GEOLOGY 12
SURFACE PROCESSES III
CHAPTER 17 NOTES
GLACIERS
Name __________________
Objectives
1. To acquire an understanding of glacial processes.
2. To learn to recognize the many types of glaciers and glacial landforms.
3. To acquire an understanding of how these landforms are developed.
Quick Glacier Facts
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Presently, 10% of land area is covered with glaciers.
Glaciers store about 75% of the world's freshwater.
Glaciated areas cover over 15,000,000 square kilometers.
Antarctic ice is over 4,200 meters thick in some areas.
During the last Ice Age, glaciers covered 32% of the total land area.
If all land ice melted, sea level would rise approximately 70 meters worldwide.
Glacier ice crystals can grow to be as large as baseballs.
The land underneath parts of the West Antarctic Ice Sheet may be up to 2.5
kilometers below sea level, due to the weight of the ice.
North America's longest glacier is the Bering Glacier in Alaska, measuring 204
kilometers long.
The Malaspina Glacier in Alaska is the world's largest piedmont glacier, covering
over 8,000 square kilometers and measuring over 193 kilometers across at its
widest point.
Glacial ice often appears blue because ice absorbs all other colors and reflects blue.
The Kutiah Glacier in Pakistan holds the record for the fastest glacial surge. In 1953,
it raced more than 12 kilometers in three months, averaging about 112 meters per
day.
Antarctic ice shelves may calve icebergs that are over 80 kilometers long.
Almost 90% of an iceberg is below water--only about 10% shows above water.
The Antarctic ice sheet has been in existence for at least 40 million years.
From the 17th century to the late 19th century, the world experienced a "Little Ice
Age", when temperatures were consistently cool enough for significant glacier
advances.
Note: Refer to the Diagrams of Glacial Features when reading through the following
information.
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Introduction
Glaciers are large masses of ice on land composed of recrystallized snow moving
slowly under the influence of gravity. During the Pleistocene Epoch (1.6 million years
ago to 10 thousand years ago) there were many episodes of glacial advance and glacial
retreat. Times of glacial advance are called glacial periods, while times of glacial
retreat are called interglacial periods. About 18,000 years ago at the maximum extent
of ice, glaciers covered about 30% of the earth's land area.
Today, glaciers cover approximately 10% of the earth's land area. Glaciers on
Greenland and Antarctica account for about 97% of the earth's glacial ice. The
remaining 3% consists of ice on polar islands, glaciers along the perimeter of Alaska
and Scandinavia, and mountain glaciers in the Alps, Himalayas, Cascade Range, Rocky
Mountains, and other major mountain ranges. Today, about 75% of the earth's fresh
water is contained in glacial ice. If all of this ice were to melt, sea level would rise
dramatically (over 70 meters or 230 feet) flooding vast coastal areas.
Two climatic conditions are necessary for the growth and maintenance of glaciers. First,
temperatures must be below freezing at least part of the year. Second, there must be
enough accumulation of snow to equal or exceed ablation which is the loss of ice
through melting and sublimation (ice directly becoming water vapor without melting).
These two conditions are most often met in high latitudes (near polar regions) and high
elevations (tops of some mountains). The presence of glacial deposits and records of
glacial erosion at lower latitudes and lower elevations than where glaciers are found
today indicates that climates were much cooler during much of the Pleistocene Epoch.
Types of Glaciers
There are two major types of glaciers, glaciers
which cover large continental areas and
glaciers which are largely restricted to alpine
(mountain area) valleys. Large continental
glaciers are called continental ice sheets or
ice caps. Glaciers in alpine valleys are called
alpine glaciers.
Ice sheets or ice caps are large masses of ice which flow outward from an area of
maximum thickness and maximum ice surface elevation. Both have a flattened domeshaped cross-section. Ice sheets and ice caps generally are not confined by local
topography, but rather they bury the topography and are free to move across it. Ice caps
are generally less than 50,000 square kilometers in area. Examples are found on
Iceland and Baffin Island. Ice sheets have areas greater than 50,000 square kilometers.
The Greenland and Antarctic ice sheets are modern examples.
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Alpine glaciers are generally confined within mountain valleys. They are much smaller
than ice sheets and ice caps. Alpine glaciers may be classified into several types. The
smallest glacier is a niche glacier which is located in a shallow hollow on a steep slope.
A cirque glacier occupies a depression usually near the head of a valley. A valley
glacier is much like a river of ice and is formed in a pre-existing stream valley. If a
cirque grows to extend down a valley, it becomes a valley glacier.
Movement of Glacial Ice
A glacier flows from its accumulation zone, the area where there is a net gain of snow
and ice, to its ablation zone, the area where there is a net loss of snow and ice by
melting and sublimation (sublimation is the direct evaporation of ice without first
melting). The boundary between these two zones is the equilibrium line, and it is here
that the ice discharge (volume which flows by during an interval of time) is greatest.. In
a valley glacier, if accumulation exceeds ablation the glacier will grow and advance
(move to extend farther down the valley). If ablation exceeds accumulation, the glacier
will shrink or retreat up the valley.
The movement of ice occurs in two ways. The first is internal flow. Internal flow occurs
by the sliding of ice crystals past one another and is usually greatest near the middle
surface of a glacier. Close to the sides and base, drag against the margins slows
internal flow. All glaciers experience internal flow. The second means of glacial
movement is basal sliding. This occurs where the pressure of the overlying ice causes
melting to occur. This produces a film of water which reduces the friction between the
base of the glacier and the bed of the glacier which is the bedrock beneath it. Where
glaciers are frozen to their bed, basal sliding does not occur. Glaciers which move by
combined basal sliding and internal flow move faster than glaciers which move by
internal flow alone. Glaciers move faster at the center and surface than at the sides.
Ice flow may produce vertical to nearly vertical, wedge-shaped cracks called
crevasses. These cracks may range in size from a few centimeters to over 10 meters in
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width and up to about 40 meters in depth. Becoming injured by falling into a crevasse is
a hazard for anyone travelling across the surface of a glacier, especially because
crevasses may be hidden by snow and difficult to detect. Crevasses most commonly
form where the ice flows over irregularities in the bedrock beneath and along the sides
of the ice. Ice falls form where a glacier descends abruptly and produce abundant
crevasses.
Processes of Glacial Erosion
Glaciers are highly effective at eroding landscapes. They are able to erode many times
faster than can rivers. Glacial erosion principally occurs in two ways, glacial abrasion
and glacial plucking (also called glacial quarrying). Both of these occur during ice
movement by basal sliding. When a glacier is frozen to its bed and not sliding, abrasion
and plucking tend not to occur.
Glacial abrasion occurs when rock fragments trapped in the base of a sliding glacier
scrape against bedrock like sandpaper. The high pressure from thick overlying ice
makes abrasion highly effective at eroding rock. Ice does not actually abrade rock
directly because it is softer than the rock. Smaller fragments may smooth and even
polish the bedrock surface while larger rock fragments may produce grooves in the
bedrock called glacial striations which are oriented parallel to the ice flow. Glacial
striations may be used to map out the directions in which ice flowed in a glacially eroded
valley.
Glacial plucking occurs when a glacier breaks off and removes large blocks of rock
from its bed often on the down-flow side of a bedrock high. Plucking is most effective on
rocks which are fractured. Plucked blocks often break away along fractures. These
fractures may have formed prior to glaciation, or they may have formed during glaciation
through frost wedging, a process in which water seeps into rock and freezes. The
expansion of water during freezing produces fractures.
Landforms Produced by Glacial Erosion
Alpine glaciation has sculpted many spectacular and scenic areas. In many areas,
alpine valley glaciers occupied valleys originally eroded by streams. In mountainous
regions, stream valleys characteristically have a V-shaped cross-section. Glacial
erosion straightens and deepens these valleys. Glaciers also change the crosssectional shape to U-shaped. This occurs because glaciers also tend to downcut (erode
downward) more effectively than they erode laterally (sideways) often producing broad
valley bottoms and steep cliffs along valley sides.
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At the head of a glaciated valley, a steep-sided, bowl- or amphitheater-shaped feature
called a cirque is often found. Cirques are the erosional representation of the upper
accumulation zone of a glacier or the product of erosion by cirque glaciers. After a
glacier melts, the bottom of a cirque is often occupied by a small lake called a tarn.
A horn forms where three or more cirques come together to produce a steep-sided
peak. The Swiss Alps are noted for their many spectacular horns including the
Matterhorn.
Descending from a horn there are usually several sharp ridges called aretes. An arete
forms where two adjacent valley glaciers erode the divide between the valleys. A col is
a low point or U-shaped notch in an arete formed where a cirque is eroded into a ridge.
Cols sometimes provide passes through mountain ranges.
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When smaller tributary glaciers downcut at more slowly than does the larger main
glacier, the main valley becomes deeper than the tributary valleys. Thus the floors of the
smaller tributary valleys often meet the larger main valley high above the floor of the
main valley. This feature is called a hanging valley. Hanging valleys are often the sites
of waterfalls where a stream plunges over a steep cliff at the end of a tributary valley to
the floor of the main valley. The movement of ice down a valley also erodes ridges
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which once separated tributary streams. Where such a ridge ends abruptly at a cliff it is
called a truncated spur. Hanging valleys are often found together with truncated spurs.
A fjord is a long, steep-sided, deep glacial valley which has been flooded by the sea.
Some fjords exceed 1200 meters in depth. This great depth suggests that coastal valley
glaciers eroded their beds to well below sea level. Examples are found in southern
Alaska and along the west coast of British Columbia. Although lacking steep-sides,
Puget Sound in western Washington is a very large fjord created at the end of the
Pleistocene epoch.
Erosion by continental ice sheets scours vast areas. Where an ice sheet flows over preexisting stream valleys, it may erode enormous U-shaped troughs. The Great Lakes of
the United States and Canada are examples.
Glacial meltwater may also erode. Meltwater flowing beneath a glacier in the form of
subglacial streams may produce localized erosion beneath a glacier. Large volumes of
meltwater from a continental ice sheet may erode large valleys called meltwater
channels alongside or beyond the terminus of a glacier. After the ice sheet has
retreated, these channels may be occupied by steams which are obviously too small to
have eroded their valleys. Such streams are called underfit or misfit streams. Many
examples of meltwater channels and underfit streams are found south and east of
Tacoma, Washington.
Coulees are channels or canyons formed by rapid release of water from an icedammed lake. During the last continental glaciation of North America, floodwaters
released from glacial Lake Missoula of Idaho and Montana rushed across eastern
Washington many times eroding numerous coulees and forming many now dry
waterfalls in an area called the Channeled Scablands.
Glacial Sediment Transport and Deposition
Glaciers are capable of eroding and transporting enormous volumes of material. The
general term for this material when it is deposited is glacial drift. Unsorted, unstratified
(non-layered) glacial drift deposited directly from a glacier is called till (or unstratified
drift). Till is very poorly sorted, and if it becomes lithified (cemented), it forms a type of
conglomerate. Particle sizes in till range from clay-sized glacial flour (rock pulverized to
a very fine powder) to large boulders. Boulders the size of a house have been
transported by glaciers and found in till deposits. When ice transports a large boulder of
one rock type to an area with a different type of bedrock, the bolder is called a glacial
erratic.
(See erratic photos.)
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Valley or Alpine Glacier
Continental Glacier or Ice Cap
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Stratified (layered) glacial sediments transported and re-deposited by glacial melt water
and wind are called stratified drift. Stratified drift deposited by glacial meltwater is often
called outwash. Outwash is largely deposited by braided streams on an outwash
plain. Outwash is moderately to well sorted, and in many glaciated areas it forms
important aquifers, Glacial flour carried in meltwater imparts a gray to greenish color to
rivers in glaciated areas. Glacial silt may be carried great distances by winds from
outwash plains to produce loess deposits. These deposits form productive farmland.
Examples of loess deposits are found in eastern Washington State and in the
Mississippi River valley.
Moraines, deposits of rock debris deposited along or near the margins or glaciers, are
the most common features of glacial deposition. Moraines are sometimes represented
on topographic maps by a stippled or dotted pattern. The irregular topography often
associated with moraines contrasts with the smoother topography of an outwash plain.
A ground moraine is a broad blanket of till which represents sediment carried within ice
near the bottom of a glacier and later deposited as the result of ablation. Till in ground
moraines is highly compacted because of the weight of overlying ice when is deposited.
This and its poorly sorted nature make ground moraine till a good aquitard.
Lateral moraines form along the margins of valley glaciers. They are composed of till
and rock debris from which has fallen from the valley sides. After a glacier melts, lateral
moraines often remain as ridges of sediment along valley sides. When two valley
glaciers flow together and join, their intervening lateral moraines coalesce to form a
medial moraine.
End moraines are accumulations of till deposited at the end or terminus of a glacier
which is where most ablation occurs. The longer the terminus of a glacier remains in
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one position (as long as ablation equals accumulation), the more till is deposited and
the larger the end moraine becomes. If climate changes and accumulation begins to
exceed ablation, the snout of a glacier begins to advance overriding its previously
deposited end moraine. The end moraine deposited by a glacier at its point of farthest
advance is called a terminal moraine. If ablation begins to exceed accumulation,
melting will cause the terminus of the glacier to recede. An end moraine deposited at
the terminus of a glacier which has receded is called a recessional moraine. If a
glacier recedes intermittently (alternately recedes and remains stationary), a series of
recessional moraines may be formed.
Glacial till and stratified drift may be molded into elongate, rounded hills called
drumlins. Drumlins are formed when advancing ice overrides previously deposited
moraines and outwash. They have a long axis which is oriented parallel to the direction
of ice movement. The cross-section of a drumlin cut along its length has a steeper slope
(more closely spaced contour lines) and a shallower tapering slope (more widely
separated contour lines). The shallower slope tapers in the direction of ice movement.
Most drumlins are produced by continental glaciers, and broad areas containing
abundant drumlins are sometimes called drumlin fields.
Meltwater flowing beneath and on the surface of a glacier produces various deposits
consisting largely of sand and gravel. Subglacial streams (streams flowing near the
base of a glacier) may deposit sand and gravel in tunnels beneath the ice. When the
glacier melts, this sediment is left behind as a winding ridge called an esker which
resembles a raised railroad bed. Water flowing across the surface of a glacier may
deposit sediment in depressions in the surface of the ice or near glacial margins. When
the ice melts, the sediment deposited in depressions forms small hills called kames
while the sediment deposited near glacial margins forms deposits on valley sides called
kame terraces. In some glaciated areas, including Langford and Colwood, eskers,
kames, and kame terraces are mined for their sand and gravel for use in various types
of construction.
Kettles are depressions formed as blocks of ice become detached from a receding
glacier and melt in the outwash plain. Kettles often contain lakes called kettle lakes.
Kettle lakes and ice-marginal lakes, lakes at the margin or terminus of a glacier, are
sites for deposition of fine silt- and clay-sized sediments. Sometimes they may contain
finely-layered sediments called varves.
Each varve consists of a pair of light and
dark layers which represent a year's
deposition. The lighter layer is deposited
during the summer. The darker layer is
deposited during the winter when the lake
freezes over. At this time, the water is almost
completely still. This allows dark-colored
organic material and the finest sediments to
settle to the bottom of the lake.
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Pleistocene Ice Ages
Ice sheets or ice caps are large accumulations of ice that flow laterally under their own
weight.
As the weight of accumulating snow and ice press down, the ice is forced to flow horizontally. In map
view, the ice flows radially away from the center of the ice cap. Because the ice cap is so heavy, it weighs
down on the continent on which it sits, pushing the continental lithosphere down into the mantle below.
Ice Ages are intervals of time when large areas of the surface of the globe are covered
with ice sheets (large continental glaciers). The growth and retreat of glaciers occurs in
response to changes in global climate. A number of factors interact to produce
conditions favoring the formation of ice sheets. Some of these factors include
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changing continental positions
uplift of continental blocks
reduction of CO2 in the atmosphere
changes in the Earth's orbit
If we look at all of Earth history we find that there were long periods in which there were
no polar ice caps. However, there have many glacial advances and retreats during the
last billion years.
Earth's climate has been growing gradually cooler for the last 50 million years. About 15
million years ago this cooling accelerated, causing large ice sheets to expand and cover
the Antarctic continent. Two million years ago ice sheets were present in the northern
hemisphere as well. Remnants of these ice sheets remain buried in the Antarctic and
Greenland.
On the North American continent we recognize four distinct periods of glaciation within
the last 1.6 million years. The most recent glacial event ended 10,000 years ago. This
period of time is called the Pleistocene Epoch. There were 3 main centers of
accumulation. These ice sheets met to cover all of Canada and most of the northern
states. Evidence of these glaciations remains in the form of rounded mountain
tops, moraines, erratics, drumlins, eskers, kames, kettles, roche moutonées and
striations.
NOTE: Many useful links are posted on our website. Use these links to familiarize yourself with
glacial terms and view images of the various features. There will definitely be glacier questions
on the Provincial exam. The more familiar you are with the features and the processes that
formed them the better prepared you will be to answer these questions.