THE WORK OF MOVING ICE :
SOURCE : NIOS & NCERT
SNOW – FIELDS :
• Wherever the rate of snow melting or its
evaporation is lower than the rate of snowfall in a year, snow accumulates into
great mass of ice.
• Permanently snow covered regions of this type are
called snow fields.
• Snow fields occur in polar regions and on high
mountainous areas.
• Snowfields are always found above the snow line.
• Snow line is an imaginary line above which there
is permanent snow.
• The height of the snow line is not uniform and
is affected by latitude, amount of snowfall, direction of winds and slope.
GLACIER :
• A moving mass of ice and snow is called a glacier.
• Its velocity is very low and it moves from a few
centimetres to a few metres in a day.
Types of Glaciers :
(i) continental glaciers and
(ii) valley glaciers.
(i) Continental Glaciers:
• A thick ice sheet covering vast area of land is
called a continental glacier.
• Glaciers of this type build up at the centre and
move outward in all directions.
• Continental glaciers of today are found mainly in
Antarctica and Greenland.
• The precipitation in these regions occurs in the
form of snow.
(ii) Valley Glaciers :
• When a mass of ice from the high mountainous
regions starts moving down into the pre-existing valleys, it is called a valley
glacier or a mountain glacier.
• The shape of the valley glaciers depends on the
valley it occupies.
• Where the valley is broad, the glacier spreads
outwards and where the valley is narrow, the glacier contracts.
• The longest glacier in India is the Siachen
Glacier in Karakoram range which is 72 kilometres long.
• Gangotri Glacier in Uttarakhand is 25.5 kilometres
long.
• The two important rivers of India, the Ganga and
Yamuna, originate from Gangotri and Yamunotri glaciers respectively.
LANDFORMS PRODUCED BY GLACIER :
(A) Erosional work of glacier :
• As a glacier moves over the land, it drags rock
fragments, gravel and sand along with it.
• These rock fragments become efficient erosive
tools. With their help glacier scrapes and scours the surface rocks with which
it comes in contact.
• This action of glacier leaves behind scratches and
grooves on rocks.
The landforms created by glacial erosion are:
I. Cirque (or Corrie), (firn):
• Snow collects at the upper end in a bowl shaped
depression, is called cirque.
• Layers of snow in the process of compaction and
recrystallization are called firn.
• Sometimes the deepest parts of these hollows are
occupied by accumulated water, to form Corrie Lake (or Tarn).
II. ‘U’ shaped Valley:
• The glacier does not carve a new valley like a
river but deepens and widens a pre-existing valley by
smoothening away the irregularities.
• In this process the glacier broadens the sides of
the valley.
• The shape of the valley formed in this manner
resembles the letter ‘U’.
• Such a valley is relatively straight, has a flat
floor and nearly vertical sides.
III. Hanging Valley :
• Just like tributary streams of river, there are
tributary glaciers also which join the main glacier after moving over their
mountainous path.
• These tributary glaciers like the main glaciers
carve U shaped valleys.
• However, they have less volume of ice than the
main glaciers and thus their rate of erosion is less rapid.
• As a result their valleys are smaller and not as deep as that of the main glacier.
• Due to this difference in deepening; the valley of
the tributary glacier is left at a higher level than that of the main glacier.
• The valley of the tributary glacier just looks
like hanging downwards at the point of its confluence with the main valley.
• This type of a topographical feature is called a
hanging valley.
• This feature is visible when ice has melted in
both the valleys.
• When the ice in the hanging valley melts, a
waterfall is formed at the point of confluence of this stream with the main
river.
Iv. Eskers :
• Glaciers can also contain sinuous flows of
meltwater that occur in ice tunnels at the base
of the ice.
• The beds of these subsurface glacial streams are
composed of layers of sand and gravel.
• When the ice melts from around the meltwater
tunnels, the beds of sand and gravel are deposited on the Earth's surface as
long twisting ridges known as
eskers.
V. Drumlins :
• feature of continental glaciation are hill shaped
deposits of till known as drumlins .
• A couple theories exist to explain their
formation.
• The most excepted theory suggests they form when
saturated ground sediments oozes up into hollows at the base of an advancing
glacier.
• The sediment is then stretched out and moulded
into a streamline form as the ice moves forward.
VI Kettle hole :
• When glaciers are rapidly retreating, numerous
blocks of ice can become detached from the main body of the glacier.
• If glacial drift is then placed around the ice, a
depression on the surface called a kettle hole can be created when the ice
melts .
• Kettle holes are commonly found on moraine and
outwash plain deposits.
VII.Outwash plain :
• Outwash deposits are formed when sand is eroded,
transported, and deposited by meltwater streams from the glacier's snout and
nearby till deposits to areas in front of the glacier.
• Outwash plain develops when there are a great
number of meltwater streams depositing material ahead of the glacier.
VIII.Kame :
• Glaciofluvial deposits are also associated with
the melting of stagnant ice at the front of the glacier.
• Where sediment rich water flows into a crevasse or
depression in the ice, a conical shaped pile of sand and gravel, known as a
kame .
(B) Transportation work of Glacier :
• Although the glacier moves very slowly, it drags
with it large boulders and rock fragments.
• Glacier gets this material from the mountain
slopes, valley sides, valley bottom and from air.
• This material is called the load of glacier.
(C) Depositional work of Glacier :
• When the glacier melts or retreats, it deposits
its load in different parts.
• The debris thus deposited are called moraines.
Depending upon their location in the valley.
Types of moraines:
(i) Terminal Moraine :
• When the glacier melts, the debris are deposited
at the end of the valley glacier in the form of a ridge.
• Morainic material ranges from fine clay to large
angular boulders.
(ii) Lateral moraine:
• The moraine which is deposited on either side of a
glacier is called lateral moraine.
(iii) Medial moraine:
• When two glaciers join each other their lateral
moraines also join.
• Moraines thus formed on the confluence of two
glaciers are called medial moraines.
(iv) Ground moraine:
• It consists of deposits left behind in areas once
covered by glaciers.
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