Glaciers
I.
Glaciers are defined as a mass of compacted and recrystallized ice and snow flowing under its
own weight on land
A. There are generally two types of glaciers- alpine or valley glaciers and continental glaciers
B. This does not include sea ice or icebergs because they are floating ice
C. Glaciers presently cover about 10% of the Earth's surface
1. Many glaciers are found in North America, especially Alaska and Canada in
mountainous areas
2. The Andes, the Alps and the Himalayas in Asia support glaciers
3. Even Mt. Kilamajaro in Africa is high enough to have glaciers even though it is at
the equator
4. Greenland and the Antarctic are almost entirely covered by glacial ice
D. Glaciers are slow but active agents of erosion, transport, and deposition
1. They deeply scour the landscape and produce distinctive landforms
2. Most glaciation and the effects of it were formed during the Pleistocene epoch from
1.6 million to 10,000 years ago
E. Glaciers are an important part of the Hydrologic Cycle
1. About 2.15% of the earth's water is in the form of glacial ice
2. Many glaciers flow directly into the ocean where they calve into large chunks that
flow out to sea as icebergs
3. Some glaciers flow from higher elevations to lower areas where they melt, producing
runoff and surface streams
4. Some alpine glaciers are important sources of fresh water form streams and valleys
II.
Glaciers are composed of ice, a crystalline structure considered to be a type of rock, one that
easily deforms
A. It forms as snow which contains 80% air and 20% water, compacts, freezes, and thaws and
refreezes
1. This process produces a granular type of ice called firn p.401
2. As the firn is buried it is compacted and converted to glacial ice consisting of 90%
solids
3. When it reaches a critical thickness of 40m, or 125 feet, the pressure is enough to
cause deformation and flow
4. Glaciers primarily move by plastic flow p.402
5. Basal slip is occurs as glaciers slide over the wet surface of meltwater which reduces
the friction
B. Valley or alpine glaciers are those that are developed on a mountain valley p.403
1. They have tributaries like streams or rivers
2. They move from higher elevations to lower elevations and are small compared to
continental glaciers
C. Continental glaciers are also called ice sheets and cover vast areas
1. They flow outward from a central area of accumulation
2. The direction of flow is determined by the thickness of the ice sheet
3. Currently only Greenland and Antarctica contain continental glaciers
4. Large ice sheets covered the North American continent during the Pleistocene
resulting in erosion and depositional landforms seen today in Canada and northern US
5. Ice caps are also recognized and are essentially smaller glaciers
III.
Glaciers have a glacial budget resulting from the accumulation and wastage of glacial material
A. The upper part is called the zone of accumulation where the glacier receives snow and ice
IV.
V.
B. The lower part is the zone of wastage where losses from melting, sublimation, and calving
exceeds the rate of accumulation p.405
C. The glacial process
1. In winter the glacier receives snow and is completely covered by spring
2. During spring and summer, the snow begins to melt and forms firn, first at low
elevations then higher as summer progresses
3. The elevation which the snow recedes is called the firn limit
4. The firn limit usually changes each year in response to the changing weather
5. If the firn limit stays the same it is said to have a balanced budget, the distal end or
terminus of the glacier, remains stationary
6. If the firn limit advances down the glacier, the glacier has a positive budget- the
additions exceed losses and the terminus advances
7. If the budget is negative, the glacier recedes and its terminus moves uphill
8. Even though a glacier is in decline, it continues to move toward the terminus
9. If a negative budget persists long enough the glacier will become stagnate or
eventually stop flowing or disappear
Rates of Glacial Movement
A. Valley glaciers move faster than continental glaciers ranging from a few cm to 10's of meters
per day
1. Valley glaciers move faster because on steep slopes than ones on shallow slopes
2. The main glacier moves faster than the tributaries because the discharge is greater
3. Valley glaciers are subject to basal slip and move faster in summer months
4. Flow rates within the ice varies
a) Flow rate increases from the top down to the firn limit
b) Flow rate slows from the firn limit to the terminus
5. Valley glaciers resemble streams in that the fastest flow rate is in the middle
a) Friction on the valley walls slow it down
b) Ice on the valley floor moves slower than sides
6. Flow rates increase upward from the base until the 40m or so from the top
a) The upper part is the rigid part of the glacier that moves as a result of the
plastic flow and basal slip of the base
b) The upper surface behaves as a brittle solid as seen in the crevasses on the
surface as the glaciers moves over a step on the valley floor or turns a corner
c) The glacial ice is stretched and broken that reaches down to the zone of plastic
flow
d) In some places the glacier descends over a steep cliff, breaking up into blocks
and spires where an ice fall develops
7. Glacial surges are bulges that move through alpine glaciers several times faster than
normal
B. Continental glaciers move at a flow rate of cm to meters per day
1. Even a slow rate has a total cumulative effect over a hundred years or so
2. These glaciers move slowly because they exist at colder latitudes and the ground is
frozen beneath them so little basal slip occurs
3. Most of the movement is by plastic flow
4. Some glaciers like in Greenland move faster when they are trapped between mountain
ranges and can exceed 100m per day
5. Ice streams in Antarctica have been studied that have sediment beneath them that
contain water that helps flow rate increase
Glacial Erosion and Transport
VI.
A. Glaciers are moving solids and can erode and transport enormous quantities of sedimentary
materials
1. Glaciers can transport large boulders over long distances before depositing them as
glacial erratics
2. Erosion of large amounts of soil materials as in bulldozing
3. Plucking is the result of glacial ice freezing in cracks in the bedrock and eventually
pulling it loose
4. Glaciers smooth the upper side of a rock projection and plucks the lower by
repeatedly freezing and pulling away the material
5. Glaciers move over the basement rock and form smooth surfaces by abrasion and will
develop glacial polish
6. Glacial striations are straight scratches produced on bedrock during glacial
movement as ice and rock material move downslope
7. Abrasion also produces fine material called rock flour which gives streams and lakes
developed form the glacier a milky appearance
B. Continental glaciers can derive sediment from mountains protruding through them or from
wind blown dust
1. Most of the material is formed at the surface they move over
2. This is transported at the lower part of the ice sheet
C. Valley glaciers derive sediment form all parts f the ice, but usually along the sides and base
1. Most sediment are from mass wasting events
2. Other sources are abrasion, plucking
Erosional Landforms from Alpine Glaciation
A. U-shaped glacial troughs are one of the most distinctive features of valley glaciation
1. These valleys are deepened, widened, and straightened by glaciers
2. They have steep vertical walls and broad flat valleys
3. Truncated spurs of ridges are formed by glaciers that cut off the ridge extending into
the main glacial valley
4. Steps and basins are formed where the glacier eroded rocks of varying hardness
5. Many of these basins contain lakes
B. In the Pleistocene the sea level was about 130m or 400 feet lower than today
1. Glaciers flowing to the sea eroded much lower than they do today
2. Fjords are formed when the glaciers melted and the ocean filled the basins where the
glaciers were
3. Fjords are formed in higher latitudes where glaciers can survive even at low
elevations like in Alaska, western Canada , Scandinavia, Greenland, and southern Chile
4. Glaciers can erode considerable distance even under water
5. A glacier 500m thick can stay in contact with the seafloor and effectively erode to a
depth of 450m before the buoyant effects cause the ice to float
6. Some fjords in Norway and Chile have been scoured to a depth of 1300m
C. Hanging valleys are tributaries that have their valley floor higher than the main valley
1. Where the two valleys meet, the mouth of the tributary is above the main valley floor
usually forming a waterfall or plunging precipice
2. The glacier in the main valley erodes much more than the tributary ones
3. When the glaciers are gone, they are left as hanging valleys
D. The upper reaches of a glacier develops spectacular erosional features
1. Cirques are steep walled bowl-shaped depressions at the upper end of the valley
a) Typically steep-walled on three sides and open to the glacial trough
b) Many have a lip or threshold at the lower end and can contain a cirque lake
c) Frost wedging and plucking enlarge the depression
d) Rotating and scouring the ice moves outward over a threshold leaving a
depression for the formation of a cirque lake or tarn
e) With continued headward erosion, the cirque widens laterally developing
aretes and horns
2. Aretes are narrow serrated ridges at the headwall of a glacier
a) Cirques on opposite sides of the ridge erode headward forming steep walls
b) Two parallel glaciers can reduce the dividing ridge into thin spine of rock
3. Horns are steep-walled pyramidal peaks formed by headward erosion of cirques
a) These are impressive peaks like the Matterhorn in Switzerland and the Grand
Teton in Wyoming
b) They must form from at least three cirques that erode headward
c) This produces a majestic triangular peak with three or more cirques on its
sides
E. Continental Glaciers erode the land to a smooth and rounded appearance
1. These glaciers erode and abrade the mountains and ridges that protrude into the
glacier to smooth and rounded features
2. They produce topography that is flat with rounded hills rather than sharp angular
features of valley glaciation
3. Continental glaciation strips off the soil and unconsolidated sediment developing
exposures of striated and polished rock
4. This landform is common in Canada, particularly in the Canadian Shield area, in
Maine and the northern US
5. These areas have drainage patterns that have been completely disrupted and have not
reestablished themselves into streams and rivers
6. They form deranged drainage systems of numerous lakes and swamps, low relief,
extensive bedrock exposures, and little or no soil called scoured plains
7. Drumlins are elongated hills formed when continental glaciers reshape previously
deposited till
a) They develop features that look like an inverted spoon and can be 1km long
and 50m high
b) The steep end is at the advancing side and the gently sloping end points in the
direction of the ice movement
c) Drumlins are used to determine the direction of movement by their asymmetry
d) They usually form as a field of drumlins and there can be thousands of them
e) Their origin is thought to have been from the glacier reshaping the till deposits
or as flooding in the outwash plain reshaping the till
8. Valley trains are formed in valley glaciers
a) They release huge amount of water and form braided streams
b) These streams are restricted to the valley or glacial trough and form narrow
deposits of valley trains
9. Kettle Lakes form when a block of ice is left behind from a retreating glacier
a) It is usually buried within the till
b) When it melts, it leaves a depression which fills with water if the ice extended
below the water table
c) Kettles can form in outwash plains and end moraines
10. Kames are conical hill composed of stratified drift
a) They are formed when sediments deposit in a depression on the glaciers
surface
VII.
b) As the ice melts the deposit is lowered intact to the ground
c) Kames can also develop in cavities of ice
11. Eskers are long sinuous ridges of stratified drift
a) They meander and have tributaries with sharp crests and steep sides
b) Some are over 100km long
c) They can be formed by valley and continental glaciers
d) Stratification and sorting indicates deposition by running water and were
probably deposited by streams running in tunnels beneath the ice
Glacial deposits
A. All the sediment deposited by glaciers is called glacial drift
B. There are generally two types of glacial drift, till and stratified drift
C. Till is sediment deposited directly by glaciers
1. It is unsorted, essentially not separated by size, shape, or density
2. It is not stratified and has no layering
3. Till of valley glaciers and continental glaciers have the same shape and look, except
that continental glaciers move material much farther
D. Stratified drift is sorted by size and density and has layering
1. Deposited by braided streams in outwash plains and riverbeds
2. These deposits derive their water and sediment directly from the glacier
E. Moraines are landforms of glacial till
F. End moraines are formed at the terminus of a glacier
1. The glacier can become stabilized at one place, depositing its load in a pile at the end
of the glacier
2. This does not mean that the glacier stops flowing, but that it has reached a balance
between its zone of accumulation and the zone of wastage
3. If the glacier should advance it will override and redistribute the former moraine
G. If the glacier should retreat, it will form a ground moraine until it becomes stabilized again
forming a new moraine called a recessional moraine
H. Terminal moraines are formed at the farthest extent of the glaciers- these are found in
southern Ohio as a series of low hills that formed along the glacial front
I. Lateral moraines develop along the sides of valley glaciers
1. They are from material in the valley walls falling onto the glacier's surface
2. They develop long lines of material at the sdes and middle of some valley glaciers
J. Medial moraines are developed when two or more glaciers merge and the lateral moraines
are trapped between ice
1. Large glaciers will have dark stripes that indicate that a glacial tributary has merged
with the main glacier
2. You can tell how many tributaries the glacier has by the number of medial moraines
K. Stratified drift is deposited by running water and exibits sorting and layering
1. It is formed in both valley and continental glaciers but is moer extensive in
continental glaciation
2. Glaciers release huge amounts of water with excessive sediment that forms braided
streams that radiate out from the glacial front
3. The outwash plain is an extensive area where these braided streams deposit their load
4. Glaciers can sometimes release huge amounts of water when a volcano erupts beneath
it and melts the ice- this happened in Iceland in 1996 causing the largest flood in 60 years
L. Glacial Lakes are common in glacial areas
1. They form as scoured out areas by glaciers, water accumulating behind moraines,
blocked streams, or kettles lakes
VIII.
2. Fine grained deposits called varves form in these lakes
3. They are alternating light and dark layers that represent a yearly episode of deposition
4. The dark layer forms in the winter when the lake was frozen over and fine particles in
suspension settled out
5. The light layer is clay ad silt deposited during spring and summer
6. The number of varves represents the number of years the lake existed
7. Dropstones are rocks that have fallen into the clay deposits from melting icebergs
floating on the surface and embedding are into the fine layers
Pleistocene Glaciation
A. Began about 1.6 million years ago and consisted of several episodes of glaciation separated
by intervals of warmer periods, called interglacials
1. At least four major glacial periods are recognized in North America, with 6 or 7
major periods seen in Europe
2. Deep sea cores have recognized 20 warm-cold cycles
B. Best estimates are that the period ended about 10,000 years ago
1. There is disagreement whether we are at the end of the glacial periods or just in an
interglacial period
2. Cycles of glaciation began about 40 million years ago when southern latitudes began
to cool
3. Ice sheets began to form about 2-3 million years ago in the Northern Hemisphere
C. Ice sheets covered about three times as much of the Earth's surface as they do now as much
as 3 km thick
1. Large areas of the North American continent were covered by ice , as well as
Scandinavia, Great Briton, Ireland, and Northern Russia
D. Climates varied worldwide, and temperatures were not as frigid everywhere as near the
glaciers
1. During advancing glaciation, oceans were lower and lowered evaporation reduced the
amount of rainfall in many areas
2. The cold belts in the northern latitudes expanded southward, compressing the tropical
zones
3. Rain that now falls on the Mediterranean would fall on the Sahara in North Africa,
creating lush healthy forests where now is desert
4. The southwest US received rain that now falls in the Pacific Northwest creating
wetter climates and numerous lakes
E. The basin and range area in Nevada and California received large amounts of rain and
developed large lakes called pluvial lakes
1. Lake Bonneville was the largest of these lakes, the Great Salt Lake is but a remnant
of this lake
2. Death Valley was filled with Lake Manly, and it maintained a level of 178m deep and
145km long. Important borate deposits were formed when the lake evaporated
3. Lake Lahontan near Reno, Nevada filled a huge depression
F. Proglacial Lakes develop along the margins of glaciers
1. Before the glaciers, no lakes existed in the Great Lakes region
2. It was flat lowlands with broad river valleys that drained to the north
3. The glaciers deeply scoured the valleys into basins that below the sea level
4. The glaciers extended far south and covered the area of the Great Lakes during their
farthest extent
5. When the ice sheet retreated, parts of the area was uncovered and filled with water by
about 14,000 years ago
G. So much water was stored in ice that the sea level fell 130m or about 400 feet
1. Extensive areas of the continental shelves were exposed
2. Land bridges between Alaska and Asia formed allowing movement of Early
Americans and other animals
3. The British Isles were connected to Europe
4. The base level of streams was lowered and rivers eroded deeper into the shelves and
the streams adjusted to the new level
5. When the sea level rose again, the sea flooded the valleys making important harbors
6. If the glaciers today would melt the sea level would rise 70m or 200 feet
H. Causes of Glaciation
1. There is some cyclic occurrence of glaciation, but it has been difficult to develop a
comprehensive theory
2. Only a few periods of glaciation exist in the geologic record and each is separated by
long periods of mild climate
3. Tectonic activity and the slow movement of continents is probably related to these
changes
4. Moving plates carry the continents to higher latitudes where ice sheets can exist,
glaciers do not form on open sea areas
5. Plate collisions and the changing ocean circulation could combine to create long term
climate changes
6. The cyclic nature of the most recent glaciation has occurred on the scale of tens to
hundreds of thousands
7. The Milankovithch Theory has been proposed in the 1920's
a) It says that the minor irregularities of the Earth's rotation and orbit could alter
the solar radiation to cause climatic changes
b) This depends on the three parameters of Earth's orbit
(1) Orbital eccentricity, which is the degree it departs from a perfect circle
(2) Calculation indicate a 100,000 year cycle which corresponds to 20
warm-cold cycles during the Pleistocene
(3) The second factor is the angle of the earth's axis and a line
perpendicular to the plane of the elliptic
(4) This angle shifts 1.5 during a 44,000 year cycle
(5) The third factor is the precession of the equinoxes, which cause the
equinoxes to shift around the Earth's orbit every 22,000 years
c) Continuous changes in these parameters cause the amount of heat to any
latitude to change over time
d) Variations such as the Little Ice Age in the 1500's to 1800's are not explained
by this theory
e) Relationships to volcanic activity have been proposed but do not fit the cyclic
nature
f) Information from astronomical observations have come up with explanations
that include the periodic encounter with cometary debris that shields the solar
energy to the Earth
g) This would include dust clouds that are in the path of the Earth's orbit
h) It could also be the result of periodic encounter with comets that collide with
the Earth as a result of cyclic changes in the Ort cloud outside the solar system
i) To put the Milankovitch Theory in place it may have been the reason the
Earth has come out of a glacial period by warming the planet slightly to initiate
interglacial warming