Lesson 8: Sierra Nevada
In this week's lesson we'll explore the geology and mineral resources of one of California's most spectacular landscapes, the Sierra Nevada. Much of this mountain range is underlain by the Sierra Nevada batholith, a great mass of granite that formed about 100 million years ago as magmas forced their way into the crust above an ancient subduction zone. As these magmas solidified deep underground the heat they released circulated groundwater that scavenged gold from the surrounding crustal rocks and concentrated it into the rich veins of the Mother Lode. After subduction ended about 80 million years ago, uplift and erosion stripped away most of the rocks that once lay above the batholith. During the past 2 million years ice-age glaciers have sculpted the range's granite core into the landscape of jagged peaks (like Eriksson Crags, right) and deep valleys that define the Sierra Nevada today.
In chapter 8 Harden briefly describes the uplift history of the modern Sierra Nevada as well as the intrusive and metamorphic events that shaped the region during the Mesozoic. She goes on to explain how lode and placer gold deposits were formed and mined, and how glaciers have shaped the modern range during the Pleistocene "ice age" of the past two million years. Finally, this week's exercise will give you an opportunity to learn how geologists estimate when and how much water mountain snowpacks—like those of the Sierra Nevada—are likely to produce during a given year.
As you read about the geology and geologic history of the Sierra Nevada 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 refer 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 the ages and locations of lava flows in the western Sierra, like Table Mountain, enable us to place limits on when the crest of the modern range rose to its present height.
- Briefly outline the geologic history and setting of the Sierra Nevada batholith—what tectonic setting did it form in, approximately when did it form (in Ma), and how do the ages and emplacement depths of plutons generally differ within it .
- Determine the name of a granitic rock from a knowledge of the relative amounts of quartz, plagioclase, and potassium feldspar it contains (see Fig. 8-12, p. 168 and our tutorial on classifying granitic rocks).
- Contrast the types of metamorphic rocks found in the eastern part of the Sierra with those found in the western part and briefly describe what these differences tell us about the different types of crust the batholith was emplaced into across the range.
- Explain how a lode gold deposit differs from a placer gold deposit, and briefly describe how each is formed.
- Outline the general temperature and precipitation conditions that determine whether a glacier will advance or retreat.
- Identify common landforms created by glacial erosion (cirques, U-shaped valleys, horns, roches moutoneés, glacial striations, and glacial polish) and deposition (moraines) in the Sierra Nevada.
Reading and Browsing Assignment
- Read Chapter 8, focusing on the topics outlined in the learning objectives above.
- Browse through "The Geologic Story of Yosemite Valley" and "Bedrock Geology of the Yosemite Valley Area" on the U.S. Geological Survey's Yosemite National Park Geology website. There is a lot of detail in these readings, so just focus on the major topics outlined in the learning objectives above.
- To learn a little more about how geologists are using oxygen from ancient rainwater to piece together the uplift history of the Sierra Nevada, check out this recent article from the San Francisco Chronicle.
- To gain a little more in-depth information about the occurrence of gold in the northwestern Sierra browse through this site on gold-bearing veins in the Mother Lode.
- Finally, for a brief introduction to how glaciers form and carve U-shaped valleys check out the USGS-NPS Glaciers site. Note that only the first two parts of the site are active. You can also learn to recognize a variety of glacial landforms by reviewing the USGS-NPS glacier diagrams.
Exercise 8: Snowpack Monitoring (Due by 9:00 AM on 7-Mar-2011)
Calfornia depends on the Sierra Nevada snowpack for about two-thirds of its water. Although this year's snowpack is about 125% of average for this time of year, you can understand how important it is for hydrologists to be able to predict the runoff from the mountains accurately so that they can plan summer deliveries to cities and farms. Your job this week is to use data from automated snowpack monitoring stations to estimate the likely timing and amount of runoff from a mountainous area, just as Department of Water Resouces hydrologists are doing for the Sierra Nevada right now. Please load up your Hazard City CD and work through version 2 of the Snowpack Monitoring exercise. Pay careful attention to the approximate dates at which snowmelt begins and ends at each SNOTEL site, and be sure to jot down any details about your observations that might be helpful in answering the questions the exercise poses. I suggest that you print the form provided on the "Report" page and fill it in before you go to the Etudes "Tasks and Tests" area to submit your answers for Exercise 8. (Note that there are only five questions this week, so each is worth two points.)
Quiz 8: Sierra Nevada (Due by 9:00 AM on 7-Mar-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 8 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about the Sierra Nevada and each is worth one point. If you can answer all of them correctly it means that you know your way around the geology of the Sierra pretty well and are ready to start learning about the enigmatic Klamath Mountains 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.