Chapter Summary• Weathering refers to processes of the rock cycle that flatten mountains.

Chemical and physical processes are involved in the breakdown. Four key fac-tors control the rate at which weathering occurs: 1) properties of the parentrock, 2) climate, 3) the presence or absence of soil, and 4) length of exposureto the atmosphere (see Table 16.1).

• How chemical weathering works is well illustrated by three examples. First, thechemical weathering of feldspars, which are the most abundant silicate miner-al in the Earth’s crust, illustrates how water with the help of carbonic acid cantransform feldspars into clay minerals and dissolve silica and salts (cations).Second, the reaction of calcite and other carbonate minerals that make up lime-stone exemplifies the role naturally acidic water plays in dissolving rock. Third,the reaction of oxygen with the iron in ferromagnesium minerals like pyroxeneillustrates oxidation.

• Physical weathering involves a variety of processes that break rock into frag-ments. Physical weathering is promoted by chemical weathering which weak-ens grain boundaries within the rock. Physical weathering also promotes chem-ical weathering by increasing the surface area of the broken rock fragments.Frost wedging, crystallization of minerals like salts, and life processes play amajor role in breaking rock apart.

• Soils, the residue of weathering, are a product of chemical weathering of rockthat has continued a very long time. Soil formation is most effected by climate.The composition of the parent rock, life processes, topography, living organ-isms, and time are also important factors in soil formation. The twelve soiltypes (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 whenthe pull of gravity exceeds the strength of the slope materials. Such movementscan be triggered by earthquakes, absorption of large quantities of water duringtorrential rain, undercutting by flooding rivers, human activities, or other geo-logic processes.

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• The three most important factors enhancing the potential for mass movementsare: the steepness of the slope, the nature of the rock making up the slope, andthe water content. Although steep slopes are prone to mass movements, slopesof 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 oversteep-ened or denuded of vegetation. Erosion by rivers and glaciers and human activities can oversteepen slopes and, thereby, enhance the potential for massmovement.

• The composition, texture, and geologic structure of the slope material areother important factors influencing the potential for slope failure. For exam-ple, rocks with high clay content tend to be weak and may liquefy. Titled lay-ers of sedimentary or volcanic rocks are more likely to fail along beddingplanes when the bedding parallels the slope. Failure of foliated metamorphicrocks 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 lubri-cating planes of weakness in the slope.

• The hazards and damage associated with mass movements can be minimizedby careful geological assessment, engineering, and land use policies thatrestrict development on unstable slopes.

Learning ObjectivesIn this section we provide a sampling of possible objectives for this chapter. No class couldor should try to accomplish all of these objectives. Choose objectives based on your analysisof your class. Refer to Chapter 1: Learning Objectives—How to Define Your Goals for YourCourse in the Instructional Design section of this manual for thoughts and ideas about how togo 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.

170 PART II CHAPTER 16

Freshman Survival Skills• The amount of material in this chapter makes it even more important that stu-

dents preview the chapter prior to lecture. To reinforce previewing you mightconsider assigning Practice Exercise 2 in Chapter 16 of the Student StudyGuide 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 aschematic, table, or flowchart demonstrating how all the material in Chapter16 fits together will be greatly appreciated by students. Such a summary willalso 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

Chapter 16: Weathering, Erosion, and Mass Wasting—Interface BetweenClimate and TectonicsMajor Factors Controlling Rates of Weathering

Properties of parent rockMineral stabilityRock structure

ClimateRainfallTemperature

Presence or absence of soil and vegetationThickness of soil layerOrganic activity

Length of exposureChemical Weathering

HydrationRole of carbon dioxideDissolutionOxidation

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 SouthernCalifornia. (writing/critical thinking)

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.

Physical WeatheringPlants and burrowing animalsFrost action—heat and coldMineral crystallizationAlternating heat and coldExfoliation and spheroidal weathering

Soils are residues of weathering processesSoils as Geosytems

Inputs: weathered rocks, organisms, and dustProcesses (see Figure 16.11)Outputs—12 soil types (see Table 16.3)

Mass wasting—Processes that move rock and soil downhill via gravityFactors that influence mass movements (see Table 16.4)

Nature of slope materials (see Figure 16.13)—consolidated vs. unconsolidatedAngle of reposeSurface tensionCohesion

Amount of water in the slope materialsLiquefaction

Steepness and instability of the slopeRock fabric and structure, e.g., jointing and other planes of weakness

Events that trigger mass wasting EarthquakesRunoff and infiltration from stormsSlope becomes steeper

UndercuttingConstructionCut/fill

172 PART II CHAPTER 16

Products of Chemical WeatheringClaysOxidesSaltsSilica and quartz sand

Factors Controlling Weathering RatesChemical stabilitySolubilityRate of dissolutionRelative stability of common rock-forming mineralsPositive feedback between physical and chemical weathering

Teaching Tips Cooperative/Collaborative Exercises and In-Class ActivitiesRefer to Chapter 4: Cooperative Learning Teaching Strategies in the Instructional Design sec-tion of this manual for general ideas about conducting cooperative learning exercises in yourclassroom.

Coop Exercise 1: Factors in Rock WeatheringThe following review question from Chapter 16 in the Student Study Guide can be used as abasis for a Think/Pair/Share exercise.

In the days of the Pharaohs of Egypt, a cherished status symbol was the obelisk, a stonecolumn decorated with hieroglyphs (designs carved into the stone—usually sandstone). In1879, the obelisk of Thothmes III from the temple of Heliopolis, Egypt, was moved to CentralPark 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 desertsun for almost 4,000 years. Why did the stone obelisk deteriorate “so quickly” when it wasmoved to New York City?

Coop Exercise 2: Evaluation of Slope StabilityExhibit to the class the cross-section shown below (from Practice Exercise 3: Evaluation ofSlope Stability in the Student Study Guide, available in the Understanding Earth eBook). Give

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

RockfallRock slideRock avalanche

Unconsolidated mass movementsCreepSolifluctionEarthflowDebris flowMudflowDebris avalanche

Submarine mass movementsOrigins of Mass Wasting

Natural causes of landslidesHuman Activities that promote or trigger slides

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.

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 preview-ing consider assigning Practice Exercise 2 in Chapter 16 of the Student StudyGuide 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

• A good summary at the end of lecture where you show a schematic, table, orflowchart demonstrating how all the material in Chapter 16 fits together willbe greatly appreciated by students. Such a summary will also be helpful dur-ing your review session prior to the next exam. Exercises 1 and 2 of theStudent Study Guide for Understanding Earth may be useful to you in pro-viding such a summary. Exercise 1 covers weathering and Exercise 2 coversmass wasting. For your convenience both exercises are shown in the next section (homework). Completed versions of these exercise are availableonline if you wish to download them as lecture slides or handouts.Alternatively, they could be assigned as a credit assignment or completed inclass 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 devel-op a useful visual summary of information on physical and chemical weathering and soil pro-files. 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 tomodel and discuss the kind of thinking that allows one to construct such charts.

174 PART II CHAPTER 16

in prior to the lecture.

Exercise 2: Inventory of the Different Kinds of Mass WastingThe authors discuss eight different kinds of mass wasting. As an aid to learning the circumstancesthat favor each of these types of mass movement, use your textbook to fill in the blanks in thetable 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 thephotos and figures of each type of mass wasting in your textbook. If you are a visual learner, thisactivity may be vital. Also, to get a kinesthetic feel for these movements imagine yourself tryingto outrun each movement. Indicate in the space labeled “Speed” whether you could escape themass movement by walking, running, or moving as fast as a speeding auto.

Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 175

Parent RockGRANITE

containing quartz,feldspars, mica,and amphiboles

High relief—mountainous

Desert

Tropical

Low relief—flat land

?

?

?

?

Does physical or chemical weathering dominate?

Does physical or chemical weathering dominate?

What is the mineral composition of the weathering products?

What is the mineral composition of the weathering products?

Desert Tropical

Topics for Class Discussion• How does physical weathering aid chemical weathering?

Note: The relationship between physical and chemical weathering is a greatexample of positive feedback.

• How is carbonic acid produced and how does it affect weathering?

176 PART II CHAPTER 16

Composition of SlopeKind of Mass (consolidated vs. unconsolidated Wasting and wet vs. dry) Characteristics

otua gnideeps a ro gninnur :deepSehcnalava kcoRSlope angle: steep slopesTriggering event(s): earthquakesNote: occur in mountainous regions where rock isweakened by weathering, structural deformation, weakbedding, or cleavage planes

:deepSpeerCSlope angle: any angleTriggering event(s): noneNote:

:deepSswolfhtraESlope angle: any angleTriggering event(s): intense rainfallNote: fluid-like movement

:deepSwolf sirbeDSlope angle: any angleTriggering event(s): Note:

mostly finer rock materials with some Speed: coarser rock debris and large amounts Slope angle:

cihportsatac ro llafniar esnetni :)s(tneve gnireggirTretaw fomelting of ice and snow by a volcanic eruptionNote: contains large amounts of water

Debris avalanche water-saturated soil and rock Speed: Slope angle: Triggering event(s): Note:

gniklaw :deepSpmulSSlope angle: any slopeTriggering event(s): rainfallNote:

surface layers of soil Speed: walkingSlope angle: any angleTriggering event(s): Note: occurs only in cold regions when water in thesurface layers of the soil alternately freezes andthaws. Water can not seep into the ground becausedeeper layers are frozen.

• A granite contains much more feldspar than quartz. How is it that sand derivedfrom the weathering of granite may be composed mostly of quartz?

• The Chemical Weathering of Minerals in the Bowen’s Reaction SeriesGenerally, minerals that crystallized at higher temperature are less stable onthe Earth’s surface than minerals that formed at lower temperatures. For exam-ple, the ferromagnesium minerals tend to chemically weather the fastest.

• Seawater contains 2.20 pounds of dissolved solids per cubic foot. Freshwaterlake 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 thewater

— 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 linesof evidence that would indicate the property was susceptible to mass move-ment? Answer outline:

— Recognizing the signs for mass movement—what a prospective homebuyer should look for:

evidence for old landslideshummocky terraina scarplack of older vegetationpoorly sorted, angular blockslack 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 rockbedding planesfoliationjointing

— damage to existing structurescracks in the wallscracks radiating from corners of window or door frames

Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 177

Interesting Facts• The Chemical Weathering of Minerals in the Bowen’s Reaction Series

Generally, minerals that crystallized at higher temperature are less stable onthe Earth’s surface than minerals that formed at lower temperatures. Forexample, 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 thenutrients away. Clay minerals and organic matter help to hold nutrients insoils so they are not easily washed away, but at the same time, easy for plantroots to extract.

• When water freezes it expands 9 percent and exerts about 15 tons per squareinch of force.

• Water molecules in their crystal form have an open-structured hexagonalarrangement that results in the expansion of water upon freezing. Ice is there-fore less dense than water.

• Bauxite—Aluminum Ore; mostly Al(OH)3 + AlO • OH

Bauxite is an uncommon mineral formed by the breakdown of aluminum-bearing rocks under special conditions of intense chemical weathering typicalof 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

178 PART II CHAPTER 16

+

+

+1

2+

• Aluminum is the second most abundant metallic element (after silicon) in theEarth’s crust, where it averages 8.3 percent. Aluminum has been producedcommercially for about 100 years. Refining aluminum is very energy inten-sive. The energy it takes to produce the aluminum in a soda can is equivalentto the energy used by a 100 watt light bulb over a period of 100 hours. Gemquality 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 iodineare trace elements within soils and important trace nutrients for human health.Recently glaciated lands are typically deficient in iodine because the glacial meltwaters leached the iodine from the soil. Since iodine is vital for the healthy oper-ation of the thyroid gland, iodine is added to table salt to prevent goiters—theenlargement of the thyroid gland due to an iodine deficient diet.

Teaching ResourcesStudent Study Guide Highlights In Part I, chapters provide strategies for learning geology. Ideally, students would read thesechapters 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

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

Weathering, Erosion, and Mass Wasting: Interaction Between the Climate and Plate Tectonic Systems 179

(part of the Understanding Earth e-Book)