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Chapter 17
Glacial and Periglacial
Processes and Landforms
Geosystems 5e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen
LAB #3
Assignment #3 (book report) is due next week!!
Key Learning Concepts:
After this lecture you should be able to:
1. Differentiate between alpine and continental glaciers and
2.
3.
4.
5.
describe their principal features.
Describe the process of glacial ice formation and portray the
mechanics of glacial movement.
Describe characteristic erosional and depositional landforms
created by alpine glaciation and continental glaciation.
Analyze the spatial distribution of periglacial processes and
describe several unique landforms and topographic features
related to permafrost and frozen ground phenomena.
Explain the Pleistocene ice age epoch and related glacials and
interglacials and describe some of the methods used to study
paleoclimatology.
1. Describe the location of most of
the freshwater on Earth today.
A large measure of the freshwater on Earth is
frozen, with the bulk of that ice sitting restlessly
in just two places–Greenland and Antarctica. The
remaining ice covers various mountains and fills
alpine valleys. More than 29 million km3 of
water, or about 77% of all freshwater, is tied up
as ice.
2. What is a glacier? What is implied about
existing climate patterns in a glacial region?
A glacier is a large mass of perennial ice, resting on land
or floating shelf-like in the sea adjacent to land.
Glaciers form by the accumulation and recrystallization
of snow. They move under the pressure of their own
mass and the pull of gravity. Today, about 11% of
Earth's land area is dominated by these slowly flowing
ice streams. During colder episodes in the past, as much
as 30% of continental land was covered by glacial ice
because below-freezing temperatures prevailed at lower
latitudes, allowing snow to accumulate. Relative to
elevation, in equatorial mountains, the snowline is
around 5000 m; on midlatitude mountains, such as the
European Alps, snowlines average 2700 m; and in
southern Greenland snowlines are down to 600 m.
3. Differentiate between an alpine glacier
and a continental glacier.
With few exceptions, a glacier in a mountain range is called an
alpine glacier, or mountain glacier. It occurs in several subtypes.
One prominent type is a valley glacier, an ice mass constricted
within the confines of a valley. Such glaciers range in length from
only 100m to over 100 km. The snowfield that feeds the glacier
with new snow is at a higher elevation. As a valley glacier flows
slowly downhill, the mountains, canyons, and river valleys beneath
its mass are profoundly altered by its passage. A continuous mass
of ice is known as a continental glacier and in its most extensive
form is called an ice sheet. Two additional types of continuous ice
cover associated with mountain locations are designated as ice caps
and ice fields. Most glacial ice exists in the snow-covered ice
sheets that blanket 80% of Greenland and 90% of Antarctica.
4. Name the three types of continental glaciers.
What is the basis for dividing continental glaciers
into types? Which type covers Antarctica?
The three types are ice sheet, ice cap, and ice
field. Both ice caps and ice sheets completely
bury the underlying landscape, although an ice cap
is somewhat circular and covers an area of less
than 50,000 km2. Antarctica alone has 91% of all
the glacial ice on the planet as an enormous ice
sheet, or more accurately several ice sheets acting
in concert. An ice field is the smallest of the
three.
5. Trace the evolution of glacial ice from
fresh fallen snow.
Snow that survives the summer and into the following winter begins a
slow transformation into glacial ice. Air spaces among ice crystals are
pressed out as snow packs to a greater density. The ice crystals
recrystallize under pressure and go through a process of re-growth and
enlargement. In a transition step to glacial ice, snow becomes firn,
which has a granular texture. As this process continues, many years
pass before denser glacial ice is produced. Formation of glacial ice is
analogous to formation of metamorphic rock: sediments (snow and firn)
are pressured and recrystallized into a dense metamorphic rock (glacial
ice).
The formation of glacial ice is analogous to formation of a metamorphic
rock with sediments (snow and firn) pressured and recrystallized into a
dense metamorphic rock (glacial ice). The time taken for such a process
is dependent on climatic conditions as noted in the text. A dry climate
such as that found in Antarctica may produce glacial ice in 1000 years,
whereas a wetter climate may take only a few years to complete the
formation process.
6. What is meant by glacial mass balance? What are
the basic inputs and outputs underlying that balance?
A glacier is fed by snowfall and is wasted by losses from its upper
and lower surfaces and along its margins. A snowline called a firn
line is visible across the surface of a glacier, indicating where the
winter snows and ice accumulation survived the summer melting
season. A glacier's area of excessive accumulation is, logically, at
colder, higher elevations. The zone where accumulation gain ends
and loss begins is the equilibrium line. This area of a glacier
generally coincides with the firn line, except in subpolar glaciers,
where frozen surface water occurs below the firn line. Glaciers
achieve a positive net balance of mass–grow larger–during colder
periods with adequate precipitation. In a glacier's lower elevation,
losses of mass occur because of surface melting, internal and basal
melting, sublimation, wind removal by deflation, and the calving,
or breaking off, of ice blocks. In warmer times, the equilibrium
line migrates to a higher elevation and the glacier retreats–grows
smaller–due to its negative net balance.
7. What is meant by a glacier surge? What do
scientists think produces surging episodes?
Some glaciers will lurch forward with little or no
warning in a glacial surge. This is not quite as abrupt as
it sounds; in glacial terms, a surge can be tens of meters
per day. The Jakobshavn Glacier in Greenland, for
example, is known to move between 7 and 12 km a year.
The exact cause of such a glacial surge is still being
studied. Some surge events result from a buildup of
water pressure in some layers of the glacier. Sometimes
that pressure is enough to actually float the glacier
slightly during the surge. As a surge begins, icequakes
are detectable, and ice faults are visible along the
margins that separate the glacier from the surrounding
stationary terrain.
8. How does a glacier accomplish erosion?
Glacial erosion is similar to a large excavation project,
with the glacier hauling debris from one site to another
for deposition. As rock fails along joint planes, the
passing glacier mechanically plucks the material and
carries it away. There is evidence that rock pieces
actually freeze to the basal layers of the glacier and,
once embedded, allow the glacier to scour and
sandpaper the landscape as it moves, a process called
abrasion. This abrasion and gouging produces a
smooth surface on exposed rock, which shines with
glacial polish when the glacier retreats. Larger rocks in
the glacier act much like chisels, working the
underlying surface to produce glacial striations parallel
to the flow direction.
9. Describe the evolution of a V-shaped stream
valley to a U-shaped glaciated valley. What kinds
of features are visible after a glacier retreats?
(Figure 17-10). Illustration (a) shows a typical river valley with
characteristic V-shape and stream-cut tributary valleys that exist before
glaciation. Illustration (b) shows that same landscape during a later
period of active glaciation. Glacial erosion and transport are actively
removing much of the regolith (weathered bedrock) and the soils that
covered the preexisting valley landscape. Illustration (c) shows the same
landscape at a later time when climates have warmed and ice has
retreated. The glaciated valleys now are U-shaped, greatly changed from
their previous stream-cut form. You can see the oversteepened sides, the
straightened course of the valley, and the presence of hanging valleys and
waterfalls. The physical weathering associated with a freeze-thaw cycle
has loosened much rock along the steep cliffs, falling to form talus cones
along the valley sides during the postglacial period. See Figure 17-10 for
labeled details of the features that are formed as a result of the formation,
growth, passage, and retreat of an alpine glacier.
Figure a: A preglacial landscape
with V-shaped stream-cut valleys.
Figure b: The same landscape
filled with valley glaciers.
Figure c: When the glaciers retreat, the new
landscape is unveiled.
10. How is an iceberg generated?
Where a glacier ends in the sea, large pieces
break off and drift away as icebergs. These are
portions of a glacier at drift in the sea. When
large pieces of an ice shelf break off, such as the
portions of the Ross ice shelf in the west
Antarctic area have done during the past few
years, enormous tabular islands are formed as a
type of iceberg.
11. Differentiate between two forms
of glacial drift—till and outwash.
Where the glacier melts, debris accumulates to
mark the former margins of the glacier—the end
and sides. Glacial drift is the general term for
all glacial deposits. Direct deposits appear
unstratified and unsorted and are called till. In
contrast, sorted and stratified glacial drift,
characteristic of stream-deposited material, is
called outwash (see 2nd slide) and forms an
outwash plain of glaciofluvial deposits across
the landscape.
Glacial Till
Glacial Outwash
12. What is a morainal deposit? What moraines are
created by alpine and continental glaciers?
Glacial till moving downstream in a glacier can form a
marginal unsorted deposit known as a moraine (see
Figure 17-7). A lateral moraine forms along each side
of a glacier. If two glaciers with lateral moraines join,
their point of contact becomes a medial moraine.
Eroded debris that is dropped at the glacier's farthest
extent is called a terminal moraine. However, there
also may be end moraines, formed wherever a glacier
pauses after reaching a new equilibrium. If a glacier is
in retreat, individual deposits are called recessional
moraines. And finally, a deposition of till generally
spread across a surface is called a ground moraine.
Figure 17.7: Examples of a Medial Moraine, End
Moraine, and Terminal Moraines.
Picture of an End Moraine: End moraine of a piedmont glacier (large valley
glaciers meet to form an almost stagnant sheet of ice) , Bylot Island, Canada.
The sharp-crested ridge of till (end moraine) was pushed up at the ice margin
during the glacier's maximum advance, probably during the Little Ice Age.
Lateral and terminal moraines of a valley glacier, Bylot Island, Canada. The
glacier formed a massive sharp-crested lateral moraine at the maximum of its
expansion during the Little Ice Age. The more rounded terminal moraine at
the front consists of medial moraines that were created by the junction of
tributary glaciers upstream.
Columbia Glacier in Alaska encroached on a forest as it surged in
the early twentieth century, pushing trees, soil, rocks and debris as it
advanced. This photograph was taken in 1914, several years after
the glacier had begun to surge, creating a new terminal moraine.
Columbia Glacier is located in the Chugach Mountains of Alaska.
13. What are some common depositional features
encountered in a till plain?
With the retreat of the glaciers, many relatively flat
plains of unsorted coarse till were formed behind
terminal moraines. Low, rolling relief and deranged
drainage patterns are characteristic of these till plains.
As the glacier melts, this unsorted cargo of ablation till
is lowered to the ground surface, sometimes covering
the clay-rich lodgement till deposited along the base.
The rock material is poorly sorted and is difficult to
cultivate for farming, but the clays and finer particles
can provide a basis for soil development.
14. Contrast a roche moutonnée and a drumlin regarding
appearance, orientation, and the way each forms.
Two landforms created by glacial action are streamlined
hills, one erosional (called a roche moutonnée) and the
other depositional (called a drumlin). A roche
moutonnée is an asymmetrical hill of exposed bedrock.
Its gently sloping upstream side has been polished smooth
by glacial action, whereas its downstream side is abrupt
and steep where rock was plucked by the glacier (See
Figure 17-16). A drumlin is deposited till that has been
streamlined in the direction of continental ice movement,
blunt end upstream and tapered end downstream.
Multiple drumlins (called swarms) occur in fields in New
York and Wisconsin, among other areas. Sometimes their
shape is that of an elongated teaspoon bowl lying face
down (See Figure 17-17).
Figure 17.16:
Glacial
Erosion
Streamlined
Rock. Roche
Mountonnée,
as exemplified
by Lembert
Dome in
Yosemite
National Park,
California.
A drumlin in New York State and Drumlin field in Snare
Lake, Canada (Figure 17.17).
15. In terms of climatic types, describe the areas on Earth
where periglacial landscapes occur. Include both higher
latitude and higher altitude climate types.
Periglacial regions occupy over 20% of Earth's land
surface (See Figure 17-18). The areas are either near
permanent ice or are at high elevation, and have ground
that is seasonally snow free. Under these conditions, a
unique set of periglacial processes operate, including
permafrost, frost action, and ground ice.
Climatologically, these regions are in Dfc, Dfd
subarctic, and E polar climates (especially ET tundra
climate). Such climates occur either at high latitude
(tundra and boreal forest environments) or high
elevation in lower-latitude mountains (alpine
environments).
Figure 17.18: Permafrost
Distribution. Distribution of
Permafrost in the Northern
Hemisphere. Alpine
permafrost occurs in high
elevations in Mexico, Europe,
Japan, and even Hawaii. Subsea permafrost occurs in the
ground beneath the Arctic
Ocean along the margins of the
continents.
16. Define two types of permafrost, and differentiate their
occurrence on Earth. What are the characteristics of each?
When soil or rock temperatures remain below 0°C (32°F) for at
least two years, a condition of permafrost develops. An area that
has permafrost but is not covered by glaciers is considered
periglacial. Note that this criterion is based solely on temperature
and not on whether water is present. Other than high latitude and
low temperatures, two other factors contribute to permafrost: the
presence of fossil permafrost from previous ice-age conditions and
the insulating effect of snow cover or vegetation that inhibits heat
loss.
Permafrost regions are divided into two general categories,
continuous and discontinuous, that merge along a general transition
zone. Continuous permafrost describes the region of the most
severe cold and is perennial, roughly poleward of the –7°C mean
annual temperature. Continuous permafrost affects all surfaces
except those beneath deep lakes or rivers. Continuous permafrost
may exceed 1000 m in depth averaging approximately 400 m.
17. Describe the active zone in permafrost regions, and
relate the degree of development to specific latitudes.
The active layer is the zone of seasonally frozen
ground that exists between the subsurface
permafrost layer and the ground surface. The
active layer is subjected to consistent daily and
seasonal freeze-thaw cycles. This cyclic melting
of the active layer affects as little as 10 cm depth
in the north and up to 2 meters in the southern
margins of the periglacial region, and 15 m in the
alpine permafrosts. (See Figure 17-19).
Figure 17.19: Periglacial Environments. Cross section of a periglacial
region in northern Canada, showing typical forms of permafrost, active
layer, and talik (discussed in detail in the next question).
18. What is a talik? Where might you expect to
find taliks and to what depth do they occur?
A talik (derived from a Russian word) is an unfrozen
portion of the ground that may occur above, below, or
within a body of discontinuous permafrost or beneath a
body of water in the continuous region. Taliks are
found beneath deep lakes and may extend to bedrock
and soil under large deep lakes (Figure 17-19). Taliks
form connections between the active layer and
groundwater, whereas in continuous permafrost
groundwater is essentially cut off from water at the
surface. In this way, permafrost disrupts aquifers and
causes water supply problems.
19. What is the difference between
permafrost and ground ice?
In regions of permafrost (permanent frost),
subsurface water that is frozen is termed ground
ice. The moisture content of areas with ground ice
may very from nearly absent in regions of drier
permafrost to almost 100% in saturated soils.
From the area of maximum energy loss, freezing
progresses through the ground along a freezing
front, or boundary between frozen and unfrozen
soil.
20. Describe the role of frost action in the formation
of various landform types in the periglacial region.
The 9% expansion of water as it freezes produces strong
mechanical forces that fracture rock and disrupt soil at and
below the surface. Frost-action shatters rock, producing angular
pieces that form a block field, accumulating as part of the arctic
and alpine periglacial landscape, particularly on mountain
summits and slopes.
If sufficient water undergoes the phase change to ice, the soil
and rocks embedded in the water are subjected to frost-heaving
(vertical movement) and frost-thrusting (horizontal motions).
Boulders and slabs of rock generally are thrust to the surface.
Soil horizons may appear disrupted as if stirred or churned by
frost action, a process termed cryoturbation. Frost action also
produces a contraction in soil and rock, opening up cracks for
ice wedges to form. Also, there is a tremendous increase in
pressure in the soil as ice expands, particularly if there are
multiple freezing fronts trapping unfrozen soil and water
between them.
21. Relate some of the specific problems humans
encounter in developing periglacial landscapes.
Human populations in areas that experience frozen
ground phenomena encounter various difficulties.
Because thawed ground in the active layer above the
permafrost zone frequently shifts in periglacial
environments, the maintenance of road beds and
railroad tracks is a particular problem. In addition, any
building placed directly on frozen ground will begin to
melt itself into the defrosting soil. Thus, the melting of
permafrost can create subsidence in structures and
complete failure of building integrity (See Figure 1724).
Figure 17.24: Permafrost melting and structure
collapse. Building failure due to improper
construction and the melting of permafrost.
22. What is paleoclimatology? Describe Earth's past climatic
patterns. Are we experiencing a normal climate pattern in this era,
or have scientists noticed any significant trends?
Paleoclimatology is the science of past climates. The
most recent episode of cold climatic conditions began
about 1.65 million years ago, launching the Pleistocene
epoch. At the height of the Pleistocene, ice sheets and
glaciers covered 30% of Earth's land area, amounting
to more than 45 million km2. The Pleistocene is
thought to have been one of the more prolonged cold
periods in Earth's history. At least 18 expansions of
ice occurred over Europe and North America, each
obliterating and confusing the evidence from the one
before.
23. Define an ice age.
The term ice age is applied to any such extended period
of cold, even though an ice age is not a single cold
spell. Instead, it is a period of generally cold climate,
called a glacial, interrupted by brief warm spells,
known as interglacials. There is a worldwide retreat of
alpine glaciers, higher snowlines in Greenland, and at
least three accelerating ice streams on the West
Antarctic ice sheet. The reduction in alpine glacial
mass balances is particularly true of low and middle
elevation glaciers. This trend in ice mass reduction
may be attributed to the present century-long increase
in mean global air temperatures. Additionally, over the
past ten years, we have experienced the eight warmest
years in instrumental history.
25. Describe the role of ice cores in deciphering
past climates. What record do they preserve?
Ice core analysis has opened the way for a new chronology
and understanding of glaciation. Chemical and physical
properties of the atmosphere and snow that accumulated each
year are frozen into place. Locked into the ice cores are the
air bubbles from past atmospheres, which indicate ancient
gas concentrations, such as greenhouse gases, carbon dioxide
and methane. For examples, during cold periods, high
concentrations of dust are present, brought by winds from
distant dry lands, acting as condensation nuclei. Past volcanic
eruptions are recorded in this manner. The presence of
ammonia indicates ancient forest fires at lower latitudes and
the ratio between stable forms of oxygen is measured with
each snowfall.
Movie: Glaciers
Many of the world’s most beautiful landscapes
were made by glaciers. This program shows how,
explaining glacial formation, structure,
movement, and methods of gouging and
accumulating earth. The program provides images
of glaciers and glacial landforms such as
moraines, and discusses how study of glaciers
may help us understand ice ages and the
greenhouse effect.
http://www.learner.org/resources/series78.html
End of Chapter 17
Geosystems 5e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen