CHAPTER 21

Glaciers: The Work of Ice

Chapter Summary• Glaciers form in cold and snow climates where snow accumulation exceeds the abla-

tion of ice due to melting, sublimation, wind erosion, and iceberg calving. Glacial icemoves by plastic flow and slip along the base, which may be lubricated by melt water.The rate of ice flow varies typically from meters per year to meters per week.

• Isostasy refers to buoyant floating. When an object such as ice is dropped into a solu-tion of greater density, such as water, the less dense object is buoyed up by a force thatis equal to the mass of solution it displaces. When this upward buoyant force exceedsthe gravitational force that pulls the object downward we have a condition of isostasy.Isostasy insures that sea level will not rise when icebergs or ice shelves melt. As theice melts it simply replaces the water that it formerly displaced.

• Glaciers are described as advancing or retreating depending on the balance betweensnow accumulation and ablation; refer to Figure 21.8. When ablation exceeds accu-mulation, the shrinking glacier “retreats” as the toe or terminus migrates up slope.When accumulation exceeds ablation, the expanding glacier “advances” as its toe orterminus migrates down slope.

• Glaciers are powerful agents of erosion and deposition. Glaciers erode by scraping,plucking, and grinding rock.

• Landscapes sculptured by ice have distinctive features that have provided geologistswith evidence for reconstructing the position of ice sheets during the ice ages and deci-phering the existence of ice ages throughout Earth’s history. U-shaped and hangingvalleys, moraines, aretes, cirques, drumlins, kames, eskers, erratics, and striated rockcharacterize a glacial landscape.

• Ice-laid deposits of rock material are called till, consisting of heterogeneous mixtureof rock, sand, and clay. Accumulations of till are called moraines, each type of moraineis named for its position relative to the glacier that formed it. Ancient tills, called tillitesprovide evidence for ancient glaciations numerous times during Earth’s history.

• Water-laid deposits from glaciers are called outwash, consisting of sand, gravel, andfine rock flour.

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that exposed large areas of continental shelf; during the interglacial intervals, sea level roseand submerged the shelves.

• Although the causes of the ice ages remains uncertain, the general cooling of the Earthleading to glaciation appears to have been the result of plate tectonics that gradually movedcontinents to positions where they obstructed the general transport of heat from the equa-tor to polar regions.

• The favored explanation for the alternation of glacial and interglacial intervals is the effectof astronomical cycles, by which very small periodic changes in Earth’s orbit and axis ofrotation alter the amount of sunlight received at the Earth’s surface. There is also evidencethat decreased levels of carbon dioxide in the atmosphere diminished the greenhouse effectand triggered glaciation.

Learning ObjectivesIn this section we provide a sampling of possible objectives for this chapter. No class could or shouldtry to accomplish all of these objectives. Choose objectives based on your analysis of your class. Referto Chapter 1: Learning Objectives—How to Define Your Goals for Your Course in the InstructionalDesign section of this manual for thoughts and ideas about how to go about such an analysis.

Knowledge• Understand how glaciers form and how they move.

• Understand how geoscientists study climate change.

• Understand why geologists think that ice ages have occurred in the ancient geologic past.

• Understand how glaciers erode bedrock, transport and deposit sediments, and shape thelandscape.

• Can define what is meant by “the ice ages” and understands the factors that caused them.

Skills/Applications/Attitudes• Sees clearly how human activities are linked to climate change (attitude) and can

articulate examples of the human activity/climate change connection in a short answerquestion.

• Can recognize, in the field or from photos, the characteristic landforms of a glaciallysculptured region.

• Can recognize, in the field or from photos, the major kinds of rock deposits associatedwith glacial ice and periods of glaciation.

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• The ice sheets of the last major advance were gone approximately 10,000 years ago,the beginning of the Holocene Epoch. Studies of the geologic ages of glacial depositson land and sediments of the seafloor show that the Pleistocene glacial epoch consist-ed of multiple advances (glacial intervals) and retreats (interglacial intervals) of thecontinental ice sheets. Each advance corresponded to a global lowering of sea level

Sample Lecture Outline Sample lecture outlines highlight the important topics and concepts covered in the text. We

Chapter 21: Glaciers—The Work of IceGlacial Ice (metamorphic rock)

SnowFirnIce

Types of GlaciersContinental ice sheetsMountain/valley glaciers

How Do Glaciers Form?ClimateAltitudeLatitudeGlacial Budget: Growth = Accumulation Minus Ablation

Zone of accumulationZone of ablation

MeltingCalvingSublimationWind erosion

Movement of IcePlastic flowBasal slipSurgesIce streams

Isotasy, Ice Shelves, and Sea Level ChangeIsostasy: Dynamic balance between buoyancy and gravityWhen an iceberg melts, sea level does not change

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General Education Skills • Write a letter to the editor of a local newspaper criticizing an article with the following

headline: Antarctic Ice Shelves Melting: Sea Levels Expected to Rise in the Future.(writing/critical thinking)

• Write a brief review of an article that deals with global climate change.

Freshman Survival SkillsAs the end of the semester approaches students will appreciate helpful ideas for preparing for and tak-ing exams. Hint: You will find lots of test taking and exam preparation tips in the Student Study Guide(available in the Understanding Earth e-Book) and in the teaching tips section of many chapters in this manual.

• Show slides with exam preparation tips as background during your lecture.

• Provide students a thorough exam review session close to the time of the final.

• Show slides with test taking tips during your exam review section.

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.

EskersDrumlinsKettlesKamesVarves

PermafrostIce Ages: the Pleistocene Glaciations

Multiple advances and retreat of iceCauses of climate changeClimate since the last glacial period

Record of Ancient Glaciation

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

Coop Exercise 1: Glacial Advances and RetreatsFrom the Student Study Guide for Understanding Earth, Chapter 21, Practice Exercises.

Students commonly misunderstand and get confused about advancing and retreating glaciers.

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Erosional and Abrasional FeaturesStriations/glacial polishU-shaped valleyHanging valleyFjordsCirqueHornAreteRoche moutonée

Ice and Melt Water Deposited Sediments/DriftGlacial tillRock flourErratic bouldersOutwashLoess

Depositional FeaturesMoraines (end, terminal, lateral, medial)

This exercise is good for an in-class Think/Pair/Share activity. Divide students into pairs andtell them to write an answer to the following question.

A glacier advances, halts, and retreats. Will the glacier continue to deposit material at its snoutwhile it is halted, and even while it is retreating? Discuss.

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You can use the following summary as a slide to provide students feedback to their respons-es to Coop Exercise 1.

Rarely does a glacier remain stationary, especially in the summer. Driven by the force of grav-ity, glacial ice and the rock material that it carries move down hill. The words advancing, retreat-ed, and halted are used to describe the movement, or location, of the toe or terminus of the gla-cier and do not actually refer to the movement of glacial ice within the glacier. The terminus ofthe glacier will remain stationary (halted), retreat up the valley, or advance down the valleydepending on the glacial budget (show students Figure 21.8). For example, if the snow accu-mulating in the upper reaches of the glacier equals the loss (ablation) of glacial ice from thelower and warmer reaches of the glacier, the size of the glacier will remain constant, and theglacial terminus will remain stationary. Nevertheless, the glacial ice is still flowing down slopewith rock material and may pile up a sizeable end moraine (show students Figure 21.11).

Coop Exercise 2From the Student Study Guide for Understanding Earth, Chapter 21, Review Questions.

The following review question might generate considerable discussion between student pairsand within the class because all three factors contribute to climate change but do so over dif-ferent time frames. Debate may center on the relative role each factor may have played incausing the last ice age.

Which of the following may have influenced climate fluctuations during the ice ages?

A. variations in Earth orbital characteristics

B. changes in the composition of the atmosphere

C. plate tectonic movements of the continents

D. all of the above

General Education Skills AssignmentClimate Change—Possible long-term causes (writing/critical thinking)Students write a short review of one of three Scientific American articles that discusses pos-sible long-term causes for global climate change. The following list contains selections thatare readable and has worked well in the past, however, instructors are encouraged to substi-tute similar material with which you may be familiar.

Prerequisite: Each article establishes possible links between geologic processes and climatechange. The greenhouse effect is an integral part of the models proposed by these articles. So, be sure that students are familiar with the greenhouse effect before assigning this ashomework.

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Assignment: Read one of the three articles, listed below. Write a short review of the article.Briefly review the principle theme(s) of the article. Your review should be between one to threepages.

“Plateau Uplift and Climate” by W. F. Ruddiman and J. E. Kutzbach, ScientificAmerican, March 1991.

“Large Igneous Provinces” by M. F. Coffin and O. Eldholm, Scientific American,October 1993.

“The Supercontinent Cycle” by R. D. Nance, T. R. Worsley and J. B. Moody,Scientific American, July 1988.

Helpful Hints for Students• Start by outlining the important ideas and evidence cited by authors that sup-

port their ideas.

• Write the review as though you were an author of the article. Think of yourreview as the executive summary. There is no need to mention the authorsthroughout the review. Be sure the title and author(s) of the article are includ-ed in the title of your report.

• Each article in some way links one or more geologic processes to global cli-mate change. Be sure to discuss how climate change may be linked (caused by)geologic processes on Earth. Include in your discussion evidence cited by theauthors to support their model for the cause(s) of climate change.

• Consider illustrating your discussion with diagrams.

Freshman Survival SkillsAs the end of the semester approaches students will appreciate helpful ideas preparing forfinals and taking exams. Hint: You will find lots of test taking and exam preparation tips inthe Student Study Guide (available in the Understanding Earth e-Book) and in the teaching tips section of many chapters in this manual. Appendix A is a study plan for preparing for exams and Appendix B offers helpful ideas about preparing for final exam week.

• Show slides with exam preparation tips as background during your lecture.

• Provide students a thorough exam review session close to the time of the final.

• Show slides with test taking tips during your exam review section.

• Finally, if you have a class Web site, consider posting some materials or linksthat will help students prepare for exams. Hint: There are some useful examprep and test taking tips in Chapter 21 of Student Study Guide. See StudentStudy Guide Highlights below.

Topics for Class Discussion• How is the advance and retreat of glaciers linked to changes in sea level?

• What is the evidence for rock debris, covering northern United States andsouthern Canada, being deposited by glaciers?

• Why do geologists think there was more than one glacial advance during thePleistocene Epoch?

• How can the centers from which the ice sheets advanced be located?

• Compare and contrast the North and South Pole of the Earth.

• What would be the effect on sea level of melting the ice at the North Pole? TheSouth Pole?

• What distinguishes a glacial till from other types of rock materials like allu-vium, lake sediments, and soil?

• Oxygen isotope ratios as a paleo-ice recorder.

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The match in the change of the oxygen isotopic ratios with the change in sun-light reaching Earth due to orbital variations provides convincing evidence forthe role orbital variations played in driving climate change during the ice ages.Consider reviewing this topic in some detail with your students, using the fol-lowing illustrations:

• What were the diverse effects of an ice age? Refer to the box below.

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

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Diverse Impacts of the Ice Ages

• Modification of river drainages ice blocks or deflects drainagerivers carry greater discharges

• Formation of the Great Lakes• Pluvial period in S.W. U.S.A.

Lake Bonneville, LahontanLake Searles and Manly

• Ice dams and floods due to catastrophicfailure of dams, e.g., ChannelScablands, E. Washington

• Sea level changesBering Straits land bridge

• Isostatic adjustment of crustrebound of crust after glaciation

• Changes in plant and animal communities

(part of the Understanding Earth e-Book)

In Part II, Chapter 21: Glaciers—The Work of Ice

Before Lecture:

Preview Questions and Brief Answers

Vital Information from Other Chapters

During Lecture: List of Landscape Features Discussed in This Chapter

After Lecture:

Check Your Notes

Study Tip: Use the Pictures in Chapter 21 as a Virtual Field Trip

Intensive Study Session

Exam Prep:

Chapter Summary

Practice Exercises: The Glacial Sculptured Landscape

Your Personal Budget as a Metaphor for a Glacial Budget

Glacial Advances and Retreats

Review questions

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