Lesson 10: Subduction Zones II: Sierra Nevada and Alpine Glaciation
This week's lesson, like lesson 9, focuses on parklands that give us windows into the processes that take place at subduction zones. This week, however, we'll consider the rocks formed by the roughly ninety percent of subduction zone magmas that do not reach Earth's surface. Instead of erupting to build volcanoes, these magmas cool and crystallize slowly underground to form bodies of coarse grained granite and diorite that may later be exposed by uplift and erosion. In many cases this erosion is accomplished in part by alpine glaciation, so we'll also examine this process and how it has shaped the landscapes of California's Yosemite and Sequoia-Kings Canyon National Parks.
As you read through pages 123-125 of chapter 5 and browse through the websites below 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 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:
- Outline how plate interactions along the western margin of North America have changed during the past 100 million years to "shut off" the Sierra Nevada part of the subduction zone that is still active in the Cascades and led to the uplift and tilting of the Sierran block. (Note that, as Lillie points out, the high elevation of the modern Sierra Nevada probably reflects two distinct processes.)
- Contrast the origins of the two types of bedrock found in the Sierra Nevada: granitic and metamorphic rocks, and describe how you could tell them apart in the field.
- Describe how you would identify a joint in the field, and briefly explain how the presence or absence of joints (including sheet joints) affects the development of geologic features subsequently produced by erosion.
- Contrast the erosional "styles" of streams and glaciers and the types of valleys they carve (U- versus V-shaped and sinuous versus straight).
- Identify the following landforms that are commonly produced by alpine glaciation in photographs or diagrams: cirques; moraines; arétes; horns; and hanging-valleys.
Reading and Browsing Assignment
- Read pages 123-125 in Chapter 5 and then "The Geologic Story of Yosemite Valley" and "Bedrock Geology of the Yosemite Valley Area" on the U.S. Geological Survey's Yosemite National Park Geology website. There is a lot of detail in these web readings, so be sure to focus on the topics outlined in the learning objectives above.
- Next, read about the origins of glaciers and how alpine glaciers carve U-shaped valleys on the USGS-NPS Glaciers site. Note that only the first two parts of the site are active.
- Finally, review the USGS-NPS diagrams that show some of the common landforms produced by alpine glaciation and learn to identify the following features: cirques; moraines; arêtes; horns; and hanging-valleys.
Exercise 10 (Due by 9:00 AM on Monday, 29-Mar-2010)
After you have read about glaciers and glaciation, please load up your Hazard City CD and work through version 1 of the Snowpack Monitoring exercise. (Okay, we're only talking about snow and not glaciers in this exercise, but it's a start!) Pay careful attention to the approximate dates at which snowmelt begins and ends at each site when you review the SNOTEL records, and be sure to jot down any notes that might help you remember why you made the decisions you did. (I suggest printing the form provided, filling it in, and then adding any notes that might be helpful.) Finally, go to the Etudes "Assignments, Tasks, and Tests" tool to complete Exercise 10. There are 5 questions (just as in the report) and each is worth two points.
Quiz 10 (Due by 9:00 AM on Monday, 29-Mar-2010)
After you feel you have met the learning outcomes outlined above, please complete Quiz 10 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 the "roots" of volcanic arcs and alpine glaciation pretty well and are ready to move on to learn about parklands formed at collisional convergent boundaries next week.