Glaciers: The Work of Ice

Transcription

1 CHAPTER 21 Glaciers: The Work of Ice Chapter Summary Glaciers form in cold and snow climates where snow accumulation exceeds the ablation of ice due to melting, sublimation, wind erosion, and iceberg calving. Glacial ice moves 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 solution of greater density, such as water, the less dense object is buoyed up by a force that is equal to the mass of solution it displaces. When this upward buoyant force exceeds the 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 the ice melts it simply replaces the water that it formerly displaced. Glaciers are described as advancing or retreating depending on the balance between snow accumulation and ablation; refer to Figure When ablation exceeds accumulation, the shrinking glacier retreats as the toe or terminus migrates up slope. When accumulation exceeds ablation, the expanding glacier advances as its toe or terminus 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 geologists with evidence for reconstructing the position of ice sheets during the ice ages and deciphering the existence of ice ages throughout Earth s history. U-shaped and hanging valleys, moraines, aretes, cirques, drumlins, kames, eskers, erratics, and striated rock characterize a glacial landscape. Ice-laid deposits of rock material are called till, consisting of heterogeneous mixture of rock, sand, and clay. Accumulations of till are called moraines, each type of moraine is named for its position relative to the glacier that formed it. Ancient tills, called tillites provide evidence for ancient glaciations numerous times during Earth s history. Water-laid deposits from glaciers are called outwash, consisting of sand, gravel, and fine rock flour. 208

2 Glaciers: The Work of Ice 209 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 deposits on land and sediments of the seafloor show that the Pleistocene glacial epoch consisted of multiple advances (glacial intervals) and retreats (interglacial intervals) of the continental ice sheets. Each advance corresponded to a global lowering of sea level that exposed large areas of continental shelf; during the interglacial intervals, sea level rose and submerged the shelves. Although the causes of the ice ages remains uncertain, the general cooling of the Earth leading to glaciation appears to have been the result of plate tectonics that gradually moved continents to positions where they obstructed the general transport of heat from the equator to polar regions. The favored explanation for the alternation of glacial and interglacial intervals is the effect of astronomical cycles, by which very small periodic changes in Earth s orbit and axis of rotation alter the amount of sunlight received at the Earth s surface. There is also evidence that decreased levels of carbon dioxide in the atmosphere diminished the greenhouse effect and triggered glaciation. 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 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 the landscape. 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 answer question. Can recognize, in the field or from photos, the characteristic landforms of a glacially sculptured region. Can recognize, in the field or from photos, the major kinds of rock deposits associated with glacial ice and periods of glaciation.

3 210 PART II CHAPTER 21 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 Skills As the end of the semester approaches students will appreciate helpful ideas for preparing for and taking 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. 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 21: Glaciers The Work of Ice Glacial Ice (metamorphic rock) Snow Firn Ice Types of Glaciers Continental ice sheets Mountain/valley glaciers How Do Glaciers Form? Climate Altitude Latitude Glacial Budget: Growth = Accumulation Minus Ablation Zone of accumulation Zone of ablation Melting Calving Sublimation Wind erosion Movement of Ice Plastic flow Basal slip Surges Ice streams Isotasy, Ice Shelves, and Sea Level Change Isostasy: Dynamic balance between buoyancy and gravity When an iceberg melts, sea level does not change

4 Glaciers: The Work of Ice 211 Erosional and Abrasional Features Striations/glacial polish U-shaped valley Hanging valley Fjords Cirque Horn Arete Roche moutonée Ice and Melt Water Deposited Sediments/Drift Glacial till Rock flour Erratic boulders Outwash Loess Depositional Features Moraines (end, terminal, lateral, medial) Eskers Drumlins Kettles Kames Varves Permafrost Ice Ages: the Pleistocene Glaciations Multiple advances and retreat of ice Causes of climate change Climate since the last glacial period Record of Ancient Glaciation 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: Glacial Advances and Retreats From the Student Study Guide for Understanding Earth, Chapter 21, Practice Exercises. Students commonly misunderstand and get confused about advancing and retreating glaciers.

5 212 PART II CHAPTER 21 This exercise is good for an in-class Think/Pair/Share activity. Divide students into pairs and tell them to write an answer to the following question. A glacier advances, halts, and retreats. Will the glacier continue to deposit material at its snout while it is halted, and even while it is retreating? Discuss. You can use the following summary as a slide to provide students feedback to their responses to Coop Exercise 1. Rarely does a glacier remain stationary, especially in the summer. Driven by the force of gravity, glacial ice and the rock material that it carries move down hill. The words advancing, retreated, and halted are used to describe the movement, or location, of the toe or terminus of the glacier and do not actually refer to the movement of glacial ice within the glacier. The terminus of the glacier will remain stationary (halted), retreat up the valley, or advance down the valley depending on the glacial budget (show students Figure 21.8). For example, if the snow accumulating in the upper reaches of the glacier equals the loss (ablation) of glacial ice from the lower and warmer reaches of the glacier, the size of the glacier will remain constant, and the glacial terminus will remain stationary. Nevertheless, the glacial ice is still flowing down slope with rock material and may pile up a sizeable end moraine (show students Figure 21.11). Coop Exercise 2 From the Student Study Guide for Understanding Earth, Chapter 21, Review Questions. The following review question might generate considerable discussion between student pairs and within the class because all three factors contribute to climate change but do so over different time frames. Debate may center on the relative role each factor may have played in causing 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 Assignment Climate Change Possible long-term causes (writing/critical thinking) Students write a short review of one of three Scientific American articles that discusses possible long-term causes for global climate change. The following list contains selections that are readable and has worked well in the past, however, instructors are encouraged to substitute similar material with which you may be familiar. Prerequisite: Each article establishes possible links between geologic processes and climate change. 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 as homework.

6 Glaciers: The Work of Ice 213 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 three pages. Plateau Uplift and Climate by W. F. Ruddiman and J. E. Kutzbach, Scientific American, March Large Igneous Provinces by M. F. Coffin and O. Eldholm, Scientific American, October The Supercontinent Cycle by R. D. Nance, T. R. Worsley and J. B. Moody, Scientific American, July Helpful Hints for Students Start by outlining the important ideas and evidence cited by authors that support their ideas. Write the review as though you were an author of the article. Think of your review as the executive summary. There is no need to mention the authors throughout the review. Be sure the title and author(s) of the article are included in the title of your report. Each article in some way links one or more geologic processes to global climate change. Be sure to discuss how climate change may be linked (caused by) geologic processes on Earth. Include in your discussion evidence cited by the authors to support their model for the cause(s) of climate change. Consider illustrating your discussion with diagrams. Freshman Survival Skills As the end of the semester approaches students will appreciate helpful ideas preparing for finals and taking 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. 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 links that will help students prepare for exams. Hint: There are some useful exam prep and test taking tips in Chapter 21 of Student Study Guide. See Student Study 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 and southern Canada, being deposited by glaciers? Why do geologists think there was more than one glacial advance during the Pleistocene 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? The South Pole? What distinguishes a glacial till from other types of rock materials like alluvium, lake sediments, and soil? Oxygen isotope ratios as a paleo-ice recorder.

7 214 PART II CHAPTER 21 The match in the change of the oxygen isotopic ratios with the change in sunlight reaching Earth due to orbital variations provides convincing evidence for the role orbital variations played in driving climate change during the ice ages. Consider reviewing this topic in some detail with your students, using the following illustrations: What were the diverse effects of an ice age? Refer to the box below. Diverse Impacts of the Ice Ages Modification of river drainages ice blocks or deflects drainage rivers carry greater discharges Formation of the Great Lakes Pluvial period in S.W. U.S.A. Lake Bonneville, Lahontan Lake Searles and Manly Ice dams and floods due to catastrophic failure of dams, e.g., Channel Scablands, E. Washington Sea level changes Bering Straits land bridge Isostatic adjustment of crust rebound of crust after glaciation Changes in plant and animal communities Teaching Resources Student Study Guide Highlights (part of the Understanding Earth e-book) 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

8 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 Glaciers: The Work of Ice 215