1.17: Lab 17 - Aeolian Geomorphology and Desert Landscapes

Learning Objectives

• Explain how arid regions are classified.
• Analyze the spatial pattern of human-induced desertification risk.
• Analyze the relationship between grain transport and size distribution.
• Identify erosional and depositional aeolian landscape features.

Introduction

Named from Aeolus, the Greek god of wind, aeolian processes refer to wind’s ability to shape the surface of the planet. Aeolian processes are most effective where surface material is fine, dry, and loose. Vegetation imposes a frictional force on the wind to reduce its effectiveness. Thus, aeolian processes are most effective in an environment devoid of vegetation. These conditions are met in deserts, found on every continent of the world. In most cases, wind erosion predominates over deposition, leaving a surface of stones. Most people think of dunes when they think of deserts, but only one-quarter of Earth's deserts are either partially or completely covered by sand.

Though less extensive in area, coastlines of large bodies of water are another aeolian environment. Here, waves and currents supply weathered material that is susceptible to wind action. Aeolian processes have been enhanced by human activity over the past few centuries. Overuse of soil and grazing land resources in semi-arid and arid and seasonally-dry regions has led to extensive wind erosion and desertification.

In this lab, you will explore local regions that have been shaped through aeolian processes; understand the importance of erosion, transportation, and deposition; and learn how to identify and understand the development of aeolian features.

Part A. Arid Region Classification

With nearly one third of Earth's land surface considered a desert or arid land, meaning it receives minimal rainfall, these regions support sparse yet impressive vegetation and a limited yet unique population of animals. Desert settings in films such as The Good, the Bad and the Ugly and Star Wars convey a sense of adventure and exploration. Deserts may be hot, or they may be cold. They may be regions of sand or vast areas of rocks and gravel peppered with occasional plants. But deserts are always dry.

There are almost as many definitions and classification systems of deserts as there are actual deserts in the world. Most classifications rely on some combination of the number of days of rainfall, the total amount of annual rainfall, temperature, humidity, or other factors. In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, these areas are differentiated in one of three ways:

➢ Extremely arid lands have at least 12 consecutive months without rainfall.

➢ Arid lands have less than 250 millimeters (approximately 9.8 inches) of annual rainfall.

➢ Semiarid lands have annual precipitation rates between 250 and 500 millimeters (approximately 9.8 to 19.7 inches).^[304]

Exercise \(\PageIndex{1}\)

1. Refer to Figure 17.1, which shows the global annual precipitation in millimeters per year.

1. Locate the Sahara Desert on the map. Approximately how many millimeters of rain does the majority of this desert receive?

2. Locate the Patagonian Desert on the map. Approximately how many millimeters of rain does the majority of this desert receive?

3. List three spatial patterns of global precipitation that you observe on this map:

Figure 17.1: Global Precipitation Values. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

Desertification is the shift from perennial grasslands to shrublands that occurs globally to impact nearly 40% of the Earth's land surface and a fifth of the world's human population. Desertification results from interactions between human activities (such as livestock overgrazing) and prolonged drought. Desertification is a global problem that reduces plant productivity, biodiversity, air and soil quality, and water availability.^[306] A classic example of desertification is the 1930s Dust Bowl period in the United States, which devastated Midwestern states. It was a time when severe droughts combined with poor land-management practices to cause tremendous suffering and economic loss.^[307] Figure 17.2 is a series of three photographs from near Tucson, Arizona, that show the progression from arid grassland to desert over a 100-year period. The change is the result of grazing management and reduced rainfall in the U.S. southwest.⁴

Figure 17.2: Desertification Photo Series. Figure and text by the U.S. Climate Change Science Program, Synthesis and Assessment Products, of the USDA is in the public domain

There are about 7.1 million square kilometers (km²) of land under low risk of human-induced desertification, 8.6 million km² at moderate risk, 15.6 million km² at high risk, and 11.9 million km² under very high risk. Each of these classes represents a desertification tension zone. The major critical tension zone that requires immediate attention is the very high-risk class. There are 11.9 million km² of land with about 1.4 billion inhabitants. For reference, the very high-risk area is larger than the areas of the United States and Mexico combined. Major national conflicts are related to the reduced ability of the land to support the people in agriculture-based economies. Tension zones result from:

➢ Excessive and continuous soil erosion resulting from overuse and improper use of lands, especially marginal and sloping lands;

➢ Nutrient depletion and/or soil acidification due to inadequate replenishment of nutrients or soil pollution from excessive use of organic and inorganic agri-chemicals;

➢ Reduced water-holding capacity of soils due to reduced volume of soil and reduced organic matter content, both a consequence of erosion and reduced infiltration due to crusting and compaction;

➢ Salinization and water-logging from over-irrigation without adequate drainage; and

➢ Unavailability of water stemming from decreased supply of aquifers and drainage bodies.^[309]

Figure 17.3 shows the areas at low (shaded green), moderate (yellow), high (orange), and very high (red) risk of desertification.

Figure 17.3: Risk of Human-Induced Desertification Map. Figure by USDA Natural Resources Conservation Service is in the public domain

Exercise \(\PageIndex{2}\)

2. Refer to Figure 17.3.

1. List three spatial patterns of the risk of human-induced desertification that you observe on this map:

2. Compare Figures 17.1 and 17.3. What is the relationship between global precipitation and the risk of human-induced desertification? Explain your response in at least one sentence.

Part B. Grain Transport and Size Distribution

Sand generally begins to move when the wind achieves a velocity of about 4.5 meters per second (m/sec), which is equivalent to about 10 miles per hour. At first, sand exhibits a rolling motion called traction or surface creep. Approximately 25% of total sand transport during sand storms occurs by traction. As wind speed increases, grains are lifted into the air by wind gusts.

Once airborne, sand grains travel downwind and then drop back to the surface several centimeters from their point of origin. Finer dust particles are lifted from the surface and suspended in the air at much greater heights than heavier sand grains. With strong winds and turbulence, sand grains can be lifted as high as 2 meters (~6.5 feet) and travel a distance of 10 meters (~32 feet) or more. About 25% of the total sand transport during these events are caused by this suspension process.

When a settling sand grain impacts the surface, it sends another grain of sand into the air to travel in the downwind direction in a process called saltation. Watching this process in action makes sand appear to be bouncing along the surface (Figure 17.4). Saltation accounts for about 50% of sand transport over dunes. Once saltation begins, transport can continue under somewhat lower wind speeds.

Figure 17.4: Sediment Saltation. Figure by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

Exercise \(\PageIndex{3}\)

3. Briefly summarize the three ways that material is moved by wind and include the percentage of each mode of transportation.

4. Apply What You Learned: Air reacts in a very similar way to the way a fluid reacts. With that being said, compare how material moves along a river to how material moves in wind, and identify a few similarities.

Now that you understand the three prominent ways material can move, it is important to analyze the variability of grain sizes. It’s important to know that the word sand refers to the size of the material, not what it is made of. This is why some beach sand can be made of quartz and feldspars, while other sand can be made of seashells and volcanic rocks.

Below you will find a table from the International Organization of Standardization, which

establishes the basic principles for the identification and classification of soils based on those material and mass characteristics most commonly used for soils for engineering purposes (Table 17.1). Also note, as earlier identified, that material can move either by traction, suspension, and saltation. Traditionally, material that is coarse or very coarse (up to 2mm) will move via traction. Material that is between 0.1mm to 0.5mm will move via saltation and material that is 0.1mm or smaller will be suspended. For reference, 1 millimeter is about 0.04 inches.

Table 17.1: Grain Size Classification

Classification Type	Classification Subtype	Classification Name	Label	Size range (mm)
Very coarse soil	n/a	Large boulder	LBo	>630
Very coarse soil	n/a	Boulder	Bo	200–630
Very course soil	n/a	Cobble	Co	63–200
Coarse soil	Gravel	Coarse gravel	CGr	20–63
Coarse soil	Gravel	Medium gravel	MGr	6.3–20
Coarse soil	Gravel	Fine gravel	FGr	2.0–6.3
Coarse soil	Sand	Coarse sand	CSa	0.63–2.0
Coarse soil	Sand	Medium sand	MSa	0.2–0.63
Coarse soil	Sand	Fine sand	FSa	0.063–0.2
Fine soil	Silt	Coarse silt	CSi	0.02–0.063
Fine soil	Silt	Medium silt	MSi	0.0063–0.02
Fine soil	Silt	Fine silt	FSi	0.002–0.0063
Fine soil	Clay	Clay	Cl	≤0.002

Exercise \(\PageIndex{4}\)

5. Use Your Critical Thinking Skills: After reviewing the table, why do you think it is important that the grain size of material is recorded?

6. Use Your Critical Thinking Skills: Assuming you are in a location doing research and that the air is very still. How could you use the average grain size to explain the general wind flow for this environment?

7. Interpret the sample photographs shown in Figure 17.5. For each photograph, take note of the scale bar present. Identify the best name for each sample based on the average grain size shown in Table 17.1.

1. What is the average grain size measured and classification name of Sample A?

2. What is the average grain size measured and classification name of Sample B?

3. What is the average grain size measured and classification name of Sample C?

4. What is the average grain size measured and classification name of Sample D?

Figure 17.5: Four Microscopic Samples of Grain Sizes. Figure by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

Part C. Erosional Aeolian Landforms

Landscapes formed from the work of wind result from either the removal of fine particles or the sculpting effects of material in movement. Deflation lifts and removes loose particles from the surface. Deserts, where soils of mixed particle size have been eroded of fine-grained sediments, leave a cobblestone-like surface behind called desert lag or desert pavement. The interlocking pavement of stones protects the underlying surface from the wind. If disturbed, the surface becomes subject to erosion.

It can be surprising to learn that water is an important agent of erosion in arid lands. Not all erosional landforms are made of cobbles and stone, or are flat and desolate. Playas are large, flat, dried-up lands located in the desert basins. These features are the result of evaporation rates that exceed precipitation, often leaving behind mud cracks and salt flats atop their playa surfaces. Often in arid regions, there can be steep-sided gullies, called arroyos, that are carved by fast-moving waters during torrential desert storms. Big or small, erosional landscapes are dynamic in arid regions, with the help of both wind and water.

Check It Out! The Lowest Elevation in North America

Ever been under the ocean? How about -282 feet below the ocean? Well, check out Badwater Basin in Death Valley! This basin is an endorheic basin also known as a playa. When you scan the QR code, Google Earth will position you so that you can zoom in and see the sea level marker, but don’t forget to turn around to explore the landscape!

Lag Deposits and Desert Pavements

Due to erosion, transportation, and deposition, desert surfaces can be completely covered in sand, or have closely packed, interlocking pebbles or even a tight array of larger cobbles. Lag deposits are the result of the removal of fine-grained material, leaving behind large and irregularly-shaped material. Desert pavements are a continuation of a lag deposit, in that the larger fragments left behind become interlocked and compacted. Due to forces such as wind, water, and thermal expansion, these larger fragments will settle into a tightly packed surface—much like a paved surface, thus earning the name desert pavement. Ventifacts are rocks that have been shaped by the erosive action of wind, similar to sandblasting.

Ventifacts are important for scientists as we are able to identify consistent direction(s) of wind flow. Below is a ventifact image of a large extrusive igneous boulder that has been left behind on a lag deposit (Figure 17.6). This is a piece of basalt; note the air bubbles that are within the rock. Observe the specimen and answer the following question. This boulder is approximately 30 pounds and more than 16 inches in length. You may observe a very distinct linear pattern on the surface of this rock, somewhere between a polishing and a pitting (abrasion).

Exercise \(\PageIndex{5}\)

8. Apply What You Learned: What do you think caused these markings on the surface of the rock shown in Figure 17.6, and can you deduct a general pattern or direction of these markings?

Figure 17.6: Image of a Ventifact in Death Valley. Figure by Daniel Mayer is licensed under GNU FDL

Buttes, Mesas, and Plateaus

Often features observed in our environment are so large that it might be hard to even see them, and the best way to do so is to observe them from an aerial view. Some of the largest erosional features observed are buttes, mesas, and plateaus. Although the features might look similar, what differentiates one from another is their size.

Buttes: due to extensive erosion and weathering (by wind and water), buttes are formed when a more resilient caprock, or top layer of material that is more resistant to weathering, remains and the material around is worn away, leaving a column-shaped feature. Buttes can be hundreds of feet tall, and similar to a barstool, have a smaller flat top. The key observation for identification is that these features are taller than they are wide.

Mesas: similar to the formation of a butte, mesas have steep sides and a larger flat top; in fact, the feature might resemble that of a table. A mesa’s top is generally large enough that standing water may be found. The key observation for identification is that these features are wider than they are tall.

Plateaus: similar still to buttes and mesas in development, what differentiates plateaus is that the flat top can extend for thousands of square miles. As an example, the Colorado Plateau, which the Grand Canyon is actively carving through, covers portions of Colorado, New Mexico, Utah, and Arizona. The key observation for identification is that these features are huge, flat, elevated highland.

Exercise \(\PageIndex{6}\)

9. Refer to Figure 17.7, which shows an unlabeled arid landscape.
   1. Label a butte, a mesa, and a plateau.
   2. Label an additional two features that are shown in the diagram. Tip: review the erosional landforms discussed previously in the lab in order to identify two additional features.

Figure 17.7: Unlabeled Arid Feature Diagram. Figure by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

10. Now that you have learned how to observe and interpret the differences between buttes, mesas and plateaus, identify them on a topographic map from New Mexico (Figure 17.8).

1. Circle and label a butte, a mesa, and a plateau.

2. In one sentence, explain how you identified a butte:

3. In one sentence, explain how you identified a mesa:

4. In one sentence, explain how you identified a plateau:

Figure 17.8: Wedding Cake, New Mexico, Topographic Map. Figure by Jeremy Patrich adapted from USGS is licensed under CC BY-NC-SA 4.0

Part D. Depositional Aeolian Landforms

There are many different types of depositional landforms; the most prominent in arid regions is the dune. Depositional landforms are the visible evidence of erosion and transportation, as these landforms are made of those materials.

Sand Dune Development

Sand dunes are formed by wind moving sand particles (Figure 17.9). Let’s begin with the foundation of dune formation. Figure 17.9 shows that with the assistance of wind (transportation) the sand grains are in motion, working uphill along the stoss side of the dune until the grains reach the top. Then the heavier or coarser-grained material rolls along the leeward side and the finer grains may become suspended.

Figure 17.9: Sand Movement along a Dune. Figure by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

Exercise \(\PageIndex{7}\)

11. Refer to Figure 17.9.

1. Apply What You Learned: Assuming that no additional material is added, but the wind direction and velocity are consistent, what do you think would happen to the shape, size, and location of the dune? Your response should be one to two sentences in length.

1. Apply What You Learned: Assuming that additional material is added, and the wind direction and velocity are consistent, what do you think would happen to the shape, size, and location of the dune? Your response should be one to two sentences in length.

Sand Dune Properties

The shape and design of the dune vary with the amount or volume of sand available, as well as the direction(s) that the wind blows. If the wind blows steadily from one direction, then linear, transverse, or barchan dunes will form. If the direction that the wind blows shifts or comes from different directions, then star or network dunes will form. Figure 17.10 identifies a few different types of dunes possible based on the volume of sand supply and wind variability. The three types of wind variability are:

➢ Unimodal: the predominant winds travel in one direction;

➢ Bimodal: the predominant winds travel in two directions (for example, onshore during the day and offshore at night); and

➢ Complex: the predominant winds travel in a variety of directions.

Exercise \(\PageIndex{8}\)

12. Use Your Critical Thinking Skills to answer the following questions.

1. What is an example of where you might have experienced unimodal wind variability?

1. What is an example of where you might have experienced bimodal wind variability?

1. What is an example of where you might have experienced complex wind variability?

Figure 17.10: Sand Dunes by Wind Variability and Sand Supply. Figure by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

13. Refer to Figure 17.10. Sand supply can either be limited or limitless. Briefly identify where sand comes from, and explain why the supply of sediment could be either limited or limitless.

Now that you have a way to identify the dunes based on sand supply and wind variability, let’s now learn how to identify these dunes in the field. Although there are many more types and variations of dunes than what are listed in the figure above, for this course we will focus on the most common six dunes: barchan, parabolic, star, transverse, linear, and barchanoid ridges. Below you will find the basic definitions of each of these six dunes. After reading about each one, identify and circle: if the wind direction is unimodal or complex, and if the sand supply is limited or limitless. (Hint: be sure to refer to Figure 17.10!).

Exercise \(\PageIndex{9}\)

14. Barchan Dune: crescent-shaped dunes with the points (horns) of the crescents pointing in the downwind direction, and a curved slip face on the downwind side of the dune. They form in areas where there is a hard ground surface, a moderate supply of sand, and a constant wind direction.
    1. Unimodal or Complex?
    2. Limited or Limitless sand supply?
15. Parabolic Dunes (also called blowouts): "U" shaped dunes with an open end facing upwind. They are usually stabilized by vegetation, and occur where there is abundant vegetation, a constant wind direction, and abundant sand supply. They are common in coastal areas.
    1. Unimodal or Complex?
    2. Limited or Limitless sand supply?
16. Star Dunes: dunes with several arms and variable slip face directions that form in areas where there is an abundant sand supply and variable wind directions.
    1. Unimodal or Complex?
    2. Limited or Limitless sand supply?
17. Transverse Dunes: large fields of dunes that resemble sand ripples on a large scale. They consist of ridges of sand with a steep face on the downwind side and form in areas where there is an abundant supply of sand and a constant wind direction.
    1. Unimodal or Complex?
    2. Limited or Limitless sand supply?
18. Linear Dunes (also called seifs): long straight dunes that form in areas with a limited sand supply and converging wind directions.
    1. Unimodal or Complex?
    2. Limited or Limitless sand supply?
19. Barchanoid Ridge: consists of several joined barchan dunes and looks like a row of connected crescents. Each of the barchan dunes produces a wave in the barchanoid ridge. This occurs when the sand supply is greater than in the conditions that create a barchan dune.
    1. Unimodal or Complex?
    2. Limited or Limitless sand supply?

Sand Dune Identification

Exercise \(\PageIndex{10}\)

20. Below you will find six images of dunes (Figure 17.11). For each of the dunes shown in Figure 17.11, identify and label:
    1. the dune type,
    2. the windward side of each dune,
    3. the slip face side of each dune, and
    4. draw arrows indicating the direction or directions in which the wind is blowing.

Figure 17.11: Image of the Six Most Common Dunes to Identify Wind Direction, Variability, and Dune Shape. Figure by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

Alluvial Fans and Bajadas

Water is an important agent for erosion, transportation, and deposition in any region, but many are surprised to see how running water greatly impacts the topography in arid regions. Running water can move tremendous amounts of water and sediments during a flash flood or in torrent rains, which in turn will carve and shape the landscape. Furthermore, that material must go somewhere, and overtime these deposits will build up into large features called alluvial fans. Alluvial fans are fan-shaped deposits along the foothills of mountains, caused by flash flood material, called alluvium, being deposited at the mouth of these canyons. If the material from an alluvial fan spreads out enough to coalesce with (touch) another fan, then they are identified as bajadas. Lastly, when an alluvial fan is built by the debris flow, it is referred to as a debris cone or colluvial fan.

Exercise \(\PageIndex{11}\)

22. Figure 17.12 is a satellite image, with a topographic overlay, of an alluvial fan in Death Valley, California.
    1. Using a colored pencil or pen, outline and label the farthest extent (perimeter) of the alluvial fan. With a different color, highlight the source of the material. Hint: you are looking for the river channel that deposited the material.
    2. Using the scale bar, what is the width of the alluvial fan from the source of the material to the edge?
    3. Using the scale bar, what is the length of the alluvial fan from the southeast to the northeast?

Figure 17.12: Image of an Alluvial Fan in Death Valley. Figure by Ethan O’Connor is in public domain

Check It Out! Alluvial Fans

Have you ever seen an alluvial fan? When you scan the QR Code, Google Earth will position you so that you can zoom in and see some alluvial fans in Death Valley!

Bajadas are aprons of rocky debris that form when alluvial fans overlap on their edges to form a ramp that spreads out toward the valley floor. A bajada or piedmont slope has sediment that is partially buried along its range front.

Exercise \(\PageIndex{12}\)

22. Refer to Figure 17.13, which is a satellite image of the southeastern portion of Death Valley.
    1. Using two different colored pencils or pens, outline and label any alluvial fans and/or bajadas.
    2. In two to three sentences, describe the differences between the alluvial fans and bajadas in Figure 17.13, and how you were able to identify them.

Figure 17.13: Image of Bajadas in Death Valley National Park, California. Figure by Google and USDA Farm Service Agency is used under Google Earth’s attribution guidelines

Loess Deposits

Loess are clay- and silt-sized sediments deposited by wind. These deposits are common in arid environments and also downwind of glaciers. During Ice Ages, loess was deposited in areas adjacent to continental glaciers that essentially ground rock into a powder. Approximately 10% of Earth’s surface is covered by loess deposits (Figure 17.14), which range in thickness from a centimeter to more than 90 meters (approximately 295 feet).

Figure 17.14: Map of Global Loess Deposits. Figure by Waverly Ray is licensed under CC BY-NC-SA 4.0

Exercise \(\PageIndex{13}\)

23. Use Your Critical Thinking Skills: Why do loess deposits often make productive agricultural soils? Explain your response in one to two sentences.

Part E. Wrap-Up

Consult with your geography lab instructor to find out which of the following wrap-up questions you should complete. Attach additional pages to answer the questions as needed.

Exercise \(\PageIndex{14}\)

24. What is the most important idea that you learned in this lab? In two to three sentences, explain the concept and why it is important to know.

25. What was the most challenging part of this lab? In two to three sentences, explain why it was challenging. If nothing challenged you in the lab, write about what you think challenged your classmates.

26. What is one question that you have about what you learned in this lab? Write your question along with one to two sentences explaining why you think your question is important to ask.

27. Review the learning objectives on page 1 of this lab. How would you rate your understanding or ability for each learning objective? Write one sentence that addresses each learning objective.

28. Sketch a concept map that includes the key ideas from this lab. Include at least five of the terms shown in bold-faced type.

29. Create an advertisement to educate your peers on the important information that you learned in this lab. Include at least three key terms in your advertisement. The advertisement should be about half a page in size (about 4 inches by 6 inches).

30. One way to think of physical geography is that it is the study of the relationships among variables that impact the Earth's surface. Select two variables discussed in this lab and describe how they are related.

31. How does what you learned in this lab relate to your everyday life? In two to three sentences, explain a concept that you learned in this lab and how it relates to your day-to-day actions.

32. How does what you learned in this lab relate to current events?
    1. Write the title, source, and date of a news item that relates to this lab.
    2. In two to three sentences, discuss how the news item relates to what you have learned in this lab.
    3. In one to two sentences, discuss whether or not the news item accurately represents the science that you learned. Tip: consider whether or not the news item includes the complexity of the topic.

33. Search O*NET OnLine to find an occupation that is relevant to the topics presented in today's lab. Your lab instructor may provide you with possible keywords to type in the Occupation Quick Search field on the O*NET website.
    1. What is the name of the occupation that you found?
    2. Write two to three sentences that summarize the most important information that you learned about this occupation.
    3. What is one question that you would want to ask a person with this occupation?

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^[304] Text by USGS is in the public domain

^[306] Text by USDA Agricultural Research Service is in the public domain

^[307] Text by the Bureau of Global Public Affairs, U.S. Department of State is in the public domain

^[309] Text by USDA Natural Resources Conservation Service is in the public domain