| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Glaciers |
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- develops anywhere (on land) where more snow accumulates than melts or lost by evaporation -
glaciers are usually produced by extended cold periods -
the last ice age (the Pleistocene) has left much of its evidence on the
worlds landscape in the form of erosional
and depositional landforms Formation and Growth of Glaciers -
from snow flakes to firn to glacial ice -
glacial ice form only below accumulations of >50 m of ice Classification
of Glaciers (based on size) >50,00
km2 [Continental]
-
ice sheet - largest of continental glaciers with areal coverage of
> 1 million km2 and thicknesses of > 1 km -
totally buries the landscape (eg. in Greenland - covers 80% of Island and
in Antarctic - covers greater than 95% of Island) -
Ice caps - a large mass of ice that totally covers the high areas
of a landscape -
Ice fields - a large mass of ice that covers portions of the high
areas of the landscape <50,00
km2 (subdivided
based on landscape position) -
Outlet glaciers are glaciers fed by ice sheets and ice caps -
they extend from their margins and flow outwards - may be confined to a
valley -
Ice shelves - are outlets of ice sheets at sea -
Alpine (true valley) glaciers - glaciers that are confined to
valleys -
streams of ice may originate in the snow fields of the higher elevations
(their widths are much smaller than their lengths) -
Piedmont glaciers - glaciers that occupy broad low lands at the
bases of steep mountain and form when one or more alpine glaciers emerge
from the confining walls of a valley Glaciers and the Hydrologic Cycle -
24 million km3or 2% of the earth's water is stored as glacial
ice -
approximately 85% of the volume of fresh water is stored in glaciers and
ice sheets -
the residence time or the rate of exchange of water between the glaciers
and its ocean source is > 8,000 years distribution
- glaciers are not evenly distributed through out the world - 85% -
Antarctica - 10% - Greenland - 5% - rest of the world Movement of Glacial Ice Plastic
deformation (flow) 1)
- flows as a result of pressure melting and recrystallization and/or 2) -
through solid-state recrystallization or inter- and intra-granular
movement of ice *
pressure on ice at depth in a glacier causes it to move by plastic flow Basal
slip -
the entire glacier move by sliding over the bedrock -
aided by melt water that 1) reduces friction for sliding - 2) provides
pore water pressure to lift the ice sources
of heat to produce melt waters 1)
heat from refreezing of meltwater (latent heat of fusion) 2)
frictional heat (from plastic flow and bedrock contact) 3)
geothermal heat from the underlying bedrock (conducted upwards) 4)
presence of underlying hot springs and near surface magma bodies -
flow within the upper 50 m is passive (carried "piggyback" style
on the ice thicker than 50 m) -
within the upper 50 m, crevasses are formed if the ice moves over
irregular terrain. note
that crevasses in glaciers do not extend down below 50 m into the zone of
plastic flow Rates
of glacial movement -
variable from glacier to glacier and within glaciers Conditions
that affect the rate of glacial movement - ice thickness, ice temperature
and slope of topography -
for glaciers with similar temperature and on similar topographic slopes,
thicker glaciers move faster -
for glaciers with similar thickness and on similar topographic slopes,
warmer glaciers move faster -
for glaciers with similar thickness and temperatures, glaciers on steeper
slopes move faster movement
within glaciers - variation in flow results from friction between the
glacier, the valley walls and valley floors -
experimental evidence shows that the fastest
velocities are at the top center of the glacier movement
between glaciers - some glaciers experience periods of extremely rapid
movement called surges -
during a surge, the glacier is believed to move at flow rates of 100 times
normal causes
of surges 1) increase in basal sliding due to melting and increased water
pressure beneath the ice - 2) increase in slope (assisted by the pull of
gravity) Glacial Budget -
glaciers are dynamic - thus respond to changes in input to and output from
the glacier -
the amount of ice within a glacier is controlled by accumulation
(precipitation) and ablation (melting and evaporation) and to a limited
extent by transportation from the zone of accumulation to that of ablation Zone
of accumulation -
formation of glacial ice (snow and rain) -
glacier thickens because precipitation is grater than wastage - there is a
net gain in the volume of ice Zone
of ablation -
destruction of glacier ice (melting and calving) -
glacier thins because wastage is greater than accumulation - there is a
net loss in the volume of the ice Equilibrium
line (snow line) - a
point along the profile of the glacier where accumulation and ablation are
equal - there is no net gain or loss in ice volume -
distribution of the snow line - this controlled by elevation (expression
of temperature) -
the snow line is at sealevel in the poles and at an elevation of ~4500 m
at the tropics Behavior
of a glacier (ablation vs accumulation) 1-
if accumulation exceeds ablation - positive budget - glacier advances 2-
if accumulation is less than ablation - negative budget - glacier retreats
(recession) 3-
if accumulation equals ablation - balanced budget - glacier is stationary
(stagnant) -
note that irrespective of the glacial budget condition, glacial ice
continues to move forward even though the glacier as a whole may be
stagnant or retreating Flow
of glaciers based on the budget -
downwards in the zone of accumulation - laterally at the equilibrium line
- upwards in the zone of ablation Work Done by Glaciers -
modify the landscape extensively - produces landscapes that are unique to
glaciation I
- GLACIAL EROSION 1-
plucking - breaking, lifting and removal of fragments of bedrock and
sediments by ice (frost action) 2
- abrasion - the scouring, smoothing or polishing action of materials
embedded in the ice against the bedrock floor (sandpaper action) Controls
of glacial erosion 1-
the rate of glacial movement 2-
the thickness of the ice 3-
the shape, size, abundance and hardness of the rock materials in the ice 4-
the erodibility of the surface beneath the ice Erosional
features - Valley or Alpine Glaciers 1-
"U"-shaped glacial troughs as contrasted to
"v"-shaped stream valleys (glaciers widens, deepens and
straightens stream valleys) -
type of valleys - main glacial trough and hanging valleys 2-
Truncated spur - irregular shaped cliff caused by removal of
meanders 3- Cirques - a steep-sided, rounded bowl-shaped depression carved into a mountain wall at the head of a valley glacier 4-
Arete - a Knife-edge peak formed by sideways erosion of two cirque
glaciers 5-
Horn - a pyramid-like peak at summits formed by headwards erosion
of several coalescing cirques 6-
Cols - passage or gap formed by back-to-back erosion of cirques After
deglaciation 1-
Tarn - a lake occupying a cirque basin 2- Pater noster lakes - lakes occupying a series of bedrock depressions within a glacial trough and connected to each other by streams 3-
Water falls - falls formed by draining hanging valleys into the main
valley 4-
fiord - a glacial trough drowned by the sea or ocean Erosional
features - Continental glaciation 1-
scouring action produces glacial flour 2-
abrasion by particles in the bottom of the ice produces scratches and
grooves in the bedrock 3-
plucking action produces streamlined knobs projecting from the land
surface called roches moutonnees Importance
of erosional feature 1-
used to determine the type and extent of glaciation 2-
used to determine the direction of glacial advance -(scratches and grooves
the possible linear direction of advance and roches moutonnees the true
direction of ice advance) Glacial Transportation method
of transportation 1-
at the bottom of the glacier or on the bedrock surface 2-
within the glacier 3-
by the sides of the glacier next to valley walls 4-
in regions within the glacier from the coalescing of materials two
glaciers 5-
on top of the glacier from material input by mass wasting types
and sizes of material transported - variable types of material and sizes
depending on the nature of the bedrock and the "process"
supplying the material (types range from the whole spectrum of rock types
and size ranges from clay-size (rock flour) to very large boulder size or
greater) Glacial Deposition -
glacier ice is a much more powerful erosive and transportation agent than
water or wind because its greater viscosity enables it to carry much
larger particles -
the material deposited by glaciers are called drift (drift
is a general term for all types of glacial deposits) Unstratified
drift -
unstratified drift deposited directly by a glacier is called till characteristics
-
poorly sorted (boulder - clay size ), generally lacks stratification and
contains angular particles -
the sizes of particles in till and their mineralogical composition reflect
the type of material over which the glacier flowed -
till commonly contains large rock fragments called erratics that have been
moved far from their source and deposited on rock of a different
composition Landforms
of till: 1-
moraines - the general term used to describe a wide variety of
landforms that are deposited at the edges of glaciers (bottom, sides,
middle and end) -
lateral moraine - deposits of till along the sides of the valley
walls (common in valley glaciers) -
medial moraine - deposits of till within the valley from the
merging of two or more lateral moraines (common in valley glaciers) - the existence and character of the medial moraines are proof that merging glaciers move side by side as separate ice streams for considerable distance without mixing -
ground moraine - smooth-to-gently undulating deposits on the
bedrock beneath a glacier (all types) -
end moraine - ridge-like deposit of till at the terminus of a
glacier (all types) -
the end moraine that forms at the furthest point to which the glacier
advances is called a terminal moraine - the terminus of a glacier may melt back, reach equilibrium, and deposit another moraine - an end moraine that is deposited by a glacier back from its terminal moraine is called a recessional moraine ** A glacier has only one terminal moraine but may have several recessional moraines 2-
Drumlins - streamlined hills molded by glaciers from till deposits
(usually ground moraines) characteristics
-
some are composed of entirely of till while others are made up of till
plastered to bedrock cores -
drumlins have a "tear-drop" shape in areal view and are
elongated parallel to the direction of glacial movement -
in profile, they have a steeper slope facing the direction from which the
ice advanced (thus can be used to indicate
the direction of ice movement) -
generally 1 to 2 km long, 400 to 600 m wide and may occur singly or
in large numbers (drumlin fields eg. in NY State) -
Stratified
drift
- stratified drift is deposited by running water from melting ice (glacial
melt waters) -
the melt water is an effective sorting agent depositing large particles
near the edge of the glacier and finer particles (gravel-sand-silt-clay)
further from the glacier -
the result of melt water activity is a sheet of stratified drift called an
outwash that extends out from the glacier Landforms
of Stratified drift: 1- Outwash plain - broad extensive plains underlain by outwash from continental glaciers Valley
train - narrow and long outwash deposits from valley glaciers 2-
Kames - mound-like hill of stratified drift -
formed by the deposition of sediments in openings by melt water in
crevasses and other openings in or on the ice - when the ice melts the stratified drift is left in the form of an isolated or semi-isolated mounds Kame
terraces are flat-topped deposits of stratified drift along the valley
walls -
formed by melting of stratified drift between the glacier and the valley
walls 3-
Eskers - a long, narrow, and often sinuous ridges of stratified
drift - formed by glacial melt streams flowing in tunnels beneath a stagnant glacier 4-
Kettle hole - a depression formed by the subsidence of the surface
resulting from the melting of buried ice - if the basin is later filled
with water, it is called a kettle lake 5-
Proglacial lakes - lakes fed by melt water that accumulates in low
areas on outwash plains or those formed by the damming of valleys by
glacial deposits -
range from small kettle lake to lakes several 1000 km2 in area Glacial Theory -
the earliest explanation of the origins of glacial deposits was attributed
to the great flood of the Bible - Ignatz Venetz (1821), Louis Agassiz
(1837) and others can be credited with postulation of the theory of
glaciation -
the bases for the theory of widespread glaciation was through the
application of the principle of uniformitarianism (examination of both
drift deposits and similar material deposited today) Ice
ages
- the earth has experienced several episodes of widespread continental
glaciation -
the latest and best understood is the Pleistocene historically referred to
as the Great Ice Age -
the great ice age began approximately 1.6 million years ago signalling the
beginning of the interval of geologic time known as the Pleistocene Epoch -
Pleistocene glaciers covered about 40 million km2 or about 30%
of the earth's land surface - mostly in the northern hemisphere -
this period is marked by four glacial stages (Nebraskan, Kansan, Illinoian
and Wisconsinan) and interglacial stages (Aftonian, Yarmouth, Sangamon and
Recent) as recognized in North America Indirect
effects of continental glaciation - (advances and retreats) 1-
forced migrations 2-
changes in river drainage routes 3-
crustal depression and rebound in the centers of the ice accumulation 4-
raised shore lines and shoreline terraces 5-
rise and fall in sealevel 6-
formation of fiords -
beyond the margins of the glaciers in arid and semi-arid regions
temperature were lower and precipitation was moderate -
this resulted in the formation of pluvial lakes (with Lake Bonnevile as
the largest) -
In north America, the greatest concentration of pluvial lake occurred in
the vast Basin and Range region of Nevada and Utah Occurrence
and causes of worldwide glaciation -
the occurrence of worldwide glaciation can be explained by assembling the
continent together in the south or north pole by plate tectonics The
onset of glaciation can be explained by the astronomical theory proposed
by Milankovitch -
precession - (21,000 yrs) caused by wobbling of the earth's axis -
Obliquity - (41,000 yrs) changes in the angle that the earth axis
of rotation makes with the plane of the earth's orbit -
variation in the shape of the earth's orbit - (100,000 yrs) - changing the
earth to sun distance Other
minor triggers of glaciation -
plate tectonics - assemblage of the continents in cold regions such as the
north and south pole for extensive periods of time -
volcanic activity particulate discharge into the atmosphere thereby
reducing global temperature and triggering glaciation
Chapter Review -
definition of glaciers - conversion of snow to glacial ice -
classification and types of glaciers -
importance of glaciers in the hydrologic cycle - implications for melting
glacial ice -
movement (plastic flow and basal slip) - sources for heat for meltwater -
method of movement within the brittle zone - rate of glacial movements -
glacial budgets and terminology associated with various zones as well as
the behavior of glaciers with changing budget -
glacial erosion deposition and associated landforms -
use of glacial features that indicate direction of glacial advance (roche
moutonnee, striations, drumlins, eskers etc.). -
glacial theory and ice age. - indirect effects of glaciation and causes of
glaciation Terms that you are
responsible for. (IF WE DO NOT COVER
THEM IN LECTURE YOU
Glacier
Ice Sheet Ice
Caps
Ice Fields Ice
Shelves
Alpine Glaciers Piedmont
Glaciers
Zone Of Accumulation Zone
Of Ablation
Equilibrium Line Truncated
Spur
"U"-Shaped Glacial Troughs Cirques
Arete Horn
Cols Tarn
Pater Noster Lakes Fiord
Roches Moutonnees Drift
Unstratified Drift Till
Moraine Lateral
Moraine
Medial Moraine Ground
Moraine
End Moraine Terminal
Moraine
Drumlins Stratified
Drift
Outwash Plain Kames
Eskers Kettle
Hole
Kettle Lake Proglacial
Lakes
Ice Ages Milankovitch
Precession Obliquity |
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