Chapter 15 Eolian Processes and Arid Landscapes

1
Chapter 15Eolian Processes and Arid Landscapes
Geosystems 5e An Introduction to Physical
Geography
Robert W. Christopherson Charlie Thomsen
2
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.

3
Key Learning Concepts

• 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.

4
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 often-cited 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.

5
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.

6
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)

7
Desert pavement is formed from larger rocks and
fragments left after deflation.
8
Yardang A small wind-sculpted rock formation
caused by abrasion in Utah.
Figure 15.4
9
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.

10
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.

11
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.

12
Figure 15.6 Sand Transport Eolian suspension,
saltation, and surface creep are mechanisms of
sediment transportation.
13
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.

14
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 wind-sculpted 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 themfor example, gibber plain in
  Australia, gobi in China, and in Africa, lag
  gravels or serir or reg desert.

15
Dune Cross Section Successive slipfaces exhibit
a distinctive pattern as the dune migrates in the
direction of the effective wind.
16
9. What are the three classes of dune forms?

• We can simplify dune forms into three
  classescrescentic, 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).

17
(No Transcript)
18
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.

19
Longitudinal Dunes (left) and a Satellite photo
of Seif Dunes in Saudi Arabia (Right).
20
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).

21
Star Dunes
22
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).

23
Dome and Reversing dunes.
24
Sandy Regions of the World World wide
distribution of active and stable sand regions.
25
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 (lt0.06 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.

26
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.
27
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).

28
An example of a Bolson.
29
13. What is meant by desertification?

• An unwanted expansion of the Earths 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.

30
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
  1025 drop in agricultural productivity a high
  hazard area has a 2550 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.

31
Figure 15.24 Desertification
Figure 15.24
32
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

33
End of Chapter 15Eolian Processes and Arid
Landscapes
Geosystems 5e An Introduction to Physical
Geography
Robert W. Christopherson Charlie Thomsen