CHAPTER 16 - GLACIERS AND GLACIATION
Overview
Glaciers are long-lasting masses of compacted, recrystallized snow that move
by gravity. Glaciated terrains can be classified as alpine (mountainous regions)
and continental (covering large portions of continents). The theory of glacial
ages, proposed by Agassiz in 1837 and initially thought to be nonuniformitarian, states that much more of the land surface was glaciated in the
past than at present. Only about 10% of the earth is currently covered with
ice, and Antarctica accounts for 85% of that cover. Valley glaciers are typically
found in areas of alpine glaciation, while ice sheets are typical of continental
glaciation. Many of the landscapes, environments and resources of Canada
are the product of past glaciations.
Glacial ice is similar to metamorphic rock. Snow is compacted into granular ice
called firn. Continued compaction, thawing and refreezing causes
recrystallization and welding of ice crystals into glacial ice. Ice accumulates in
the upper part of a glacier (zone of accumulation), moves by gravity and is
lost by melting (ablation) in its lower part (zone of ablation). The equilibrium
line separates these two zones. A positive ice budget means more ice
accumulates than melts and the ice terminus advances, a negative budget
indicates more melting than accumulation and the ice terminus retreats. Valley
glaciers and ice sheets exhibit an upper rigid zone with crevasses, overlying a
lower zone of plastic flow and basal sliding. Antarctica has two separate ice
sheets that move by plastic flow.
Glaciers erode by grinding of debris (abrasion) and freezing of debris to the
base of ice (plucking). These processes produce features such as faceted
pebbles, striations, grooves, and rock flour. Erosional landscapes are
characterized by steep walled glacial troughs (U-shaped valleys), truncated
spurs, hanging valleys, rock-basin lakes, rock-basin lakes (tarns), roche
moutonnées, cirques, horns, and arêtes. Ice sheets tend to produce rounded
topography, but grooved and striated bedrock is common.
Glaciers deposit poorly sorted materials as till; erratics are boulders
transported by ice from elsewhere. Depositional landforms include drumlins,
moraines (lateral, medial, end, terminal, recessional), outwash, eskers, loess,
and varves.
The Earth has undergone episodic climate changes for the last 2 to 3 million
years, and northern North America has been affected by the repeated growth
and decay of large ice sheets during this time. Direct effects of glaciation
include scouring and removal of sediment from the Canadian Shield,
development of excellent soils from weathered till and loess, formation of the
Great Lakes, deposition of moraines (Sable Island, Long Island, and Cape
Cod), formation of drumlins, extensive glacial lake deposits in Manitoba and
Saskatchewan, the Finger Lakes in New York, and other features. Quaternary
alpine glaciation was extensive and sculpted mountainous areas, such as the
spectacular Canadian Rocky Mountains. Indirect effects of past glaciation
include the development of pluvial lakes (Great Salt Lake and Death Valley),
lowered sea levels and fiord formation, and crustal rebound. Not all glaciation
is associated with the Quaternary and ice ages have occurred in the
Precambrian, Late Ordovician and Late Paleozoic. Distribution of Precambrian
and late Paleozoic tillites (lithified till) is cited as evidence for continental drift.
More than 50% of Canada is underlain by some form of frozen ground or
permafrost. In northern regions, permafrost forms a continuous zone of
frozen ground up to 700 thick (continuous permafrost) but occurs as
discontinuous patches (discontinuous permafrost) separated by unfrozen areas
further south. Ground ice can take a variety of forms and may form features
such as ice wedges, pingoes and patterned ground. When the ice in
permafrost melts, either as a result of natural processes or human
disturbance, the overlying sediment collapses (thermokarst). Thermokarst is
a major problem for the construction of buildings, roads and pipelines in areas
underlain by permafrost. The presence of frozen ground poses particular
problems for the economic and urban development of Canada’s north.
Learning Objectives
1. Glaciers are bodies of ice that form from the accumulation of snow and
move because of gravity. Snow accumulates to form firn, which gradually
coverts to glacial ice, by compaction (pressure) and recrystallization (melting
and refreezing). This is analogous to the transformation that occurs as
sedimentary rocks change into metamorphic rocks.
2. In alpine areas glaciers form valley glaciers, restricted to flowing between
valley walls. Ice sheets and ice caps are not confined to a valley but spread
out over larger areas.
3. A glacier gains mass in the zone of accumulation that lies above the
equilibrium line and has perennial snow cover; ice is lost in the zone of
ablation, below the equilibrium line, by evaporation, melting or calving at the
terminus. Advancing glaciers have a positive ice budget (accumulation >
ablation), while retreating glaciers have a negative ice budget (accumulation <
ablation).
4. Valley glaciers move by a combination of plastic flow (ice deformation) and
sliding on a thin film of meltwater at the ice base. The upper portion of a
glacier is rigid, and fractures to produce crevasses. Ice sheets in extremely
cold regions, such as Antarctica, move by plastic flow without basal sliding.
5. Rock fragments transported at the base of a glacier are effective agents of
erosion and cause abrasion of underlying materials. Striated, smoothed and
grooved bedrock is a common feature of glaciated terrains and roches
moutonnees are produced by a combination of glacial abrasion and plucking.
Erosion by valley glaciers creates landforms such as steep-sided glacial
troughs (U-shaped valleys), truncated spurs, hanging valleys and tarns.
Cirques, horns and arêtes are formed by glacial processes in combination with
weathering and erosion.
6. Glacial deposits include poorly sorted debris called till, and better sorted
outwash materials deposited by meltwaters (glacial streams). Moraines are
ridges of till or rockfall debris formed either at the margins (lateral moraines)
or terminus (end, terminal and recessional moraines) of a glacier or ice sheet.
Drumlins, spoon-shaped hills of till or outwash with a gentle dip in the
direction of ice movement, are landforms that form beneath glaciers and ice
sheets. Meltwaters issuing from glaciers deposit sands and gravels (outwash)
in braided stream systems and eskers form as a result of outwash deposition
in tunnels below the ice.
7. Other depositional features associated with glaciation are kettles (formed by
the melt of buried ice blocks), loess (wind blown silt), and varved lake
deposits.
8. Glaciation is not restricted to the Quaternary (the past 2 million years or
so). The oldest glaciations occurred about 2.3 billion years ago and the
distribution of Late Paleozoic glacial deposits (250-350 million years old) is
used as evidence for continental drift. Mean temperature was probably only 5
degrees C lower than current temperatures at the height of Quaternary
glaciations.
9. Much of the landscape of northern North America is the result of glacialtion.
Scoured bedrock of the Canadian Shield, the Great Lakes and thick tills in
central and southern Canada and the central United States are the product of
erosion and deposition by continental ice sheets. Indirect effects of glaciation
are pluvial lakes, such as the Great Salt Lake, formed by abundant rainfall
associated with glaciation. The development of large continental ice sheets
caused global sea level to fall during glaciel events. Uplifted and tilted
shorelines in previously glaciated coastal regions and around the Great Lakes
indicate crustal rebound from removal of the weight of continental ice sheets.
Boxes
16.1 - ENVIRONMENTAL GEOLOGY - GLACIERS AS A WATER RESOURCE
Glaciers store considerable amounts of water and can be exploited as a source of
water for power generation and irrigation. Melting of glacial ice has the appeal of
occurring when it is needed during the hot months, with accumulation occurring during
the cold months. Glacier-fed rivers in Alberta provide a valuable source of water for
farmers in dry Prairie regions. Melting of glacier ice can be controlled by either
darkening the surface with coal dust to promote melting, or protecting the surface
with reflective material to retard melting. Although appealing in the short term,
tampering with the glacier's regime could adversely effect the environment and has
been resisted thus far.
16.2 - ENVIRONMENTAL GEOLOGY - WATER BENEATH GLACIERS, FLOODS, GIANT
LAKES, AND GALLOPING GLACIERS
The Bering Glacier in Alaska exhibits a 20-30 year cycle of surges where it may
advance as much as 100 meters per day and can sustain advances of 35 meters per
day for several months. Surges break the lower part of the glacier into chaotic blocks
and are attributed to a build-up of water between the glacier and bedrock. Rapid
melting of glaciers can produce catastrophic floods like the one that occurred in
Iceland in 1996. A volcano erupted beneath the glacier and released water at a rate of
45,000 cubic meters per second. The flood lasted several hours and caused
considerable damage, but monitoring by geologists prevented loss of life. Lake Vostok
is one of the largest lakes in the world (200 km long, 50 km wide, 510 m deep), and
other smaller lakes lie beneath the East Antarctic ice sheet. Lake Vostok has been
sealed beneath ice for a million years and may contain microbes from that time.
Sampling the lake without disrupting the ecosystem presents a problem.
16.3 - IN GREATER DEPTH - DRILLING THROUGH ICE SHEETS FOR A RECORD OF THE
PAST
Students are directed to the website for information concerning cores drilled through
the Greenland and Antarctic ice sheets. These cores provide an ice record for the past
250,000 years, and a record of snowfall for the past 13,000 years, that is not
complicated by lateral flow. Snow thickness and the composition of air trapped as
bubbles suggest that the climate changed very quickly after the last ice age,
increasing by 6 degrees C in just a few years. There is also a record of volcanic
eruptions reflected in ice layers. Increased volcanic activity was associated cooler
temperatures 8,000 to 9,000 years ago.
16.4 - ASTROGEOLOGY - MARS ON A GLACIER
Thousands of meteorites that fell on Antarctic ice sheets over the years have been
transported to an area where the ice is forced to move vertically against the
Transantarctic Mountains and surface melting concentrates them for recovery. These
meteorites are of interest as they provide clues as to the nature of the Earth's interior
and materials forming other planets in the solar system.
16.5 - IN GREATER DEPTH - CAUSES OF GLACIAL AGES
Students are directed to the website for more information on the causes of ice ages, a
topic of considerable interest to many scientists. A primary control on Quaternary
glacial and interglacial episodes is variation in the Earth's orbit around the sun, which
controls the amount of solar radiation reaching the Earth. In 1921, the Serbian
mathematician Milankovitch calculated the variations in orbit and predicted cooling and
warming cycles at 21,000, 41,000 and 100,000 years that have been documented in
marine sediments.
Unfortunately, orbital variation fails to explain the absence of glaciation over most of
geologic time, and other mechanisms may have contributed to the inception of glacial
ages. These include: 1) Changes in the atmosphere - increased carbon dioxide
promotes ‘greenhouse’ warming, while volcanic eruptions lower temperatures through
release of SO2 and dust; 2) Changes in the position of continents - movement of the
continents closer to the poles favors glaciation; 3) Changes in sea water circulation landmasses block sea water circulation and affect ocean temperatures.
16.6 – IN GREATER DEPTH – PERMAFROST IN URBAN AREAS
Construction of settlements, pipelines and transportation routes is particularly difficult
in areas underlain by permafrost as care must be taken not to disturb or cause
thawing of the frozen ground. Structures may be placed on piles above the ground or
on pads of insulating material to prevent heat transfer to the ground. The provision of
municipal services such as water supply, waste and sewage removal in Northern
communities requires either placement of services in insulated above-ground boxes
(utilidors) or gravel-filled underground trenches. Water is often supplied from deep
lakes that do not completely freeze in winter.
Short Discussion /Essay
1. Why are the majority of glacial deposits poorly sorted?
2. Explain the analogy between glacial ice and metamorphic rock.
3. Why is only the recessional history of a glacier preserved by moraine
formation?
4. Explain why river valleys and glaciated troughs have different forms.
5. How much would global sea level rise if 1) the north polar ice cap melted;
2) the south polar ice sheets melted; 3) all glacial ice on earth melted?
Longer Discussion/Essay
1. Explain the effects of glacial advance and retreat in areas that were never
glaciated.
2. Contrast the effects of alpine and continental glaciation on landscape
development.
3. Discuss the relationship between human-induced global warming and the
natural cycle of climate change involving repeated glacial and interglacial
cycles.
4. Would pre-Quaternary glaciations have been caused by the same processes
that caused the Quaternary glaciations?
5. Discuss the problems associated with urban development on permafrost.
Selected Readings
The Journal of Glaciology is devoted to scientific articles on glaciology. The
papers tend to be highly technical. Brief reports on current research in
Antarctica are included in the Antarctic Journal of the United States, published
by the National Science Foundation.
Alley, R.B., 2000. The Two-Mile Time Machine: Ice Cores, Abrupt Climate
Change and our Future. Princeton University Press, 229pp. The story of
global climate change as revealed by studies of ice cores drilled in Greenland.
Ausubel, J.H., 1991. -A second look at he impacts of climate change.American Scientist
79 (May-June): 210-21
Bolles, Edmund. B., 1999, The Ice Finders: How a Poet, a Professor and a
Politician Discovered the Ice Age. Counterpoint, Washington, 257pp. A wellwritten account of three men, Agassiz, Kane and Lyell, working in the 1840’s,
when the existence of past ice ages was highly controversial.
Berner, R. A., and Lasaga, A.C. 1989. -Modeling the geochemical carbon
cycle.- Scientific American 260 (March): 74-81.
Broecker, W.S. and G.H. Denton. 1990. -What drives glacial cycles?- Scientific
American 262: 48-56.
Campbell, I.D., C. Campbell, M.J. Apps, N.W. Rutter, and A.B.G. Bush, -Late
Holocene ~1500 yr climatic periodicities and their implications.- Geology 26
(5): 471-473.
French, H., 1997. Living on Ice: Problems of urban development in Canada’s
north. In (N. Eyles Ed.) Environmental Geology of Urban Areas, Geological
Association of Canada, p. 81- 91
Houghton, R.A. and G. M. Woodwell, 1989. -Global climate change,- Scientific
American 260:36-44.
Sharp, R.P., 1989. Living ice: Understanding glaciers and glaciation. New York:
Cambridge University Press
Williams, R.S., and J. G. Ferrigno (eds.), 1988. Satellite image atlas of glaciers
of the world. U.S. Geological Survey Professional Paper 1386.
Woods Hole Oceanographic Institution. 1989. -The Oceans and Global
Warming.Oceanus 32.
Slide Set
The National Association of Geoscience Teachers (NAGT) has a set of 25 slides
showing glacial features prepared by Earle McBride, University of Texas at
Austin for sale. The address is NAGT , P.O. Box 5443, Bellingham, WA 982275443.