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:

Reading and Browsing Assignment

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.