Geology 200

Study Guide

Unit 10:
Deserts and Wind Action

Overview

In Unit 10, we focus on another geomorphic agent that influences changes on the surface of the Earth: wind. Climate is an important variable that determines wind action. Results of wind action on the landscape are most notable in arid climates, defined as regions receiving less than 25 cm of precipitation per year. Landscapes in which such conditions prevail are usually defined as deserts. However, geomorphic processes in regions with arid climates are not necessarily dominated by a single process. Running water, tectonic forces, and wind all play a role in shaping the landscape. As such, desert landscapes may vary.

The unit begins by looking at the global distribution of deserts as well as their characteristics. A discussion of features found in desert landscapes follows, focusing on desert environments of the southwestern USA. Next we will look at wind erosion and transport, after which the unit concludes with a discussion of wind deposition.

Objectives

After completing this unit, you should be able to

  1. outline variables that control the distribution of deserts around the world, including atmospheric circulation, latitudinal location, distance from the ocean, and cold ocean currents.
  2. describe the characteristics of a desert environment, including drainage patterns, rainfall, runoff, and discharge patterns.
  3. outline features that characterize desert stream channels, including arroyos.
  4. outline features of landforms that result from wind erosion and sediment transport such as plateaus, mesas, buttes, and pediments.
  5. define the terms playa lake and bajada.
  6. define ventifacts, and explain how they are produced by wind action.
  7. describe the process of deflation, and outline the features it produces, including blowouts.
  8. define loess deposits, and describe how they are transported by wind.
  9. outline the conditions that promote the development of dune structures, and describe the basic morphology of a dune using the terms slip face, angle of repose, and saltation.
  10. describe different dune types including barchans, parabolic, transverse, and longitudinal dunes. For each dune type, explain the basic structure and conditions that favour its development.

Section 1: The Distribution and Characteristics of Deserts

The first part of this section looks at the global distribution of deserts. You will learn that climate, including the key factors of global air circulation and the distribution of precipitation, directly influences where desert conditions are found on Earth. Geomorphologic influences on precipitation and the effect of ocean currents on terrestrial climates are also highlighted as variables that may affect the location of deserts.

The second part of this section examines the characteristics associated with desert environments. We will discuss drainage, topography, and other general surface processes that occur in deserts.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • The introduction to Chapter 13: Deserts and Wind Action (pp. 318‑319).
  • “Where and How Deserts Form” (pp. 319-322).
  • “Some Characteristics of Deserts” (pp. 322-323).

Study Questions

  1. Why are the world’s deserts centred around the 30° North and South latitudes?
  2. What is a rain shadow zone?
  3. What are polar deserts?
  4. Why do most deserts lack through-flowing streams?
  5. What is an arroyo?

PDF icon Answer Key

Section 2: Desert Features in the Southwestern United States

To help familiarize you with the geomorphology of inland desert environments, this section examines the deserts of the southwestern USA. It will examine features that characterize the landforms of such landscapes and the processes through which such landforms develop.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Desert Features in the Southwestern United States” (pp. 323-328).

Study Questions

  1. What is the difference between a mesa and a butte?
  2. How do bajadas form from alluvial fans?
  3. How does a pediment develop from a mountain?

PDF icon Answer Key

Section 3: Wind Action: Sediment Erosion and Transport

Section 3 examines wind’s capacity to erode and transport sediment. Sediment transported by wind falls into two main categories: fine sediment and sand. Fine sediment comprises silt (diameter 0.004-0.052 mm) and clay (diameter < 0.004 mm), which are generally transported in suspension. As such, fine sediment can be transported over long distances. The other category of wind-transported sediment comprises sand. Sand includes mineral grains or lithological fragments in the size range 0.0625-2 mm. These particles are largely transported through saltation, whereby the grain particles move downwind via a series of jumps. As a result of these distinct modes of transport, sediments transported by wind are usually well sorted.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Wind Action” (up to “Wind Deposition”) (pp. 329-331).

Study Questions

  1. If you found ventifacts with three flat faces, what would you conclude about the wind activity in that environment?
  2. What does the process of deflation entail?
  3. How does a blowout form?

PDF icon Answer Key

Section 4: Wind Action and Sediment Deposition

This section addresses depositional features produced by wind action. As you learned in Section 3, wind transported sediments are generally well sorted. On one end of the spectrum are loess deposits, which comprise fine-grained sediment of silt and clay. Loess deposits can reach up to tens of metres high, and soils from loess deposits are often highly productive for agriculture. On the coarser end of the spectrum, wind-transported sands can be deposited in the form of dune structures. This section discusses a number of dune types and their depositional environments.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Wind Deposition” (pp. 331-338).

Study Questions

  1. How does high-angle cross-bedding develop in dune structures?
  2. What are wind ripples?
  3. What are the main structural differences between barchans and parabolic dunes?
  4. Which dune types form some of the largest structures, and what dimensions can they reach?

PDF icon Answer Key

Unit 10 Self Test

You have now finished Unit 10, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.

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