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Basic study page for Glaciers and
Global warming's effect on glaciers



A Glacier Story

Over the past 60 to 100 years, glaciers
worldwide have tended toward retreat.
Alpine glaciers, which are typically
smaller and less stable to begin with,
seem particularly susceptible to glacial
retreat. Whether this is due to a
predictable climate trend or because of
increased human impacts on global
climate remains to be determined.

The Franz Josef Glacier in New Zealand
retreated rapidly over a 15 year period
during the 1950s and 1960s. What makes a
glacier? Where are glaciers found? How
does the climate affect glaciers? If you
want to know the answers to these
questions and more, the Glacier Story
will take you on a quick tour.

In regions where average temperatures hover
below zero degrees Celsius, glaciers
grow with each snowstorm. Compressed by
overlying snow, buried layers slowly
grow together to form a thickened mass
of ice. The pressure created from
overlying snow squashes snow grains
together. Individual grains eventually
metamorphose, growing to the size of
rock salt. If these enlarged crystals
survive one melt season, they are
considered firn. Looking down into a
crevasse, is just one way of glimpsing
the sheer immensity of a glacier -- most
glaciers have accumulated and compressed
so much snow that they are hundreds or
even thousands of feet thick.

Firn grains are generally four to 16 times
the size of the original snow crystal
and increase in size as the weight of
the overlying snow increases. As the
grains grow, they slowly snuff out
pockets of existing air between the
grains. Over time, individual firn
grains are pressed together to form
larger crystals, ultimately forming
slabs of glacier ice. When the mass
of compressed ice reaches a critical
thickness of about 18 meters,

it begins to deform and move. Its sheer girth, in
combination with the forces of gravity,
causes a glacier to slowly move, or
flow. Glacier ice flows down mountain
valleys, fans across plains, and spreads
into the sea. As a glacier moves over
the ground surface, friction causes the
underside of the glacier to move more
slowly while overlying glacier ice moves
unimpeded. Most of the world's glaciers
are found at the poles, but glaciers
exist on all of the world's continents,

even Africa. Glaciers require very
specific geographical and climatic
conditions. Most are found in regions of
high snowfall in winter and cool
temperatures in summer. These conditions
assure that the snow accumulating in the
winter remains throughout the summer.
Such conditions typically prevail in the
polar and high alpine regions.

The amount of precipitation (whether in the
form of snowfall, freezing rain,
avalanches, or wind-drifted snow) is
important to glacier survival. In areas
such as Siberia and parts of Antarctica,
where low temperatures meet glacier
growth requirements, the lack of
adequate precipitation prevents glacier
development. In areas of glacier growth,
upon reaching a critical mass, the slabs
of ice begin to flow and dramatically
impact the surrounding environment.

The great weight and slow movement causes
glaciers to reshape the underlying and
surrounding landscape. Acting as an
enormous push broom, the ice erodes the
land surface, carrying broken rocks and
soil debris far from their place of
origin. Glaciers slowly push earth and
rock forward as they advance and leave
these same materials behind in the form
of moraines and other glacial deposition
features as they retreat. Glacier
National Park, located in Montana,

U.S.A., shows a variety of glaciated
valleys. Part of this mountain has
literally been cut away by a glacier
that has long since retreated. In the
northern half of North America, glacial
remnants from the last ice age may be
reincarnated as vegetated hillsides.

Views from an airplane window over the
midwestern states and provinces reveal
lines of eskers and herds of drumlins
dotting the landscape. Throughout
advance and retreat, glacial debris
(till) is jostled in all directions.
Till is thrust forward with the glacier,
brushed aside as the glacier pushes past
less mobile objects, such as a
mountainside, or drawn along on the
glacier's journey. As large glaciers
retreat, the underlying ground surface
is typically scoured of most materials,

leaving only scars on the underlying
surface. Glacier retreat, melt, and
ablation, result from increasing
temperature, evaporation, and wind
scouring. Ablation is a natural and
seasonal part of glacier life. As long
as snow accumulation equals or is
greater than melt and ablation, glacier
health is maintained. Over the past 60
to 100 years, glaciers worldwide have
tended toward retreat. Alpine glaciers,

which are typically smaller and less
stable to begin with, seem particularly
susceptible to glacial retreat. Whether
this is due to a predictable climate
trend or because of increased human
impacts on global climate remains to be
determined.

Unattributed photographs at the World Data Center for Glaciology, Boulder

http://nsidc.org/glaciers/story/page1.html



Tierra Del Fuego Glaciers

Tierra Del Fuego is a 150-mile
long peninsula off the southern tip of
South America, separated from the
mainland by the Strait of Magellan. It
is divided into Chilean and Argentinean
halves, and its principle mountain
range, a west-east range known as the
Cordillera Darwin, stretches across both
halves. The Cordillera is the
continuation of the Patagonian Andes,

and rises along the southern shore. The
range holds beautiful ice-packed and
misty mountains, with large glaciers
pouring down into the sea. The range is
dominated by two high peaks, both
situated in the Chilean southwestern
portion of the peninsula, within the
boundaries of the Parque Nacional
Alberto de Agostini. Impressive peaks,

beautiful forests, lakes, glaciers, and
abundant bird life are highlights of the
park. The island is vulnerably situated
at the point where two of the world's
major oceans meet, and is consequently
stormy for much of the year. The city of
Ushuaia, Argentina is located on the
southern shore, and is the southernmost
city in the world. The two dominant
peaks of Tierra Del Fuego are Monte
Darwin (8,163 ft./2,488 m.) and Monte
Sarmiento (7,546 ft./2,300 m.). For a
list of the individual mountains in the
Tierra Del Fuego, click below.
http://www.peakware.com/encyclopedia/ranges/tierradelfuego.htm


Glaciers: What Types of Glaciers are
There?

Ice Sheets Found only in Antarctica and
Greenland, ice sheets are enormous
masses of glacial ice and snow that
cover over 50,000 square kilometers. The
ice sheet on Antarctica is over 4200
meters thick in some areas, covering
nearly all of the land features except
the Transantarctic Mountains that poke
up above the ice. Ice Shelves Ice
shelves occur when ice sheets extend
over the sea, floating on the water. In
thickness they range from a few hundred
meters to over 1000 meters. Ice shelves
surround the entire continent of
Antarctica. The largest shelf is the
Ross Ice Shelf , covering over 500,000
square kilometers.

Ice Caps Ice caps are miniature ice
sheets, covering less than 50,000 square
kilometers. They form primarily in polar
and sub-polar regions, occupying high
and relatively flat regions. Mountain
Glaciers These glaciers develop in high
mountainous regions, often flowing out
of icefields that span several peaks or
even a mountain range. The largest
mountain glaciers are found in Arctic
Canada, Alaska, the Andes in South
America, the Himalayas in Asia, and on
Antarctica.

The Chickamin Glacier in British
Columbia, Canada, is a typical mountain
glacier. Several glaciers flow into it,
and the landscape is nearly covered with
ice and snow. Valley Glaciers Commonly originating
from mountain glaciers or ice fields, these
glaciers spill down valleys, looking
much like giant tongues. Valley glaciers
tend to be very long, often flowing down
beyond the snow line, sometimes reaching
sea level. Piedmont Glaciers Piedmont
glaciers occur when steep valley
glaciers spill into relatively flat
plains, where they spread out into
bulb-like lobes. The Malaspina Glacier
in Alaska, covering over 5,000 square
kilometers is one of the most famous
examples of this type of glacier. Cirque
Glaciers Cirque Glaciers are named for
the bowl-like hollows they occupy, which
are called cirques. Typically, they are
found high on mountainsides and tend to
be wide rather than long.

Hanging Glaciers Also called
ice aprons, these glaciers cling to
steep mountainsides. Like cirque
glaciers, they are wider than they are
long. Hanging glaciers are common in the
Alps, where they often cause avalanches
due to the steep inclines they occupy.
Tidewater Glaciers As the name implies,
these are valley glaciers that flow far
enough to reach out into the sea.
Tidewater glaciers are responsible for
calving numerous small icebergs, while
not as imposing as Antarctic icebergs,
can still pose problems for shipping
lanes.

Lamplugh Glacier, in Glacier Bay,
Alaska, shows the snout of a typical
tidewater glacier. Because the surface
of the ice is heavily crevassed and
jagged, small bits of ice have broken
off, are seen floating in the water.

http://www.digistar.mb.ca/minsci/geology/types.htm


How Glaciers Form and Flow

Glaciers are more-or-less permanent
bodies of ice and compacted snow that have become
deep enough and heavy enough to flow under
their own weight. Today, glaciers are
found in mountainous regions or in the
very cold areas around the poles, and
cover only about 10% of the Earth's
surface. During past glacial periods
this area increased considerably so,

although active glaciation is very
limited in Europe now, and non-existent
in the UK, much of the landscape of
northern Europe shows evidence of past
glacial activity. How glaciers form and
flow Glaciers develop where the
temperatures are cold enough to allow
snow to accumulate over a period of
years. Favorable conditions are found
around the poles and at high altitudes
in lower latitudes, i.e. mountainous
regions such Northern Scandanavia and
the Alps.

Enough snow must fall each
winter to ensure that it doesn't all
melt in the summer. This way, the amount
of snow lying on the ground gets deeper
each year as new snow is added to the
remains of last years fall. On north
facing slopes it may survive all year
without melting, whereas all the snow
may melt on south facing slopes ( in the
northern hemisphere ). This is because
north facing slopes get much less direct
sunshine than south facing ones, and
thus remain cooler. North facing corries
(French Alps), showing small snow
patches remaining in mid summer Fresh
falling snow is made up of microscopic
ice crystals and has a very delicate
structure, nothing like solid ice found
in glaciers, but after it lands it goes
through a series of alterations to
become hard ice. In fact, very cold,

and thus dry, snow will hardly stick
together at all under normal conditions
and would never be any use as ice unless
it was changed. As snow accumulates its
structure changes. Newly fallen snow is
very light and porous,with a density
from 50 to 300 kg/m, but as the snow
becomes buried by subsequent snowfalls
the ice crystals that make up the
snowflakes partially melt and sublimate,
particularly at the delicate points of
the flakes. The vapour then condenses,
and over time the flakes change into
granular ice crystals called firn, which
has a density greater than 500 kg/m3.

As more time passes the firn becomes buried
even deeper under more fresh snow and
firn, and the weight above it compresses
it into solid ice with a density of
approximately 900 kg/m3. This is the
typical 'blue' ice of glaciers. The time
needed to change snow in to glacial ice
in this way depends on several factors
such as how warm or cold it is and how
much new snow is added each year.

In fact, it can take anywhere from 5 years
in an ideal site, to over 3000 years.
The solid ice mass that is produced,
unless it is on a very steep slope, wont
start to move until its thickness
approaches approximately 50 metres. This
is because it is only when the ice is
about 50m thick, that the pressure is
enough to make the lowest, or basal, ice
undergo plastic deformation, ie: it
begins to flow like a soft or liquid
plastic. Side view near the snout of an
Alpine glacier, with a 30m wall of ice
rising above the lateral moriane.

The ice flows very slowly, moving away from
the place where the pressure is
greatest. This will be the place where
the ice is thickest. The ice will even
flow up hill if the pressure is great
enough. In many regions the areas of
accumulation of snow and ice are at high
altitudes so the glaciers tend to flow
down hill under the influence of
gravity. The speed of the flow varies
considerably, and in some cases,

especially at high latitudes where it
can be very cold, the glacier may be
frozen to the bed rock so that it flows
only by deformation under its own
weight. It is as if the ice bends and
slowly flows away like a block of
toffee. Glaciers that are frozen to the
underlying rock surface are called "cold
glaciers". In a cold glacier the ice at
the bottom of the glacier doesn't move
at all, but the speed increases towards
the surface of the glacier as the ice
deforms. The fastest moving ice is that
above about 50 metres from the surface
of the glacier. The area of deformation
in a glacier is also known as the
"region of shear" because layers of ice
are sliding over each other. In less
cold regions the basal ice of the
glacier may melt due to the pressure of
the ice and firn above it.

This allows the glacier to slide over the underlying
land. These glaciers are not frozen to
their bed and are called "warm
glaciers". In warm glaciers the maximum
velocity at the surface is greater than
in cold glaciers because it is the sum
of the velocity at which the glacier is
sliding over the bed and the velocity
due to internal deformation. The
velocity of ice flow in all glaciers
varies considerably, depending on
factors like the thickness of the ice,

the slope of the surface over which the
glacier is advancing, and the amount of
meltwater that is available to lubricate
the base of the glacier. Some glaciers
creep so slowly that their movement can
only be seen over a period of years;
others advance at several meters per
day. When the ice becomes thick enough
to flow, the glacier will begin to move
away from its source. It will keep
advancing as long as there is enough ice
in the source area to keep supplying it.

Whether it will advance along the
valley, remain stationary or retreat
again depends on the balance between the
amount of new ice collected in the
source area and the rate of melting
(called ablation) in the rest of the
glacier. The accumulation of ice is
controlled by the annual snowfall and
melting rates. The melting rate is
mainly influenced by the temperatures
and the amount of debris covering the
ice surface. The ablation, or wastage,
of a glacier takes place due to several
processes with include sublimation,

melting and, where the glacier ends in a
water body, by calving of icebergs from
the glacier snout. Melting is the most
important process in almost all
glaciers. The boundary between the zones
of accumulation and ablation is roughly
the same as the limit of year round
snow. In other words, the point up the
mountain where it becomes cold enough
for snow not to melt during the summer.

This is called the Snow Line. If the
rate of snow accumulation in the source
area is greater than the rate of wastage
then the glacier will advance, but if
the rate of accumulation is less than
the rate of wastage the glacier will
retreat. When accumulation and ablation
are equal, they balance each other, and
the snout of the glacier look as if it
isn't moving. In fact, ice is always
flowing towards the snout of the glacier
whether it is advancing, retreating or
seeming to remain stationary. Ice can
deform under pressure, but most of the
ice isn't all that good at it. Unless
the conditions are just right, it's more
likely to crack than deform.

This is especially so where the ice encounters a
sudden increase in gradient and is
stretched faster than deformation can
accommodate. This results in fractures
developing in the ice, often narrowing
at depth and widest at the surface
because the ice is being stretched with
a point of rotation at or near the
glacier base. These cracks, which
generally run at 90° to the glacier are
called crevasses. Through crevasses
meltwater and rock debris can be
transported to the base of the glacier
where they aid in the erosion of the bed
rock. Crevasse with rock debris at its
lip Crevasses are often the most obvious
features on the ice surface, appearing
either on their own or in groups. They
present a danger to climbers and others
who have to cross the ice since they are
not only deep and smooth sided, they can
be covered over by fresh snow that gives
way under foot.

http://www.zephryus.demon.co.uk/geography/resources/glaciers/origin.html




GLACIER BAY PAGE
http://sdcd.gsfc.nasa.gov/GLACIER.BAY/hall.science.txt.html
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