Transcript St Andrews Scot’s School Geography Department

•St.Andrews Scots
School
Geography
Department
Presents :
IGCSE Geography
Core Theme 2. The Natural Environment
Glacial Processes
The syllabus states that the student should be able to:
Demonstrate an understanding of the glacial processes in eroding, transporting and
deposition.
Refernce should be made to the erosional processes of plucking and abrasion,
assisted by freeze-thaw weathering.
Transport of rock materials (moraine) as sub-glacial moraine (ground moraine),
englacial morraine, lateral moraine and medial moraine should be understood.
Describe and explain the landforms associated with the processes.
A study should be made of the following glacial landforms:
Glacial troughs (U-shaped valleys with truncated spurs)
Corries (cirques)
Aretes
Pyramidal peaks
Hanging valleys
Rock basins
Roche moutonnees
Landforms resulting from depostion of moraine
terminal moraine, boulder clay and drumlins
So what is a glacier?
In cold climates and at high altitude most precipitation
is in the form of snow. Snow can collect in hollows on the
side of mountains. With more prcipitation the weight
increases and compresses the snow turning it into FIRN.
This is snow which has survived one winter’s freezing and
survived a summer’s melting. Once ice has formed, it may
begin to flow downhill (due to gravity) as a glacier.
They are like rivers of ice, flowing through valleys once
occupied by running water.
The Glacial system
Glaciers behave like a system, with inputs, stores,
transfers and outputs.
Snow from avalanches and precipitation provides
the inputs into the system. This occurs at the start
(head) of the glacier in the zone of accumulation.
As the snow accumulates and turns to ice it can be
stored within the glacier. Under the influence of
gravity the glacier can begin to move downhill,
therefore the ice is transfered.
At the end of the glacier, ice can melt creating
meltwater streams. Some will also be evaporated.
The melting of ice at the end (snout) of the glacier
is known as ablation and occurs in the ablation zone
The Glacial system
Source: ‘Integrated Approach’ by D.Waugh
A more simplistic
representation of
the glacial system
Different types of glacier
There are several different types of glacier which are classified
according to their shape, size and location.
A niche glacier
These are very
small areas of ice
which occupy
shallow hollows
and gulleys.
Corrie or Cirque glaciers
Although larger than niche glaciers, these
are relatively small ice masses which occupy
armchair-shaped hollows on mountain sides.
They often spillover from their hollows to
feed valley glaciers.
Valley glaciers
These are large masses of ice which flow down hill
from either an ice fiedls or a corrie glacier. They
usually follow river valleys but due to their
erosive power they alter the original course.
Piedmont glaciers
These are formed when several valley
glaciers spill onto lowland areas and then
spreadout, often merging together.
Ice caps
These are huge areas of ice that spread out from
central domes. Apart from the exposed tops of high
mountains (Nunataks) the whole area is buried deep
in ice. The largest ice sheet is found in Antarctica,
followed by Greenland and then in South America.
How does a glacier erode the landscape?
The speed at which a glacier erodes depends on its size and the amount of material
it can carry. The source of the rock material is from frost shattering (freeze-thaw
weathering). The glacier uses this material to erode the valley bottom and sides.
A glacier can erode the landscape by the following processes:
ABRASION
PLUCKING
Abrasion:
Material supplied by frost shattering and then picked up by the glacier is used to
scrape against the side and floor of the valley. The more material it has the greater
the rate and strength of abrasion. The glacier it like a giant piece of sandpaper
rubbing along the valley.
Plucking:
Glaciers will freeze to rock surfaces when under extreme pressure. When the
glacier starts to move it rips large pieces of rock away from the underlying surface.
These rock fragments, now frozen into the ice, can be used by the glacier to abrade
the valley.
Erosional landforms
1. Corries (or Cirques)
These are deep, steep-sided hollows formed on the
side of a mountain. They form when snow starts
to collect in small depressions and hollows on the
mountain. With more snowfall the ice is
compressed and it deepens. It eventually starts
to move downhill when the corrie glacier starts to
flow out of the hollow. As the corrie glacier
starts to move it erodes the hollow even further.
A steep back wall is created by plucking and the
hollow is deepened by abrasion. The result is a
corrie, an armchair shaped hollow. Many corries
are filled with a corrie lake (or tarn) the result of
trapped glacial water.
Large armchair
shaped hollow
Loose scree from
frost shattering
Steep back wall with
jagged rock, the
result of plucking and
frost shattering
Corrie lake (or
tarn)
2. Aretes
When two corries form back-to-back and
begin to erode towards each other, the land
inbetween them gets narrower. This knifeedge ridge is known as an arete. The
following photos show some very clear
examples.
3. Pyramidal peaks
When three or more corries erode back
into the same mountain, a pyramidal peak
results. Aretes will radiate out from the
peak .
A very famous pyramidal peak is the
Matterhorn in Switzerland.
4. Glacial troughs (or U-shaped valleys)
As glaciers move downhill they will follow the easiest
possible route which is usually an exisiting river valley.
Glaciers usually fill the whole valley (unlike rivers) and
therefore they have much more power to erode than a
river. Instead of winding around the valley floor and
the interlocking spurs, the glacier will take a direct
route, eroding the interlocking spurs as it moves.
The result is a widened, deepened and straightened
valley. The shape of the valley goes from being ‘V’
shaped to ‘U’ shaped. This is often known as a glacial
trough. The interlocking spurs have been erode to
leave truncated spurs.
The glacier would
have occupied this
river valley and
through the
processes of
abrasion and
plucking, it has
eroded it to form a
U-shaped glacial
trough.
Photos to show glacial troughs
5. Hanging valleys
Before the appearance of glaciers, tributary
rivers would have joined the main valley river at
the same height. During the ice age, the glacier in
the tributary valley would have been much smaller
than the one occupying the main valley, and so it
would not have as much energy to erode
downwards. When the ice melted and the glaciers
disappeared, the tributary valley would be left
‘hanging’ above the main valley. The only way for
the water in the tributary river to know join the
main valley river is by dropping down a very steep
drop, often as a waterfall.
Yosemite National
Park, USA
U-shaped
valley
Hanging valley
Waterfall connecting the
hanging valley to the main
valley below
This picture shows not only a perfect hanging valley,
complete with waterfall but a great example of a
glaciated trough (or U-shaped valley).
6. Ribbon
7.
Roche Moutonnees
lakes
This
occurs
when
the
glacierinmeets
a more
resistant
Whenfeature
a glacier
moves
along
itsadvancing
valley, changes
the rate
of erosion
mayrock
than
that
of the surrounding
valley.
lead to
increased
deepening of
sections of the valley floor. Areas of softer
As the
glacier
passes overincreased
the moredeepening.
resistant rock
it’sthe
base
actually
starts to
rock
may
also experience
When
glacier
retreats,
melt.
The material
it is fill
carrying
polishes
the
rock
surfaces
through
action
the deepened
sections
with melt
water
and
become
lakes.
These the
lakes
of
abrasion,
leaving
a gently has
sloping
and supplied
smooth slope.
remain
long after
glaciation
ended,
by rainfall and subsequent
streams
and rivers.
The
English
Lake
district
its character
theseThis
The downward
facing
part
of the
rock
is muchowes
steeper
and moreto
jagged.
narrow
ribbon
along
its valley
is because
the lakes
glacier
refreezes
on floors.
this surface (due to a decrease in
pressure) and plucking takes place.
Glacial deposition
Moraine is the name given to material which is carried by a glacier and later
deposited. It can refer to large boulders or very fine sediment.
There are several different types of moraine:
1.
Lateral moraine
This is material which is carried along the sides of
the glacier. The material comes from frost shattering which is taking
place higher up on the valley sides.
2.
Medial moraine
This is found in the centre of the glacier
and is the result of two lateral moraine merging as two glaciers meet.
3.
Terminal moraine
This is often a ridge of material found at the snout
(end) of the glacier and it marks the furthest extent that it travelled.
4. Sub-glacial moraine
This is material which is dragged underneath the
glacier. It is often material which has been plucked from somewhere else
and is used by the glacier to erode the valley floor.
5. Englacial moraine
This is material which is held within the
glacier. It may have got there after falling into crevasses on the
glaciers surface.
Features of glacial deposition
As already explained, glaciers have the potential
to carry very large amounts of material (moraine).
Under certain conditions this material can be
deposited to produce some distinctive landforms.
The syllabus requires students to be familiar with
the following three features of glacial deposition.
1. Boulder clay
2. Drumlins
3. Terminal moraine
1.Boulder clay
This is material which has been carried by the glacier (either as sub
glacial or englacial moraine) and then has been deposited as glacial till by
the glacier.
Till is the term often given to all material deposited by a glacier.
As the name suggests, boulder clay can be a mixture of different size
material, ranging from very fine to extremely large.
It is often deposited as the glacier retreats and it looses its ability to
transport it’s load.
2. Drumlins
These are smooth, elongated mounds of till (clay & stones) which have
their long axis parallel to the directions of the ice movement. There is
disagreement as to how drumlins form, but it is widely believed that they
are the result of the glacier becoming overloaded with material. As a
result it looses it’s ability to carry the load and so it is deposited.
Subsequent ice movements will mould this material into mounds known as
drumlins.
Often drumlins are found in great numbers and this is called a swarm of
drumlins.