Lesson 5: Volcanism in California
Building on the basic geologic concepts we've learned during the past three weeks, we're now ready to begin our detailed study of California's geology. As Harden explains on pages 63-64, California can be subdivided into about a dozen geomorphic provinces that are characterized by their distinct rock types and geologic structures. In the weeks ahead we'll explore many of these provinces as well as some larger geologic issues—from seismic hazards to water resources—that are important to the state as a whole. This week we'll learn about recent volcanic activity in California and the potential hazards it poses. Today, most of this activity occurs in the northern part of the state where subduction produces the intermediate lavas in the High Cascades province (including Mount Shasta, right) and crustal extension produces mafic lavas in the Modoc Plateau. Farther south transform motion dominates the state's tectonics, and volcanism—like that at Long Valley—is commonly localized where the lithosphere is being pulled apart at transtensional "bends" along fault systems (Fig. 1-17).
In chapter 5 Harden briefly describes the tectonic settings of several major volcanic centers in California and explains how lava compositions (felsic, intermediate, and mafic) influence the types of eruptive materials and landforms these volcanoes produce. She also outlines the eruptive histories of several well-known volcanoes, describes the types of hazards posed by different types of eruptions, and concludes with brief discussions of how we assess the extents and frequencies of volcanic hazards and make use of volcanic materials. Although this chapter introduces some of the largest volcanic centers in California, in the weeks to come we'll see that smaller volcanic centers are found in other parts of the state as well (e.g., p. 136-140; and 302-306). Finally, this week's exercise will give you an opportunity to examine deposits produced by several volcanic processes and evaluate the potential hazards each might pose to a nearby town.
As you read about the geologic setting and character of volcanic activity in California it will be useful to take thorough 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:
- Correctly identify California's geomorphic provinces on a map of the state.
- Identify the five common types of volcanoes (shield volcanoes, stratovolcanoes, cinder cones, domes, and calderas) from photographs or cross-sectional drawings, and explain how their shapes and typical eruptive styles (effusive or explosive) are related to the compositions of the lavas they produce.
- Describe features you would look for to distinguish deposits produced by ash falls (tephra), lava flows, pyroclastic flows, and debris flows in the field, and indicate which of these deposits you might expect to pose a particular threat to people living in valleys on and around a volcano.
- Contrast the processes that cause partial melting of the asthenosphere to produce magmas along subduction zones (like Cascadia) with that which occurs in extensional setting (such as Lava Beds National Monument).
- Briefly describe how rhyolite lavas—like those that have been erupted at Long Valley, Mono Craters, and high on the Medicine Lake volcano—are formed. Specifically, which part of the lithosphere melts to produce them and what provides the heat to cause this melting in extensional settings?
- Describe the two general approaches that geologists use to infer the likely timing of future eruptions at volcanoes, and briefly explain how they compliment one another. (Hint: Consider what the monitoring of phenomena like earthquakes, ground deformation, or gas release might tell you versus what you might learn from the mapping and dating of ancient volcanic deposits and calculating the recurrence intervals of past eruptions (= time period during which past eruptions have occurred ÷ number of eruptions).)
Reading and Browsing Assignment
- Read the introduction to California's geomorphic provinces and Chapter 5, and be sure to focus on the topics outlined in the learning objectives above. Do not worry about trying to remember the details of the eruptive histories of the individual volcanoes Harden describes.
- Take a few minutes to browse through the How Volcanoes Work site authored by Vic Camp at San Diego State University. The site has sections on topics ranging from eruption dynamics to volcanic landforms that will supplement our text, and each section includes a brief "self-test" that you can try.
- To learn a little more about specific volcanic hazards check out the US Geological Survey's Volcanic Hazards Program site.
- Browse through the geology section of the Mount Shasta Companion for an introduction to our local volcanic landscape. Also, feel free to visit one or more of the following sites for detailed information on other volcanoes in California: Medicine Lake volcano, Lassen Peak, and the Long Valley caldera. (Note: The Medicine Lake volcano link takes you to a page where you can download a 23 Mb PDF file. You may want to skip this one if you have a slow connection.)
Exercise 5: Volcanic Hazard Assessment (Due by 9:00 AM on 14-Feb-2011)
Several inhabited areas in California, including the Weed–Mount Shasta City area, are built on or near dormant volcanoes. In order to learn about the potential eruptive and non-eruptive hazards posed by these volcanoes, I would like you to complete Version 3 of the "Volcanic Hazards Assessment " exercise on your Hazard City CD. Like all of our Hazard City exercises, this one is pretty self-explanatory. It does, however, require careful reading and attention to detail so give yourself plenty of time. After you've read the introduction click through the different parts of the exercise using the buttons at the bottom of the screen. It will be useful to take notes on any concepts that are unfamiliar to you so that you'll have the information handy as you make your observations and begin formulating your conclusions. When you are ready to write your answers down, click on the "Report" button and print out a copy of the worksheet so that you'll know exactly what you need to do. Finally, when you have answered all of the questions from Version 3 of the exercise on the worksheet and feel that you understand volcanic threats in the Hazard City area pretty well, point your browser to the "Assignments, Tests, and Surveys" tool of our Etudes site and click on Exercise 5. The first four questions will be the ones on the worksheet, and the last six will be questions I've written that are directly related to what you've done and read.
Quiz 5: Volcanism in California (Due by 9:00 AM on 14-Feb-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 5 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about volcanism and volcanic activity in California and each is worth one point. If you can answer all of them correctly it means that you know your way around the basics of volcanism in California pretty well and are ready to start learning about California's 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.