Lesson 9: Subduction Zones I: Cascade and Aleutian Arcs
This week's lesson is the first of three in which we'll expore park landscapes formed at or near convergent plate boundaries. We'll begin by studying modern subduction zones and the processes that take place in their accretionary wedges and volcanic arcs. Subduction zones are some of the most geologically active regions on Earth as you've heard from reports about the recent earthquake in Chile and volcanic eruptions in Indonesia and the Phillipines. In the United States and its territories there are at least three active subduction zones, but this week we'll focus on the two of the best known: the Cascadia and the Aleutian subduction zones. During the coming weeks we'll consider the two other landscapes shaped by plate convergence: the "roots" of volcanic arcs and the mountain ranges raised by continental collsions. Finally, late in the semester, we'll come back to this topic and study terrane accretion which is also related to plate convergence in many circumstances.
As you read through pages 89-123 of chapter 5 please take careful notes so that you can keep track of major points and recall them more easily when we refer to them later in the semester. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the exercise and quiz described near the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Compare and contrast the two types of mountain ranges typically found parallel to a continental margin subduction zone in terms of their geologic structures and origins.
- Describe how turbidite deposition occurs and explain why adjacent sand and shale layers of similar thickness might, nonetheless, record deposition over very different lengths of time.
- Infer whether a Cascade stratovolcano has been active recently or not by inspecting its topographic profile.
- Differentiate between a volcanic crater and a caldera. (Note that the author of our text seems a little vague about this, so be sure to read the website on Mount Mazama from the U.S. Geological Survey below.)
- Differentiate between materials produced by different volcanic processes, such as: lava flows (including "pillow lavas"), airfall "ash" (tephra), debris flows, and pyroclastic flows. (You may want to review your notes on volcanism from chapter 3 before you work on Exercise 9.)
- Explain how geologists estimate the recurrence intervals of subduction zone earthquakes, and indicate whether we would expect to see the shoreline of the Pacific Northwest rise or sink and whether this would tend to make inundation from a tsunami more or less severe when the next large earthquake occurs in Cascadia.
Reading and Browsing Assignment
- Read pages 89 to 123 in Chapter 5 and focus on the topics outlined in the learning objectives above.
- For a brief review, check out this introduction to geologic processes at convergent plate boundaries from the U.S. Geological Survey and National Park Service..
- To see a volcanic debris flow in action check out the video clip of the August 1997 Whitney Creek debris flow posted online as Figure 14 on the Geologic Processes page of the Mount Shasta Companion.
- To learn a little more about the geologic history of Mount Mazama and its collapse to produce the Crater Lake caldera, browse through Mount Mazama and Crater Lake: Growth and Destruction of a Cascade Volcano from the U.S. Geological Survey.
- Finally, to learn a little more about how complex and devastating even small volcanic eruptions can be, browse through this summary of Lassen Peak's most recent (1915) eruption. (Note that, contrary to what Lillie says, Lassen Peak is not a remnant of Mount Tehama. It is actually a large dome that grew about 27,000 years ago on the flank of the old stratovolcano which itself had already been lowered by glacial and stream erosion rather than a "catastrophic" eruption.)
Exercise 9 (Due by 9:00 AM on Monday, 15-Mar-2010)
After you have read the chapter, please go to your Hazard City CD and work through the Volcanic Hazard Assessment exercise, version 1. This exercise challenges you to evaluate the threat to human lives in two neighborhoods of Hazard City that are posed by five specific volcanic hazards from nearby Lava Mountain: airfall tephra; lahars; pyroclastic flows; lava flows; and volcanic gases. Be sure to make notes on your field observations in and around Hazard City, and then use what you have learned to decide whether each hazard is likely to pose a high or low risk to the neighborhoods. Printing the form provided on the CD and recording your results on it may be helpful. Finally, take your notes and go to the Etudes "Assignments, Tasks, and Tests" tool. Use what you have learned from your study of the exercise and the chapter to complete Exercise 9. (There are eight questions from the volcanic hazard report and two others that are based on the exercise.)
Quiz 9 (Due by 9:00 AM on Monday, 15-Mar-2010)
After you feel you have met the learning outcomes outlined above, please complete Quiz 9 in the Etudes "Assignments, Tasks, and Tests" tool. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around active subduction zones pretty well and are ready to move on to learn about parklands that expose the intrusive "roots" of an ancient arc after spring break.