London.
Bagnold took Henry Ford at his word that a Model-T Ford could
handle the most difficult terrain. He drove a Model-T all over the
desert, carrying sections of chicken wire for areas where support
was needed under the wheels. Additional wind-tunnel tests
completed key aspects of his research on eolian processes.
2. Explain the term eolian and its
application in this chapter.
Wind-eroded, wind-transported, and wind-deposited
materials are called eolian (also spelled aeolian;
named for Aeolus, the ruler of the winds in Greek
mythology). The actual ability of wind to move
materials is small compared with that of other
transporting agents such as water and ice, because
air is so much less dense than these other media.
3. Describe the erosional processes
associated with moving air.
Two principal wind-erosion processes are deflation,
the removal and lifting of individual loose particles,
and abrasion, the grinding of rock surfaces with a
“sandblasting” action by particles captured in the air.
(See next slides)
Desert pavement is formed from larger
rocks and fragments left after deflation.
Yardang: A small wind-sculpted rock formation
caused by abrasion in Utah.
Figure 15.4
4. How are ventifacts and yardangs
formed by the wind?
Rocks exposed to eolian abrasion appear pitted, grooved, or
polished, and usually are aerodynamically shaped in a
specific direction, according to the flow of airborne
particles. Rocks that bear such evidence of eolian erosion
are called ventifacts. On a larger scale, deflation and
abrasion are capable of streamlining rock structures that are
aligned parallel to the most effective wind direction, leaving
behind distinctive, elongated ridges called yardangs. These
can range from meters to kilometers in length and up to
many meters in height.
5. Differentiate between a dust storm and
a sand storm.
Only the finest dust particles travel significant
distances, and consequently the finer material
suspended in a dust storm is lifted much higher
than the coarser particles of a sand storm, which
may be lifted only about 2 meters.
6. What is the difference between eolian
saltation and fluvial saltation?
The term saltation was used in Chapter 14 to describe
movement of particles along stream beds. The term saltation
also is used in eolian processes to describe the wind
transport of grains along the ground, grains usually larger
than 0.2 mm. About 80% of wind transport of particles is
accomplished by this skipping and bouncing action (See
Figure 15-6). In comparison with fluvial transport, in which
saltation is accomplished by hydraulic lift, eolian saltation is
executed by aerodynamic lift, elastic bounce, and impact.
Figure 15.6: Sand Transport; Eolian suspension,
saltation, and surface creep are mechanisms of
sediment transportation.
7. Explain the concept of “surface creep”.
Wind exerts a drag or frictional pull on surface
particles. Bagnold studied the relationship between
wind velocity and grain size, determining the fluid
threshold (minimum wind speed) required for initial
movement of grains of various sizes. A slightly
lower wind velocity suffices if the particle already
has been set into motion by the impact of a saltating
grain. Bagnold termed this lesser velocity the impact
threshold. Once in motion, particles continue to be
transported by lower wind velocities.
8. What is the difference between an erg and a
reg desert? Which type is a sand sea?
A common assumption is that most deserts are covered by
sand. Instead, desert pavements predominate across most
sub-tropical arid landscapes; only about 10% of desert areas
are covered with sand. Sand grains generally are deposited
as transient ridges or hills called dunes. A dune is a windsculpted accumulation of sand (See next slide). An
extensive area of dunes, such as that found in North Africa,
is characteristic of an erg desert, which means sand sea.
Most desert landscapes are not covered with sand but are
desert pavements, which are so common that many
provincial names have been used for them–for example,
“gibber plain” in Australia, “gobi” in China, and in Africa,
“lag gravels” or “serir” or reg desert.
Dune Cross Section: Successive slipfaces exhibit a distinctive
pattern as the dune migrates in the direction of the effective wind.
9. What are the three classes of dune forms?
We can simplify dune forms into three classes–crescentic,
linear, star dunes, and others. Crescentic dunes are
divided into four types: Barchan, Transverse, Parabolic,
and Barchanoid Ridge. Barchan dunes are crescent shaped
dunes with horns pointed downwind. Winds are constant
with little directional variability. Limited sand availability.
Transverse dunes are asymmetrical ridges which are
transverse (perpendicular) to the wind direction. Surface
has abundant sand supply. Parabolic dunes are generated by
vegetation, open end faces upwind with U-shaped and arms
shaped by the vegetation. Barchanoid ridge dunes, are
wavy, symmetrical dune ridges aligned in right angels to the
winds. Formed from coalesced barchans. (See next slides).
9. What are the three classes of dune forms?
(continued)
The next class of dunes is called linear dunes. They are
divided into two types: Longitudinal and Seif.
Longitudinal dunes are long, ridge-shaped dunes that are
aligned parallel to the wind direction and have two slipface.
Average 100 meters high and 100 kilometers long and can
reach to 400 meters high. Seif dunes (means sword in
Arabic) a sharp-crested sand dune with curved edges, often
several miles long. Runs in a series of parallel ridges; most
common in the Sahara desert.
Longitudinal Dunes (left) and a Satellite photo
of Seif Dunes in Saudi Arabia (Right).
9. What are the three classes of dune forms?
(continued)
The third class of dunes is called a Star dune (One
type only). Star dunes are giant dunes; Pyramidal or
star shaped. Slipsurfaces in multiple directions.
Resulting from winds shifting in all directions. (See
next slide).
Star Dunes
9. What are the three classes of dune forms?
(continued)
The last class is named “other” for other types.
There are two of them: Dome dunes and Reversing
dunes. Dome dunes are circular or elliptical
mounds with no slipface. Reversing dunes are
asymmetrical ridges formed intermediately between
star dunes and transverse dunes formations. Wind
direction can alter their shapes between forms. (See
next slide).
Dome and Reversing dunes.
Sandy Regions of the World: World wide distribution of
active and stable sand regions.
10. Another form of material deposits are loess
deposits. How are loess materials generated? What
form do they assume when deposited?
Pleistocene glaciers advanced and retreated in many
parts of the world, leaving behind large glacial
outwash deposits of fine-grained clays and silts
(<0.06 mm). These materials were blown great
distances by the wind and redeposited in
unstratified, homogeneous deposits named loess.
Loess deposits form some complex weathered
badlands and some good agricultural land.
11. Name a few examples of significant loess deposits on Earth.
Answer: In Europe and North America, loess is thought to be
derived mainly from glacial and periglacial sources. The vast
deposits of loess in China, covering more than 300,000 km2, are
thought to be derived from desert rather than glacial sources.
12. Desert features: Describe a desert
bolson formation from crest to crest.
A typical bolson is a slope-and-basin area between
the crests of two adjacent ridges in a dry region- the
physiology and geography combine to give the
bolson a dry climate, few permanent streams, and
internal drainage pattern (no ocean drainage).
Basin-and-range relief is abrupt, and rock structures
are angular and rugged. As the ranges erode, the
transported materials accumulate to great depths in
the basins, gradually producing extensive desert
plains. (See next slide).
An example of a Bolson.
13. What is meant by desertification?
An unwanted expansion of the Earth’s desert lands in a
process known as desertification. This now is a worldwide
phenomenon along the margins of semiarid and arid lands.
Desertification is due principally to poor agricultural
practices (overgrazing and agricultural activities that abuse
soil structure and fertility), improper soil-moisture
management, erosion and salinization, deforestation, and the
ongoing global climatic change which is shifting
temperature and precipitation patterns.
The United Nations estimates that degraded lands have
covered some 2 billion acres since 1930; many millions of
additional acres are added each year. An immediate need is
to improve the data base for a more accurate accounting of
the problem and a better understanding of what is occurring.
The process of desertification:
Figure 15-24 (next slide) is drawn from a map prepared for
a U.N. Conference on Desertification. Desertification areas
are ranked: A moderate hazard area has an average 10%–
25% drop in agricultural productivity; a high hazard area
has a 25%–50% drop; and a very high hazard area has more
than a 50% decrease. Because human activities and
economies, especially unwise grazing practices, appear to
be the major cause of desertification, actions to slow the
process are readily available. The severity of this problem is
magnified by the poverty in many of the affected regions.
Figure 15.24: Desertification
Figure 15.24
Movie: Wind, Dust and Deserts
This program shows how deserts are defined by
infrequent precipitation and how desertification
relates to proximity to the equator, proximity to
mountains, and ultimately plate tectonics. Images of
landscapes illustrate how wind creates features such
as dunes, playas, blow-outs, and even oases.
http://www.learner.org/resources/series78.html
End of Chapter 15
Eolian Processes and Arid
Landscapes
Geosystems 5e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen