Lesson 6: California's Deserts
Today, as Harden points out, about one fifth of California is desert. These lands are spread among three geomorphic provinces (Basin and Range, Mojave Desert, and Colorado Desert) but share the characteristic that each year they lose more mositure by evapotranspiration than falls as precipitation. California's deserts have played a major role in the state's history as sources of mineral wealth, and are likely to play an even larger role in its future as the development of alternative energy—especially solar energy—grows. This week's lesson looks at how wind and water are shaping California's modern desert landscapes (like the dunes at Mesquite Flat, right), and at the very different role that water has played in this landscape during much of the past two million years (Pleistocene Epoch, p. 43). Although our initial focus will be on the recent history of California's deserts, we'll revisit them next week to study the ancient sediments exposed there that tell of a bygone ocean.
In chapter 6 Harden briefly describes how and where deserts develop in California, although she highlights the origins of rainshadow deserts somewhat more than tropical ones. She goes on to explain how wind and water are shaping modern desert landscapes, and also how features such as playas, tufa towers, and evaporite deposits preserve a record of the lakes and river systems that defined the region during the Pleistocene "ice age". Finally, this week's exercise will give you an opportunity to learn how geologists map groundwater—a critical resource in desert regions—in order to understand how it moves and may carry contaminants.
As you read about the modern geology and mineral resources of California's deserts it will be useful to take detailed notes. A lot of information that bears on this week's learning objectives is presented in the chapter, and writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and spot any gaps in your knowledge. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refere back to it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Explain how climatic criteria (precipitation and evapotranspiration) are used to define a desert, and briefly describe the conditions under which both "tropical" and "rainshadow" deserts develop.
- Describe how fine sediment is eroded by deflation, transported by the wind, and ultimately deposited to form dunes.
- Recognize common desert landforms such as desert pavements, alluvial fans, bajadas, arroyos, and playas, and explain how each is formed.
- List several features one might look for to decide if a closed desert basin had once held an ancient lake, and briefly explain why such lakes were much more common in eastern California prior to about 11,000 to 13,000 years ago than they are today.
- Describe the conditions that led to the development of tufa and evaporite minerals (including borates) in the ancient lakes of eastern California.
Reading and Browsing Assignment
- Read Chapter 6, focusing on the topics outlined in the learning objectives above.
- Browse through Ritter's site on tropical deserts and be sure that you understand the difference between these deserts (which lie beneath global high-pressure systems at about 30° N and S latitude) and others that occur on either the lee sides (rainshadows) of mountains or in coastal areas adjacent to cold ocean currents. Note that the Colorado Desert of southern California (and the adjacent Sonoran Desert of Arizona and Mexico) is a "tropical desert", whereas the Mojave and Great Basin Deserts are rainshadow deserts.
- To learn about the origins of a variety of desert landforms check out some of the "field trip stops" in the U.S. Geological Survey and National Park Service's "Geology in the Parks" sites for Death Valley National Park and Mojave National Preserve. There's a lot to explore on these sites, but for this week it will be best to focus on the recent histories of both areas. For example, check out the Shoreline Butte "stop" and the entire Death Valley Today section at Death Valley, as well as the Kelso Dunes and Soda Lake "stops" at Mojave. Remember, your goal is to learn to recognize and understand the origins of common features in today's desert landscapes; we'll explore the ancient geologic features of these regions next week.
- Finally, although California's early history may be remembered for the gold rush the evaporite mineral borax has also had a long and storied history in the state. Even though the "20 mule teams" are long gone, you can read a description of one of the world's largest producing borate deposits near Boron, California—including its origin—by checking out the Kramer Borate geocaching site. Note that the geologic background given on this site used to be on the Rio Tinto Borax Company's website, but does not appear to be linked there any longer.
Exercise 6: Groundwater Contamination (Due by 9:00 AM on 21-Feb-2011)
One common feature of deserts is a scarcity of surface water, and this often means that people living in desert areas rely on groundwater. In order to learn about how groundwater moves and how geologists track groundwater contamination, please load your Hazard City CD and work through version 3 of the Ground Water Contamination assignment. This is a fairly involved project, so allow a couple of hours to complete it. I found it helpful to print out the various reports before I started calculating my water table elevations. Using a spreadsheet (like Excel) can be a quick way to do these calculations if you have access to one and are familiar with how to use it. Be sure to jot down any useful notes on the procedure you followed and have your final map in hand before you go to fill out the report at the end of the assignment. I suggest printing the report worksheet, filling it in, and keeping it handy when you go to complete Exercise 6 in the "Assignments, Tests, and Surveys" tool of the Etudes site.
Quiz 6: California's Deserts (Due by 9:00 AM on 21-Feb-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 6 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about California's deserts and each is worth one point. If you can answer all of them correctly it means that you know your way around modern desert landscapes in California pretty well and are ready to start learning about the earlier geologic history of the Great Basin and Mojave Deserts next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.
