Weathering, Erosion, and Mass Wasting: Interactions Between the Climate and Plate Tectonic Systems

Transcription

1 CHAPTER 16 Weathering, Erosion, and Mass Wasting: Interactions Between the Climate and Plate Tectonic Systems Chapter Summary Weathering refers to processes of the rock cycle that flatten mountains. Chemical and physical processes are involved in the breakdown. Four key factors control the rate at which weathering occurs: 1) properties of the parent rock, 2) climate, 3) the presence or absence of soil, and 4) length of exposure to the atmosphere (see Table 16.1). How chemical weathering works is well illustrated by three examples. First, the chemical weathering of feldspars, which are the most abundant silicate mineral in the Earth s crust, illustrates how water with the help of carbonic acid can transform feldspars into clay minerals and dissolve silica and salts (cations). Second, the reaction of calcite and other carbonate minerals that make up limestone exemplifies the role naturally acidic water plays in dissolving rock. Third, the reaction of oxygen with the iron in ferromagnesium minerals like pyroxene illustrates oxidation. Physical weathering involves a variety of processes that break rock into fragments. Physical weathering is promoted by chemical weathering which weakens grain boundaries within the rock. Physical weathering also promotes chemical weathering by increasing the surface area of the broken rock fragments. Frost wedging, crystallization of minerals like salts, and life processes play a major role in breaking rock apart. Soils, the residue of weathering, are a product of chemical weathering of rock that has continued a very long time. Soil formation is most effected by climate. The composition of the parent rock, life processes, topography, living organisms, and time are also important factors in soil formation. The twelve soil types (see Table 16.3) result from the combined effect of these five processes. Mass movements are gravity-induced transfer of large masses of material. Slides, flows, or falls of large masses of rock material downward occur when the pull of gravity exceeds the strength of the slope materials. Such movements can be triggered by earthquakes, absorption of large quantities of water during torrential rain, undercutting by flooding rivers, human activities, or other geologic processes. 169

2 170 PART II CHAPTER 16 The three most important factors enhancing the potential for mass movements are: the steepness of the slope, the nature of the rock making up the slope, and the water content. Although steep slopes are prone to mass movements, slopes of only a few degrees of slope can also fail catastrophically. Slopes become unstable when they become steeper than the angle of repose, the maximum slope angle that unconsolidated material will assume. Slopes in consolidated material may also become unstable when they are oversteepened or denuded of vegetation. Erosion by rivers and glaciers and human activities can oversteepen slopes and, thereby, enhance the potential for mass movement. The composition, texture, and geologic structure of the slope material are other important factors influencing the potential for slope failure. For example, rocks with high clay content tend to be weak and may liquefy. Titled layers of sedimentary or volcanic rocks are more likely to fail along bedding planes when the bedding parallels the slope. Failure of foliated metamorphic rocks is more likely to occur parallel to the direction of foliation. Water absorbed by the slope material contributes to instability in two ways: (1) by lowering internal friction (and thus resistance to flow) and (2) by lubricating planes of weakness in the slope. The hazards and damage associated with mass movements can be minimized by careful geological assessment, engineering, and land use policies that restrict development on unstable slopes. Learning Objectives In this section we provide a sampling of possible objectives for this chapter. No class could or should try to accomplish all of these objectives. Choose objectives based on your analysis of your class. Refer to Chapter 1: Learning Objectives How to Define Your Goals for Your Course in the Instructional Design section of this manual for thoughts and ideas about how to go about such an analysis. Knowledge Know how weathering fits into the rock cycle. Know how physical and chemical weathering work. Know how soils form as products of chemical weathering. Skills/Applications/Attitudes Understand how silicate mineral s susceptibility to chemical weathering to its atomic structure and position in the Bowen s Reaction Series. Explain how soil formation is linked to climate. Analyze a hillside plot for susceptibility to mass wasting. Understand the need for geological assessment to identify hazardous slopes, and the role slope ordinances can play in reducing slope hazards.

3 Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 171 General Educational Skills Write a set of brief (less than one page) guidelines for use by your employer (a local developer) in regard to building homes in coastline areas of Southern California. (writing/critical thinking) Freshman Survival Skills The amount of material in this chapter makes it even more important that students preview the chapter prior to lecture. To reinforce previewing you might consider assigning Practice Exercise 2 in Chapter 16 of the Student Study Guide for Understanding Earth (located in the Understanding Earth e-book) before lecture. Explain to students that since the idea is to get them to preview the chapter you will give credit only for exercises turned in prior to the lecture. For the same reason, a good summary at the end of lecture where you show a schematic, table, or flowchart demonstrating how all the material in Chapter 16 fits together will be greatly appreciated by students. Such a summary will also be helpful during your review session prior to the next exam. Sample Lecture Outline Sample lecture outlines highlight the important topics and concepts covered in the text. We suggest that you customize it to your own lecture before handing it out to students. At the end of each chapter outline consider adding a selection of review questions that represent a range of thinking levels. Chapter 16: Weathering, Erosion, and Mass Wasting Interface Between Climate and Tectonics Major Factors Controlling Rates of Weathering Properties of parent rock Mineral stability Rock structure Climate Rainfall Temperature Presence or absence of soil and vegetation Thickness of soil layer Organic activity Length of exposure Chemical Weathering Hydration Role of carbon dioxide Dissolution Oxidation

4 172 PART II CHAPTER 16 Products of Chemical Weathering Clays Oxides Salts Silica and quartz sand Factors Controlling Weathering Rates Chemical stability Solubility Rate of dissolution Relative stability of common rock-forming minerals Positive feedback between physical and chemical weathering Physical Weathering Plants and burrowing animals Frost action heat and cold Mineral crystallization Alternating heat and cold Exfoliation and spheroidal weathering Soils are residues of weathering processes Soils as Geosytems Inputs: weathered rocks, organisms, and dust Processes (see Figure 16.11) Outputs 12 soil types (see Table 16.3) Mass wasting Processes that move rock and soil downhill via gravity Factors that influence mass movements (see Table 16.4) Nature of slope materials (see Figure 16.13) consolidated vs. unconsolidated Angle of repose Surface tension Cohesion Amount of water in the slope materials Liquefaction Steepness and instability of the slope Rock fabric and structure, e.g., jointing and other planes of weakness Events that trigger mass wasting Earthquakes Runoff and infiltration from storms Slope becomes steeper Undercutting Construction Cut/fill

5 Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 173 Types of mass movement (see Figure 16.17) Rock mass movements Rockfall Rock slide Rock avalanche Unconsolidated mass movements Creep Solifluction Earthflow Debris flow Mudflow Debris avalanche Submarine mass movements Origins of Mass Wasting Natural causes of landslides Human Activities that promote or trigger slides Teaching Tips Cooperative/Collaborative Exercises and In-Class Activities Refer to Chapter 4: Cooperative Learning Teaching Strategies in the Instructional Design section of this manual for general ideas about conducting cooperative learning exercises in your classroom. Coop Exercise 1: Factors in Rock Weathering The following review question from Chapter 16 in the Student Study Guide can be used as a basis for a Think/Pair/Share exercise. In the days of the Pharaohs of Egypt, a cherished status symbol was the obelisk, a stone column decorated with hieroglyphs (designs carved into the stone usually sandstone). In 1879, the obelisk of Thothmes III from the temple of Heliopolis, Egypt, was moved to Central Park in New York City. Within sixty years the hieroglyphs were barely visible on the obelisk, while its counterpart still standing in Egypt remains in nearly perfect condition in the desert sun for almost 4,000 years. Why did the stone obelisk deteriorate so quickly when it was moved to New York City? Coop Exercise 2: Evaluation of Slope Stability Exhibit to the class the cross-section shown below (from Practice Exercise 3: Evaluation of Slope Stability in the Student Study Guide, available in the Understanding Earth ebook). Give teams of students about two to three minutes to complete an assessment of potential problems with slope instability. Have them write down their answers on a sheet of paper with the names of all students in the team. Call on a variety of teams to share their assessments. Discuss their answers. Collect answers for attendance and credit, if you wish.

6 174 PART II CHAPTER 16 Freshman Survival Skills The amount of material in this chapter makes it even more important than usual that students preview the chapter prior to lecture. To reinforce previewing consider assigning Practice Exercise 2 in Chapter 16 of the Student Study Guide for Understanding Earth before lecture. The Study Guide is available in the Understanding Earth ebook. Explain to your students that since the idea is to get them to preview the chapter you will give credit only for exercises turned in prior to the lecture. A good summary at the end of lecture where you show a schematic, table, or flowchart demonstrating how all the material in Chapter 16 fits together will be greatly appreciated by students. Such a summary will also be helpful during your review session prior to the next exam. Exercises 1 and 2 of the Student Study Guide for Understanding Earth may be useful to you in providing such a summary. Exercise 1 covers weathering and Exercise 2 covers mass wasting. For your convenience both exercises are shown in the next section (homework). Completed versions of these exercise are available online if you wish to download them as lecture slides or handouts. Alternatively, they could be assigned as a credit assignment or completed in class as a coop exercise. Homework: Visual Summary Exercises for Weathering Practice Exercises 1 and 2 from Chapter 16 in the Student Study Guide will help students develop a useful visual summary of information on physical and chemical weathering and soil profiles. These exercises are provided below. Exercise 1: Physical and Chemical Weathering Fill in the blanks in the flowchart below. Remember that it is better to teach a person to fish than to just give them a fish. Be sure to model and discuss the kind of thinking that allows one to construct such charts.

7 Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 175 Parent Rock GRANITE containing quartz, feldspars, mica, and amphiboles High relief mountainous Low relief flat land? Does physical or chemical weathering dominate?? Does physical or chemical weathering dominate? Tropical Desert Tropical Desert? What is the mineral composition of the weathering products?? What is the mineral composition of the weathering products? Exercise 2: Inventory of the Different Kinds of Mass Wasting The authors discuss eight different kinds of mass wasting. As an aid to learning the circumstances that favor each of these types of mass movement, use your textbook to fill in the blanks in the table below. Textbook figures and figure captions will help you complete the table. Hint: You probably haven t seen many of these features before, so be sure to examine the photos and figures of each type of mass wasting in your textbook. If you are a visual learner, this activity may be vital. Also, to get a kinesthetic feel for these movements imagine yourself trying to outrun each movement. Indicate in the space labeled Speed whether you could escape the mass movement by walking, running, or moving as fast as a speeding auto.

8 176 PART II CHAPTER 16 Composition of Slope Kind of Mass (consolidated vs. unconsolidated Wasting and wet vs. dry) Characteristics Rock avalanche Creep Earthflows Debris flow mostly finer rock materials with some coarser rock debris and large amounts of water Speed: running or a speeding auto Slope angle: steep slopes Triggering event(s): earthquakes Note: occur in mountainous regions where rock is weakened by weathering, structural deformation, weak bedding, or cleavage planes Speed: Slope angle: any angle Triggering event(s): none Note: Speed: Slope angle: any angle Triggering event(s): intense rainfall Note: fluid-like movement Speed: Slope angle: any angle Triggering event(s): Note: Debris avalanche water-saturated soil and rock Speed: Slope angle: Triggering event(s): Note: Slump surface layers of soil Speed: Slope angle: Triggering event(s): intense rainfall or catastrophic melting of ice and snow by a volcanic eruption Note: contains large amounts of water Speed: walking Slope angle: any slope Triggering event(s): rainfall Note: Speed: walking Slope angle: any angle Triggering event(s): Note: occurs only in cold regions when water in the surface layers of the soil alternately freezes and thaws. Water can not seep into the ground because deeper layers are frozen. Topics for Class Discussion How does physical weathering aid chemical weathering? Note: The relationship between physical and chemical weathering is a great example of positive feedback. How is carbonic acid produced and how does it affect weathering?

9 Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 177 A granite contains much more feldspar than quartz. How is it that sand derived from the weathering of granite may be composed mostly of quartz? The Chemical Weathering of Minerals in the Bowen s Reaction Series Generally, minerals that crystallized at higher temperature are less stable on the Earth s surface than minerals that formed at lower temperatures. For example, the ferromagnesium minerals tend to chemically weather the fastest. Seawater contains 2.20 pounds of dissolved solids per cubic foot. Freshwater lake water contains about 0.01 pounds of dissolved solids per cubic foot. Where do the dissolved solids in seawater come from? Weathering of silicate rocks + volcanic volatiles source of salt (dissolved solids) in the oceans How does water enhance mass movements? adding weight buoyancy weight of an object is decreased in water by the weight of the water displaced by the object Archimedes Principle hydrostatic pressure dissolution of cement, e.g., calcium carbonate cementing a sandstone undercutting by streams freeze/thaw You are about to purchase a hillside lot for use as a home site. What are lines of evidence that would indicate the property was susceptible to mass movement? Answer outline: Recognizing the signs for mass movement what a prospective home buyer should look for: evidence for old landslides hummocky terrain a scarp lack of older vegetation poorly sorted, angular blocks lack of layering (bedding) in deposit springs emerging from a slope undercutting by a river, ocean waves, or construction cut and fill foundation for building fabric of the rock bedding planes foliation jointing damage to existing structures cracks in the walls cracks radiating from corners of window or door frames

10 178 PART II CHAPTER 16 Interesting Facts The Chemical Weathering of Minerals in the Bowen s Reaction Series Generally, minerals that crystallized at higher temperature are less stable on the Earth s surface than minerals that formed at lower temperatures. For example, the ferromagnesium minerals tend to chemically weather the fastest. Clay Minerals/Ion Exchange/Soil Fertility Soil fertility results from a delicate balance between weathering processes, which release mineral nutrients, and intense weathering which can wash the nutrients away. Clay minerals and organic matter help to hold nutrients in soils so they are not easily washed away, but at the same time, easy for plant roots to extract When water freezes it expands 9 percent and exerts about 15 tons per square inch of force. Water molecules in their crystal form have an open-structured hexagonal arrangement that results in the expansion of water upon freezing. Ice is therefore less dense than water. Bauxite Aluminum Ore; mostly Al(OH) 3 + AlO OH Bauxite is an uncommon mineral formed by the breakdown of aluminumbearing rocks under special conditions of intense chemical weathering typical of tropical climates. Special conditions for the formation of bauxite include: abundant rainfall groundwater that is neither acidic nor alkaline aluminum-rich parent rocks subsurface drainage, e.g. no swamps low relief erosion is slow relative to chemical weathering

11 Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 179 Aluminum is the second most abundant metallic element (after silicon) in the Earth s crust, where it averages 8.3 percent. Aluminum has been produced commercially for about 100 years. Refining aluminum is very energy intensive. The energy it takes to produce the aluminum in a soda can is equivalent to the energy used by a 100 watt light bulb over a period of 100 hours. Gem quality crystals of aluminum oxide include rubies and sapphires. Corundum, a very hard mineral, is made from aluminum oxide. Chromium, manganese, cobalt, copper, zinc, selenium, molybdenum, and iodine are trace elements within soils and important trace nutrients for human health. Recently glaciated lands are typically deficient in iodine because the glacial melt waters leached the iodine from the soil. Since iodine is vital for the healthy operation of the thyroid gland, iodine is added to table salt to prevent goiters the enlargement of the thyroid gland due to an iodine deficient diet. Teaching Resources Student Study Guide Highlights In Part I, chapters provide strategies for learning geology. Ideally, students would read these chapters early in the course. Chapter 1: Brief Preview of the Student Study Guide for Understanding Earth Chapter 2: Meet the Authors Chapter 3: How to Be Successful in Geology (part of the Understanding Earth e-book) In Part II, Chapter 16: Weathering, Erosion, and Mass Wasting Interface Between Climate and Tectonics Before Lecture: Preview Questions and Brief Answers Learning Warm-up Tip During Lecture After Lecture: Check Your Notes Intensive Study Session Exam Prep: Tips for Preparing for Exams Chapter Summary Practice Exercises: Physical and Chemical Weathering Inventory of the Different Kinds of Mass Wasting Water s Role in Mass Wasting Review questions