Topic Overview This week, we will explore how earth’s crust deforms by folding and faulting in response to tectonic forces. Additionally, we will discuss mass wasting—the movement of sediment downslope under the influence of gravity. Additionally, we will read “Los Angeles against the Mountains”, another chapter from The Control of Nature. This chapter chronicles the conflicts between humans and nature in the housing developments located in the landslide-prone foothills of the San Gabriel Mountains. Tectonic forces put stress on rocks. When the crust is pulled apart by tensional stress, rocks are stretched and thinned. When compressional stress pushes rock together, the crust is shortened and thickened. Shear stress involves transverse motion and results in blocks of rock moving past each other. When rocks deep in the crust are subject to stress, they deform by folding because they are under great pressure and are hotter than rocks at shallow depths. Rocks close to the surface of the crust are cooler and brittle and they deform by faulting. Folding results in physical features like anticlines, synclines, domes, and basins. Tensional stress results in normal faults, and compressional stress results in reverse faults or thrust faults. Movement of rocks along a fault results in the release of seismic energy known as earthquakes. Seismic energy travels through the rock as seismic waves. In addition to the obvious destruction of buildings from the shaking of seismic waves, earthquakes can trigger other hazards such as tsunamis and landslides. Mass wasting is the downslope movement of sediment of any size under the influence of gravity and friction. The maximum steepness at which unconsolidated material (loose rock or soil) can hold its slope is called the angle of repose. When this angle is exceeded, material will move under the influence of gravity. Water is usually an important factor driving mass wasting because water adds weight to and lubricates unconsolidated material allowing it to overcome the friction holding it in place and allowing it to move downslope. Mass wasting events are severe hazards that can impact human life and property. Read the assigned chapters of the textbook (available for free at Opengeology.org/textbook) and watch the mini-lecture videos before completing the assignments and discussions. This week’s readings and videos focus on crustal deformation through folding and faulting, as well as mass wasting processes like landslides and debris flows. Textbook Readings • • An Introduction to Geology: chapters 9 & 10 The Control of Nature: Los Angeles against the Mountains. Watch Videos; https://mediaspace.regis.edu/media/Crustal%20Deformation/1_s9wduhos https://mediaspace.regis.edu/media/Mass%20Wasting/1_2x300ldk Discussion Question In “Los Angeles against the Mountains” John McPhee describes the tribulations people face when housing is built on the landslide and debris flow prone San Gabriel Mountains. Consider the following questions and then write a discussion post that summarizes your position: 1. Who pays for the cost of preventing landslide damage in Pasadena? 2. Who pays for the damages when landslides occur? Is this a fair arrangement of cost sharing? 3. How would you propose paying for landslide prevention and damages? After you have made your initial post, read and respond to a classmate’s post Week 4 Discussion – Evans 1. Who pays for the cost of preventing landslide damage in Pasadena? According to “The Control of Nature”, the Los Angeles County Flood Control District, or “FLOOD” is the agency responsible for the building of debris basins that ultimately prevent widespread landslide damage in Pasadena. This district is governed by the county of Los Angeles Board of Supervisors
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