Lesson 17: Accreted Terranes and Landslides
This is our final lesson and, perhaps fittingly, it considers our most exotic parklands. This week we'll explore accreted terranes: blocks of lithosphere foreign to North America that have been added to the continent along its convergent or transform margins. As Lillie points out, we've encountered accreted terranes before—in the Appalachians, for example—but this week we'll explore parks in Alaska and Washington state that are new to us and exemplify this style of continental growth. Because landscapes built by terrane accretion almost alway involve high mountains cut by numerous faults they are often subject to landslides, so our last exercise will focus on assessing landslide hazards.
This is also our last regular week of class, and next week we'll have our final. I will post an announcement with more information about the exam on our course's Etudes home page shortly. It will be very similar to the quizzes we have been doing throughout the semester, however, so a good way to prepare is to review your earlier quizzes and be sure that you understand any questions that were marked wrong. The final will be in two parts, each of which will be timed for an hour. You can take one part right after the other or you can take them at different times. The deadline for having both parts finished, however, will be 5:00 pm on Thursday, 20-May.
Finally, as always, as you read through chapter 11 and the supporting websites 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 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:
- Define what a terrane is, describe how you might recognize if you have crossed from one terrane to another, and how you might determine where a terrane has come from (for example, its original latitude).
- Predict whether the terranes you pass through are successively younger or older as you move northward across Alaska or westward across the Pacific Northwest, and briefly explain how these trends are related to the growth of the continent.
- Define what a metamorphic facies is, and use a metamorphic facies diagram (like the one shown Nelson's site, below) to predict what facies a rock would be in at a given pressure and temperature within the Earth.
Reading and Browsing Assignment
- Read Chapter 11, focusing on the topics outlined in the learning objectives above.
- For a slightly different take on this week's topic browse through the article on terranes in Wikipedia. Also, check out this short animation that shows how terrane accretion occurs along a convergent plate boundary.
- To learn more about the role of terrane accretion in building North Cascades National Park check out Moving Plates and Tectonic Terranes and the succeeding pages that are part of the larger North Cascades Geology website by the U.S. Geological Survey and NPS.
- Finally, for a brief overview of what can be a pretty complicated topic, check out Stephen Nelson's discussion of metamorphic facies and their relationships to different plate tectonic settings. (Note that the word "basic" in his text is equivalent to "mafic"; that is, a "basic" lava is a basalt.)
Exercise 17 (Due by 9:00 AM on Monday, 17-May-2010)
Because the rocks of accreted terranes are commonly sheared by faulting as they are thrust together to build steep mountains, landslides are a constant threat in these regions. To learn a little about how geologists assess landslide hazards, load up your Hazard City CD and start version 2 of the Landslide Hazard Assessment. Work through the exercise carefully, and note that you are really answering two questions about each building site: is it stable or unstable with respect to landslides; and, independently, s it suitable for construction? You can answer the first question using the table on the Introduction page but will need to review the soil data to answer the second. When you are all done evaluating the five properties print the worksheet on the "Report" page, fill it out, and answer the questions on Exercise 17 in the Etudes "Assignments, Tasks and Tests" tool. Please don't rush through this—thre are lots of details to keep track of—and, as always, let me know if you have any questions or problems.
Quiz 17 (Due by 9:00 AM on Monday, 17-May-2010)
After you feel you have mastered the learning outcomes outlined above, please complete Quiz 17 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 accreted terranes pretty well and have learned everything you need to know for our final exam next week.