CHAPTER 19 Winds and Deserts Chapter Summary Deserts are regions where evaporation exceeds precipitation. Desert regions on Earth are the result of: global air circulation patterns which generate a relatively stationary zone of descending, warm and dry air at about 15 to 30 north and south of the equator; distance from large bodies of water like the oceans; the rain shadow generated by high mountains block the flow of moisture-rich air; cold ocean currents which reduce air temperatures and reduce the transport of moisture by the air; and polar climates where the air is so cold that it holds very little moisture at all. Wind is a major erosional and depositional agent, moving enormous quantities of sand, silt, and dust. Turbulent air-flow erodes and transports particles by suspension, sliding, rolling and saltation. As velocity decreases, sediment is pulled out of air by gravity and deposited as a blanket or dunes of sand and dust. Only about 20 percent of the area of desert regions are covered by sand. The distinctive types of sand dunes are governed by the amount of sand available, the strength of the wind, and the variability in wind direction. Loess, wind blown dust, is another important eolian deposit. Geologic and topographic features associated with desert regions include: sand dunes, loess deposits, evaporite (salt) deposits in playas, desert pavement, ventifacts (sandblasted rocks), alluvial fans and alluvial sands and gravels, pediments, distinctive soils (pedocals, rich in calcium carbonate), and rusty orangish brown colors to weathered rock surfaces. Some of these features are preserved in the rock records and provide geologists clues to ancient desert regions that no longer exist. 194
Winds and Deserts 195 Learning Objectives In this section we provide a sampling of possible objectives for this chapter. No class could or should try to accomplish all of these objectives. Choose objectives based on your analysis of your class. Refer to Chapter 1: Learning Objectives How to Define Your Goals for Your Course in the Instructional Design section of this manual for thoughts and ideas about how to go about such an analysis. Knowledge Know how turbulent flow and forward motion combine to lift finer particles into the wind and carry them by suspension, sliding, rolling, and saltation. Know how winds deposit sand and dust. Know the relationship between the shape of sand dunes, the supply of sand, the strength and variability in wind direction. Know what factors contribute to the existence of desert regions on Earth. Know what features are characteristic of desert landscapes. Skills/Applications/Attitudes Interpret wind direction and reconstruct paleowind directions using dune shape, ripples, and cross-bedding within the dune. Know how human activities may play a role in desertification. Know how ancient deserts can be recognized by geologists studying the rock record. General Education Skills Write a two-page position paper exploring the causes of desertification for a United Nations committee. Discuss at least one possible natural and one possible human cause for the expansion of desert conditions. Be sure to identify the time span (decades, centuries, millennium, or however long), over which each possible cause operates. Freshman Survival Skills Encourage students to develop time-management skills by putting up a slide referring them to the time-management tip in the Before Lecture section of Chapter 19 of the Student Study Guide, available in the Understanding Earth e-book (time management/ previewing) Sample Lecture Outline Sample lecture outlines highlight the important topics and concepts covered in the text. We suggest that you customize it to your own lecture before handing it out to students. At the end of each chapter outline consider adding a selection of review questions that represent a range of thinking levels.
196 PART II CHAPTER 19 Chapter 19: Winds and Deserts Deserts are where evaporation exceeds precipitation Wind Turbulence Wind belts Coriolis effect Wind Transport Suspension Saltation Rolling Sliding Wind transported materials Loess/dust Sand Wind erosion Ventifacts Deflation Desert pavement Wind deposits Sand dunes Barchan Transverse Blowout Linear Loess/dust Factors Contributing to Desert Conditions Global air circulation Distance from oceans Topography Ocean currents Desert weathering Rusty color Desert varnish Sediments Alluvial Eolian Evaporites Desert landscape features Dry washes/wadis Alluvial fans Pediments Playas dry lake beds
Winds and Deserts 197 Teaching Tips Cooperative/Collaborative Exercises and In-Class Activities Refer to Chapter 4: Cooperative Learning Teaching Strategies in the Instructional Design section of this manual for general ideas about conducting cooperative learning exercises in your classroom. Coop Exercise 1: Quick Think/Pair/Share Exercises The following question from the Student Study Guide could be used in Think/Pair/Share exercise. While hiking through a dune-filled coastal plain on a windless morning, you become surrounded by a dense fog and realize you are lost. You know you are near a shoreline, and that the beach will lead you back to camp, but you don t know the correct direction. You recognize the crescent-shaped dunes that wrap moderate depressions as blow-out dunes! According to your compass, the tapered arms of the dunes point south. Then, remembering that in this region strong, gusty winds come onto the coastal plain off the ocean, you immediately remember which direction to head to get to the beach: A. North B. South C. East D. West Hint: Refer to Figure 19.12 You are lost in the Goblin Desert. The nearest town is due south. Winds blow from the south to the north but there is no wind blowing today. Furthermore, the sun is totally obscured by clouds. Without a compass, which direction are you going to hike given the orientation of the barchan dune shown below? A. Direction A B. Direction B C. Direction C D. Direction D Hint: Refer to Figure 19.12
198 PART II CHAPTER 19 Sample Exercise from the Student Study Guide Exercise 1: Sand Dune Types Complete the table below by filling out the blank spaces. Figure 19.12 and the section in your text on Dune Types will be helpful. Dune Type Characteristics Sand Supply Wind Direction/Strength Barchan Transverse Blowout limited to moderate unidirectional/gusty Linear See Figure 19.8 Freshman Survival Skills Assignment Refer students to the following study tip in the Student Study Guide or put it up as a slide before your lecture while students are arriving in the classroom. Time-Management Tip: Preview Just Before Lecture The best time to preview is right before lecture. Why? Because the preview questions will be fresh in your mind as the lecture begins. If you can arrive thirty minutes before lecture that is ideal. But even five minutes of previewing done just before the lecture begins will improve your ability to understand the material. Here s a program for using whatever time you have efficiently. It is prudent to include a few exercises to help the freshmen in your class learn how to learn and to reinforce mastery of the basics of good preparation for college-level lectures. In the Student Study Guide we laid out a strategic approach for students to use to master this chapter (quoted in full below). Consider delivering this material to your students in a brief lecture segment or converting it into an assignment. How Much Time Do You Have Before Lecture Begins? How To Use It 30 minutes or more With this much time you can dig deep into the chapter. Do as many of the following as your time allows. Read the Chapter Preview questions and brief answers Read the suggestions for During Lecture Study the key figure(s) for this chapter (usually shown at the beginning of the Student Study Guide chapter) Study and annotate any additional figures, hints, or suggestions alluded to in the Chapter Preview If time allows, do the Practice Exercises 15 to 20 minutes Do a brief but intense preview: Read the Chapter Preview questions and brief answers Read the suggestions for During Lecture Study the key figure for this chapter (always shown at the beginning of the Student Study Guide chapter) 5 to 10 minutes Read the Chapter Preview questions and brief answers. Focus on getting the questions clearly in mind. Then listen for answers during lecture. Even five minutes of previewing helps!
Winds and Deserts 199 Topics for Class Discussion Geoscientists are studying fossil sand dunes to estimate paleowind directions (wind directions hundreds of millions of years ago). Explain how the dune shapes and cross-bedding within the fossil dunes can be used in this way. There are several types of dunes. How are the dune types influenced by wind direction and velocity, quantity of sand available, and amount and type of vegetation or other obstacles to sand movement? Consider an in-class exercise, homework, or extra credit exercise that asks students to discuss what evidence might be found in the rock record for ancient deserts. Possible answers include: Geologic Record of Deserts Sediments Loess Evaporites Sandstone Sedimentary structures Large-scale, planar cross-bedding Ripples Raindrop imprints Salt casts Physical weathering dominates over chemical weathering Preservation of less stable minerals, e.g., feldspars Poorly developed paleosols Rusty color to sediments Paleosols rich in carbonate and deficient in organic (plant) matter Lack of fossils Teaching Resources Student Study Guide Highlights (part of the Understanding Earth e-book) In Part I, chapters provide strategies for learning geology. Ideally, students would read these chapters early in the course. Chapter 1: Brief Preview of the Student Study Guide for Understanding Earth Chapter 2: Meet the Authors Chapter 3: How to Be Successful in Geology In Part II, Chapter 19: Winds and Deserts Before Lecture: Preview Questions and Brief Answers Time Management Tip Box Vital Information from Other Chapters During Lecture: Warm-up Activity After Lecture: Check Your Notes: Have You Intensive Study Session
200 PART II CHAPTER 19 Exam Prep: Chapter Summary Practice Exercise: Sand Dune Types Review questions Additional Web Sites The two Web sites below are an excellent source for additional information and images. They may also be useful as a reference for student assignments. http://pubs.usgs.gov/gip/deserts/ http://www.desertusa.com/glossary.html