Chapter 20
Glacier Systems and the Ice Age
OVERVIEW
This chapter examines the role of glacial ice as a denudation and landforming agent.
Glacial ice had a major impact on the landscapes of midlatitude and subarctic regions
during the past Ice Age and still covers many high latitude and high elevation areas of the
Earth.
 Glaciers are natural bodies of land ice that have, or have had in the past, the ability to
flow. They form where winter snowfall exceeds summer ablation over long periods of
time.
 Glaciers erode the land surface by plucking and abrasion. Eroded material is
incorporated into the glacier, transported, and eventually deposited when the ice melts.
 Alpine glaciers form in cirques in high mountain locations and often flow down preexisting stream valleys carving them into U-shaped glacial troughs.
 Snow builds up in the zone of accumulation found at the upper end of the glacier
where layers of snow undergo compaction and recrystalization to produce firn.
Beneath the surface of the glacier, the ice acts as a plastic substance and will flow
slowly. Glaciers also move by basal sliding.
 Glaciated landscapes tend to be very rugged. Freeze-thaw weathering and glacial
erosion produce cirques, arêtes, and horn peaks.
 The erosional capacity of glaciers also produces large quantities of depositional
materials. Piles of unsorted debris that form along the end and sides of glaciers are
moraines.
 Ice sheets are accumulations of ice that cover large areas and extend over major
topographic features. Greenland and Antarctica are sites of present-day ice sheets.
 Ice shelves are extensions of ice sheets that float on ocean water. Icebergs are pieces
of ice that break free from ice shelves and glaciers to float in the ocean.
 Continental ice sheets expand and contract during an ice age, causing alternating
periods of glaciation, deglaciation, and interglaciation.
 During the Late-Cenozoic Ice Age, extending over the past two to three million years,
continental ice sheets have grown and melted up to thirty times.
 Much of North America and Europe, as well as parts of Asia and South America, were
covered with ice during the most recent episode of ice sheet expansion, the Wisconsin
Glaciation.
 The erosive action of alpine glaciers and ice sheets produces grooved, scratched, and
polished surfaces on more resistant rock, and strips away regolith and weaker rock.
 Glacial drift refers to all those sediments that are deposited by glaciers. Unstratified
drift deposited directly from glaciers is called till. Those sediments derived from
glaciers, but modified by transportation by meltwater, are called stratified drift.
 Some of the more common landforms made up of till deposits are moraines, till
plains, and drumlins.
 Stratified drift features include outwash plains, which form where braided meltwater
streams issuing from glaciers deposit sediment over a wide area. Sediment deposited
by meltwater streams flowing in ice tunnels beneath a glacier form ridge-like eskers.
 Kames are stratified drift deposits that originate as deltas in meltwater lakes near
glacier margins.
 Three possible causes of the Late-Cenozoic Ice Age are:
 a change in continent positions due to plate tectonic activity
 an increase in the number and severity of volcanic eruptions
 a reduction in solar energy output
 Cycles of glaciation appear to be related to cyclic changes in the Earth’s axial tilt and
distance from the sun.
 Global warming has the potential to change both ablation and snowfall on the Earth’s
ice sheets. The net effect of these changes on global sea level is uncertain.
KEY TERMS
ablation
alpine glacier
abrasion
plucking
ice sheet
cirque
moraine
glacial trough
fiord
glaciation
deglaciation
interglaciation
Late-Cenozoic Ice
Age
Wisconsin
Glaciation
glacial drift
till
unstratified drift
STUDY QUESTIONS
What conditions lead to glacier formation?
How and why do glaciers flow?
What is the difference between an alpine glacier and an ice sheet?
Sketch a diagram to illustrate the major erosional landforms produced by an alpine
glacier.
5. Describe an alpine glacier as a matter flow system. Under what conditions will the
glacier front advance? Under what conditions will it retreat?
6. Where are major ice sheets found in the world today? How large are they? How thick
are they?
7. How is the Wisconsin Glaciation related to the Late-Cenozoic Ice Age?
8. How extensive were continental ice sheets during the most recent glaciation?
9. How do major glaciations affect global sea level? Why?
10. What is isostatic rebound, and how has it affected the coastline of North America?
1.
2.
3.
4.
11. What
is glacial drift and what are the two major types of glacial drift that are
recognized?
12. Describe the major landforms formed by the deposition of till.
13. Describe the major landforms produced by the deposition of sediments from glacial
meltwater.
14. What might have caused the Late-Cenozoic Ice Age?
15. What is the Holocene Epoch, and how has the global climate fluctuated over this
time?
16. How might global warming affect the world’s major ice sheets, and how might this, in
turn, affect global sea levels?
CHAPTER QUIZ
Multiple Choice Questions
1. The cycle of continental ice sheet expansion and contraction that has occurred over the
last two to three million years is called the:
a) Holocene Epoch
b) Wisconsin Glaciation
c) Late-Cenozoic Ice Age
d) Milankovitch Cycle
2. Which of the following is a glacial landform composed of till?
a) arête
b) moraine
c) fiord
d) esker
3. The term ablation refers to the:
a) build-up of snow and ice on a glacier
b) loss of snow and ice by melting and evaporation
c) advance of the terminus of a glacier
d) flow of glacier ice under the influence of gravity
4. The current interglaciation began approximately:
a) one hundred years ago
b) 1000 years ago
c) 10,000 years ago
d) 100,000 years ago
5. ___________are smoothly rounded oval hills that occur in swarms behind the terminal
moraine of a glacier.
a) Eskers
b) Drumlins
c) Recessional moraines
d) Deltas
True/False Questions
1.
2.
3.
Most of the state of Texas was covered by ice during the Wisconsin Glaciation. (T/F)
Glaciers move by both basal sliding and plastic flow. (T/F)
Glacial ice accumulates when ablation exceeds accumulation. (T/F)
4.
An iceberg is formed when a piece of glacial ice breaks off an existing glacier or ice
shelf. (T/F)
Short Answer Questions
1.
2.
3.
4.
5.
What essential property is necessary for an ice mass to be a glacier? How is this
property achieved?
Glaciers always flow from the zone of accumulation to the zone of ablation, yet the
terminus of the glacier can move forward or retreat backward. How is this possible?
What are the principal mechanisms by which glaciers erode the land surface over
which they move?
How do terminal and recessional moraines differ in terms of their usual characteristics
and mode of formation?
What are the three possible causes of the Late-Cenozoic Ice Age?
Short Essay Questions (1 - 2 paragraphs)
Describe the process of isostatic rebound, and discuss how this has affected the coasts
of North America.
2. The world’s present ice sheets may be affected by global warming, and this, in turn,
might affect global sea levels. Discuss the possible inter-relationship between global
warming, ice sheets, and sea level.
1.
Internet Resources
1. Geomorphology from Space – Glaciers:
<http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_9/GEO_CHAPTER
_9_TABLE.HTML>
2. Glacier photo collection - National Snow and Ice Data Center:
<http://nsidc.org/NSIDC/CATALOG/ENTRIES/G00472.html>
3. Postglacial drainage evolution of Saskatchewan – animated:
<http://www.usask.ca/geography/atlas/glace.html>
4. All About Glaciers – extensive discussion and excellent photo gallery:
<http://nsidc.org/glaciers/information.html>
5. The 2001 Tokositna Glacier Surge:
<http://www.nps.gov/dena/home/glacier/tok.html>
6. Studying the Trapridge Glacier surges:
<http://www.geop.ubc.ca/Glaciology/Trapridge.html>
7. Glacial landform identification tutorial: <http://www3.cerritos.edu/Earthscience/tutor/glacial_landforms.htm>
8. Illustrated glossary of alpine glacial landforms:
<http://www.uwsp.edu/geo/faculty/lemke/vgd_alpine/contents.html>
9. Ice ages and glaciations: <http://www.hartwick.edu/geology/work/VFT-sofar/glaciers/glacier1.html>
10. NOVA Online – Cracking the Ice Age: <http://www.pbs.org/wgbh/nova/ice/>