Lesson 9: Mass Movements and Subsidence
This week's lesson on mass movements and subsidence introduces a geologic process that causes serious damage and loss of life here in California each year. Many of these mass movements (like the La Conchita landslide, right) occur in either the Coast Range or the Southern California mountains. In the Coast Range the soft sheared rocks of the Franciscan Formation are especially susceptible to failure, whereas in the Southern California wildfires driven by Santa Ana winds destroy vegetation and expose steep slopes to winter rains. Even in Siskiyou County, however,
rains have triggered two significant debris flows on Mount Shasta during the past 10 years, and each year they also trigger landslides on slopes in the Klamath Mountains
where logging and road building have removed vegetation and modified
drainage patterns (see Fig. 10.9b on page 338 in our textbook).
Keller opens chapter 10 with background information on landslides and related phenomena and then explores some of their implications for humanity. For example, you'll learn how recognize the conditions that promote landslides and what can be done to minimize the chance that they will occur. This week's exercise asks you to examine several building sites and identify which are potentially susceptible to landslide hazards and so not suitable for construction.
As you read through this chapter on mass movements be sure to take careful notes on the topics addressed by the learning objectives so that you'll have ready access to them when we refer back to them later in the semester. Also 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:
- Recognize common types of landslides and mass movements based on the nature of movement, material, and speed (i.e., Figure 10.5).
- Predict the type of slope (e.g., free face vs. complex slope) that is likely to develop in an area based on the hardness of the rock exposed, the climate, and the amount of vegetation.
- Describe how changes in the characteristics of a potential landslide (such as the area of its slip surface, the unit weight of the rock, and the shear strength of the slip surface) affect the slide's factor of safety.
- Evaluate the likely roles of material type, topography, vegetation and water in contributing the likelihood of mass movement in a given setting.
- Analyze a landslide-prone situation depicted in a photograph, map, or diagram and outline strategies that might be implemented to prevent or minimize the hazard from mass movement.
- Predict likelihood of subsidence in a region based on information about the type of underlying rock or sediment and the abundance of subsurface fluids.
Reading and Browsing Assignment
- Read Chapter 10, focusing on the topics outlined in the learning objectives above.
- For an overview of types and causes of landslides and some practical information on what you can do to protect yourself and your family before, during, and after a slide check out the US Search and Rescue Task Force's landslide site.
- On the USGS Landslide Hazards site, check out the Real-time Monitoring of an Active Landslide Above Highway 50 in the Sierra Nevada. From the picture of the Mill Creek slide shown on the home page of the site what factors can you recognize that likely contributed to its development?
- Finally, to see a 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.
Exercise 9: Landslide Hazard Assessment (Due by 9:00 AM on 17-Oct-2011)
After you have studied the chapter and mastered the learning objectives outlined above please log into the Hazard City site and work through version 2 of the Landslide Hazard Assessment exercise. It's a pretty straightforward exercise, but there are lots of details to keep track of for each building site. When I worked through it I made a table listing the sites across the top and the various criteria down the left-hand side. Then, as I gathered information I just filled in my data table. When I was done it was really obvious which sites were suitable for building and which were not. Write your results out (I suggest printing the form provided and just filling it in) and then go to the "Assignments, Tests, and Surveys" area of the Etudes site to complete Exercise 9.
Quiz 9: Mass Movements and Subsidence (Due by 9:00 AM on 17-Oct-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 9 in the Etudes "Assignments, Tests, and Surveys" area. 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 basics of mass movements pretty well and are ready to move on to a more detailed look at coastal processes next week. Remember, answers will be available for review after 10:00 AM next Tuesday so please look over the results of your quiz and contact me if you have any questions about either content or scoring.