Lesson 7: Volcanism and Volcanic Hazards
This week's lesson on volcanoes and volcanic activity is our second to focus on a specific geologic hazard. As Keller points out, volcanoes (like Volcán Poas in Costa Rica, right) are widely distributed across the globe and produced eruptions that claimed more than 20,000 lives during the last two decades of the 20th century. Those of you who live near College of the Siskiyous also know that Mount Shasta, a dormant composite volcano that last erupted about 200 years ago, dominates the local landscape and poses a real threat to the thousands of people who live in the cities at its base. Like the other High Cascade volcanoes Mount Shasta's probability of erupting during any given year is estimated to be small (about 1 in 250-300) but, as the eruptions of Lassen Peak and Mount Saint Helens during the past century have shown, even a single eruptive episode can continue for years and drastically change the natural and human landscapes around a volcano.
In chapter 8 Keller introduces the common types of volcanoes, describes how their characters depend on the types of the lavas they produce, and contrasts the processes that produce lavas in different tectonic settings (decompression melting versus addition of volatiles). He goes on to review the common types of eruptive products and the hazards each poses before closing chapter with a discussion of the observations we can use to assess whether an eruption of a given volcano may be imminent. In this week's exercise you'll have an opportunity to study deposits around Lava Mountain and decide which, if any, of its eruptive products may pose a threat to the people of Hazard City.
As you read through chapter 8 and browse through the websites on volcanoes and volcanic hazards below it will be helpful to take careful notes so that you can keep track of major points and have them readily available when you want to refer back to them. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz described 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:
- Recognize the five common types of volcanoes from photographs or cross-sectional drawings (shield volcanoes, composite volcanoes [stratovolcanoes], cinder cones, domes, and calderas), and explain how the shape and typical eruptive style of each (effusive or explosive) is related to the compositions and temperatures of the lavas it produces. (Note that calderas can be formed by eruptions of both mafic and felsic lavas. Calderas that form on mafic shield volcanoes usually develop in response to flank eruptions that are not typically dangerous (p. 257). Those that develop above shallow rhyolite magma reservoirs where gas-rich lavas are erupted explosively along steep ring fractures are very dangerous, and are what Keller describes as "caldera eruptions" (p. 265-266).)
- Predict which type of volcano is likely to develop in a specific tectonic setting (e.g., mid-ocean ridge, oceanic hotspot, subduction zone, continental hotspot) and explain your decision in terms of the type(s) of lava typically produced in this setting.
- Describe the criteria you could use to distinguish deposits produced by tephra (ash) falls, lava flows, and debris flows in the field, and indicate which of these deposits you would expect to pose a threat mostly to people living in valleys on and around a volcano.
- Indicate which of the following phenomena associated with volcanic eruptions: seismicity, heat flow, topographic deformation, gas emissions, and the geologic history of a volcano would be most useful for making short-term forecasts and which would be more useful for making longer-term forecasts and establishing land-use plans.
Reading and Browsing Assignment
- Read Chapter 8, focusing on the topics outlined in the learning objectives above.
- 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.
- Learn about the eruptive history and likely future hazards posed by Mount Shasta by browsing through materials on the Cascade Volcano Observatory website or by reviewing the geology section of the Mount Shasta Companion by yours truly.
Exercise 7: Volcanic Hazard Assessment (Due by 9:00 AM on 3-Oct-2011)
After you have read the chapter, please go to the Hazard City site and work through version 2 of the Volcanic Hazard Assessment. This exercise challenges you to evaluate the threat to human lives in two neighborhoods posed by five specific volcanic hazards: airfall tephra; lahars (debris flows); pyroclastic flows; lava flows; and volcanic gases. Be sure to make notes on your field observations at the sites in and around Hazard City and then use them to decide whether each hazard is likely to pose a high or low risk to the people in the neighborhoods. It's helpful to print the form provided on the site and fill it in so that you know exactly what you are are looking for. Finally, take your notes and report and go to the "Assignments, Tests, and Surveys" of the Etudes site. Use your results and what you have learned from studying the chapter to answer the questions in Exercise 7.
Quiz 7: Volcanism and Volcanic Hazards (Due by 9:00 AM on 3-Oct-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 7 in the Etudes "Assignments, Tests, and Surveys" area. There are ten questions that address the learning outcomes outlined above. Each is worth one point, and if you can answer all of them correctly it means that you know your way around the basics of volcanism and volcanic hazards pretty well and are ready to move on to a more detailed look at streams and flooding next week. Please contact me if you have any questions or there are concepts you're not sure about.