The Birth and Death of
Glaciers
Glaciers are the products of climate, and they are born only in places
where the climate and various other factors are found in just the right
combination. Glaciers do not form on the lowlands of the middle
latitudes, but they do form and thrive in mid-latitude hghlands such
as the Alps and Himalayas. They exist close to sea-level on some
islands of the Canadian Arctic and on certain sub-Antarctic islands.
But not all of these islands support glaciers, and there are vast areas
on the very edges of the Ardic Ocean which are thought never to have
been glaciated. Clearly glaciers do not simply prefer high-altitude
and high-latitude areas. Other factors also play an important part,
and before we look at how glaciers grow and behave it will be
worthwhile to spend a few moments considering the situations in
whch they are born.
BIRTH IN THE MOUNTAINS
Most glaciers are born in the mountains because here conditions are
especially suitable for snow to collect and survive from one year to
the next. Once snow has collected and survived the summer melting
season it will accumulate and a snowpatch will begin to grow. The
rate of accumulation, and the rate at which a snowpatch grows into a
glacier, depends upon the interaction of many different factors,
including temperature, latitude, altitude and aspect.
Snowfall exerts a very potent control over the occurrence of
glaciers. In general the areas which receive the greatest amounts of
snowfall are very favourable for the birth or growth of glaciers, but it
should be remembered that a high snowfall at a locality is of little use
if it is all melted away during the summer. Many parts of the Alps,
the Scottish mountains and the Rockies receive high snowfall totals
every year, but this snowfall is inadequate for the birth of glaciers
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because of the typically warm summers experienced by those areas.
In other words, the snow which falls is "ineffective" from a
glaciological point of view. On the other hand if an area is high
enough to reduce summer temperatures, for example in tropical
mountains, high snowfalls may be "effective" and glaciers are formed.
In some polar areas summer temperatures seldom rise above freezing,
and here quite small snowfall amounts are sufficient for glacier birth
and growth. For example, central Antarctica and the northern part
of the Greenland ice sheet are more arid than many of the world's subtropical desert areas, but the occasional snowfall is still quite
adequate to maintain the ice sheet surface.
Highland icefields and valley glaciers flowing down towards the
ice-covered Hare Fjord, Ellesmere Island, Canada. (Source: Canadian
Dept of Energy, Mines and Resources)
From the foregoing it will be clear that glaciers need relatively
low temperatures in order to survive and thrive. Many glaciers exist
in areas where mean annual air temperatures are as low as -10
degrees C or even -50 degrees C, but mean annual air temperature is
probably not all that important; after all, so long as the air
temperature is below zero snow can accumulate, and so long as air
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temperature is above zero melting can occur. The most critical factor
therefore seems to be the mean air temperature during the summer
months, and so long as this is not much above zero glacier survival is
possible. Where summer temperatures are substantially above zero
glaciers can only survive where there is more winter snowfall than can
be melted away.
It might be supposed that most glaciers exist close to the
poles. T h s is not strictly so. True, the regional snowline (i.e. the line
at whch the amount of snow falling is equal to the amount of summer
melting) does fall gradually from the tropics towards the poles, and it
is true that the polar lands support more glaciers than the tropics or
sub-tropics. But we have already mentioned the fact that many polar
areas are extremely arid, and some authorities believe that the more
humid middle latitudes (especially around 50 degrees - 60 degrees N
and S) are more suitable for the birth of glaciers than the Arctic and
Antarctic.
Altitude has an important effect upon the location of glaciers,
and most of the smaller glaciers of today are mountain glaciers. They
occupy collecting grounds in the hghlands, and they often flow down
spectacular and beautiful mountain valleys. The glaciers of the
tropics, in particular, would not exist without the presence of high
mountains which have the effect of lowering local air temperatures
and ensuring that most of the annual precipitation falls as snow
rather than rain. For example, Mexico has only three mountains
which support glaciers, and these are all above 5000 m. In East
Africa there are glaciers on Mount Kenya, Mount Kilimanjaro and in
the Ruwenzori, but none of the ice extends below an altitude of 4400
metres.
The shape of the land surface (or its relief) determines the
precise location of glaciers withn those areas which are climatically
suitable. Hollows or depressions in the mountains are particularly
suitable for the accumulation of snow, for they collect not only
"direct"precipitation but also windblown snow from exposed plateau
surfaces and mountain sides. If the slopes around a hollow are steep
enough more snow may be added by the mechanism of avalanching,
which is a perfectly normal process in all steep mountain country
where snowfall totals are high. The highest peaks in a particular
upland area may never support glaciers because their slopes are too
steep, but gentler mountain summits perhaps 300 m lower in altitude
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may be able to retain the snow which falls, causing glaciers to form.
Certain types of glaciers can only occur where there are certain types
of land surface w h c h favour their formation. Cirque glaciers and
mountain valley glaciers are formed in steeply undulating mountain
country, but ice caps normally form in gently undulating plateau
areas, as for example in parts of Norway and Iceland.
Quite closely connected with relief is the factor of aspect or
orientation. Those of us who live in the northern hemisphere know
that south-facing slopes tend to be sunny and warm, w h l e northfacing slopes tend to be shady and cool. It follows that glaciers,
which are products of cold climates, will tend to concentrate on the
northern sides of mountains wherever possible. Another factor is
prevailing wind direction. Since many of the northern hemisphere
glaciated areas experience prevailing south-westerlies, it turns out
that most snow collects on the lee side of mountains and in hollows
whch face north-east. Hence many of the small glaciers of the midlatitude hghlands have this orientation.
A final factor in glacier formation is distance from the nearest
ocean. Most people are familiar with the idea that as one travels
inland from the coast, rainfall (or snowfall) totals tend to diminish.
Thus, all other things being equal, glaciers have a greater chance of
survival close to a coast where there is adequate moisture. Further
inland only the highest mountain areas will receive heavy snow-falls.
If the prevailing winds have to cross mountain barriers close to the
coast they may lose most of their moisture before reachng the inland
mountains, and the latter may remain quite arid (and unglaciated)
because of the rain-shadow effect.
From the foregoing we can see that the birth of glaciers
depends on many different factors that interact with one another to
create conditions suitable for glaciation. Each glacier takes so long
to form that a study of its present-day environment gives us only
part of the answer to our question "Why is it there?" Really we have
to know about the glacier's past environment as well; we now know
that small glaciers may exist today because of the climatic conditions
which prevailed hundreds of years ago, while large glaciers may exist
because there happened to be suitable climatic conditions several
thousands of years ago.
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SNOW INTO ICE
Glaciers grow through the accumulation of snow and ice to
considerable thicknesses. As mentioned above, most of the snow
which feeds glaciers comes from direct precipitation, wind drifting
(called deflation), and avalanching from steep mountain slopes. It is
worth looking briefly at the process by which snow is converted into
ice, and also at the characteristics of the ice which makes up the bulk
of a glacier.
(dl Compact granular rmw
firn or ndvd
(f
*
0
0
0
0
0
0
How snow crystals are turned into glacier ice as they are buried
deeper and deeper byfieshfalls ofsnow. This process may take more
than 5000 years in Antarctica, but on mid-latitude glaciers a decade
may be sufficient.
When fresh snowflakes fall onto a glacier surface they have a
delicate and beautiful hexagonal form. Snow crystal forms come in
an almost infinite variety, and their complexity and size varies
according to the temperature and other conditions prevailing at the
time of the snowfall. The largest and most delicate crystals fall
during calm weather when the air temperature is close to zero. Under
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colder conditions and when there is a strong wind blowing, crystals
tend to be smaller and more granular, partly because they are
damaged in collisions with one another.
Once on the glacier surface the crystals undergo a gradual
conversion to glacier ice. As soon as they are buried by other
snowflakes they lose some of their delicate tracery and become
simpler in outline. Eventually the six "arms" are lost by melting and
the snowflakes become snow granules. These may be less than 1
mm in diameter. As more snow layers build up on the glacier surface
the pressure on the buried granules increases. Most air is expelled
and the granules are packed closer and closer together. At sub-zero
temperatures the changes in granule shapes occur simply because of
pressure and physical damage. Where it is warmer, melting and
refreezing occurs. There is more melting at the points of contact
between granules, and the water so produced is recrystallised, causing
them to grow. When this process has continued for a year or more
the enlarged crystals are called firn (neve in French). By now the
crystals are several millimetres in diameter, and there is very little air
trapped between them. As they continue to grow they become
welded together and impermeable to air or infiltrating meltwater; the
trapped air from the original snow-pack can only exist as small air
bubbles in the ice. This is now true glacier ice, and the individual
crystals sometimes reach the diameter of a football.
Quite
commonly the thick annual snow layers are transformed into narrow
ice layers (or foliations) deep withn the glacier.
Not all of the ice which makes u p the bulk of a glacier comes
from this slow process of conversion. In cool humid environments
such as the Antarctic Peninsula, the sub-Antarctic islands and even
western Iceland, glaciers may receive great additions of a special type
of ice called rime ice. This is formed by the direct freezing of water
droplets from the atmosphere when they come into contact with a
cold surface. Generally rime ice forms in great cauliflower-shaped
masses on rock summits and valley sides, but when these masses
break off and fall down they help to build up the glacier surfaces
below. Another type of ice is called superimposed ice; this is
formed at relatively high temperatures (above or at freezing-point)
when meltwater from the glacier surface infiltrates into the snow and
firn beneath and freezes when it reaches the underlying ice. Some ice
caps in the Canadian Arctic are made almost entirely of
superimposed ice, whch forms in a new layer every year.
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Foliated or banded ice exposed in the snout of the Cook Glacier,
South Georgia. The bands are mostly -- but not a l w a y s -- annual
accumulation layers. (Source: C. Clapperton)
We should not be too earth-bound in our ideas concerning the
formation of ice. There is perfectly respectable ice on the planet
Mars, differing mainly in that it is made of solid carbon dioxide. It
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blankets large parts of the Martian polar areas, and glacial
astronomers (or should they be called "astronomical glaciologists"?)
are now studying the characteristics of the two ice caps. These ice
caps show up remarkably clearly on the photographs transmitted
back to Earth from spacecraft.
On earth, the time taken for the conversion of fresh snow to
glacier ice varies greatly from place to place. On some mid-latitude
glaciers where annual snowfall totals may be as high as 5-10 m, the
process may take less than five years. On the cold glacier surfaces of
the Greenland and Antarctic ice sheets the process may take more
than 5000 years. Also the depth of the firn-ice transformation
varies greatly, from 20 m or less on mid-latitude glaciers to 60 m or
more on true polar glaciers. These differences help us to understand
the ways in which different types of glacier behave, and they also
provide us with guidelines to the dynamism or activity of glaciers in
different environments. Clearly those glaciers which have the highest
rates of accumulation and which convert snow to glacier ice at the
greatest speed need to evacuate this surplus material at the highest
possible rates if they are to remain in any sort of equilibrium with
their environment. This means that the ice has to move away
downslope relatively rapidly. Conversely glaciers whose surfaces
build up but slowly tend to behave in a sluggsh fashion.
GLACIERS LARGE AND SMALL
It follows that if there are many different types of glacier ice, so there
must be many different types of glacier. Glaciologists have spent a
great deal of time over the years in attempting to classify the subjects
of their study, and for our part we cannot understand the events of
the Ice Age unless we know a little about the types of glaciers which
have come and gone over the centuries.
One of the most popular ways of distinguishing glaciers has
been through the use of their temperature characteristics. It has been
realised for many years that the temperature of glacier ice about 10
metres below the surface can give a guide to the mean annual air
temperature of a locality. For example, the mean annual air
temperature at the South Pole is about -51 degrees C, and this figure
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was arrived at originally not by prolonged meteorological
observations but by measuring the "10 m ice temperature" in a deep
pit. Ice which is at such low temperatures tends to behave in a
different way from ice in temperate latitudes, and glaciologists
commonly refer to the two types of ice as cold ice and temperate ice.
The major difference between these is explaii~edby reference to the
pressure melting point of ice. The melting point of snow and ice is
zero centigrade at sea-level, but on glaciers it varies with altitude and
with depth beneath the surface. In general, the thicker the ice the
lower the pressure melting point, and beneath thick glaciers such as
the Antarctic ice-sheet meltwater can exist at a depth of 2500m
around -1.6 degrees C.
An image of the north polar ice cap of Mars, taken on the Mariner 9
voyage in 1972. The ice cap is made of carbon dioxide ice. (Source:
NASA)
Ice which is always below the pressure melting point is
referred to as cold ice, while ice which is sufficiently close to the
pressure melting point to contain water for some of the time is called
temperate ice. The ice in the centre of the Greenland ice sheet is cold;
at a depth of 500 m it has a temperature of about -23 degrees C, but
although the temperature rises with increasing depth it is still no
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hgher that - 10 degrees C when the ice sheet bed is reached at a depth
of c.1300 m. Many of the Alpine and North American glaciers are
made largely of temperate ice,allowing a layer of water to exist on the
glacier bed throughout most of the year and allowing substantial
amounts of water to exist in the body of the glacier itself.
A vertical aerial photograph of a polar glacier snout on Ellesmere
Island, Canada. This is the Lower Gilrnan Glacier, near Lake
Hazen. (Source: Royal Canadian Air Force)
It is no easy matter to define a glacier as simply "cold1'or
"temperate" for it may be made u p of different types of ice in its
upper and lower parts. In Antarctica the ice is cold close to the
surface and temperate close to the bed; in Svalbard glaciers are warm
beneath the collecting grounds and cold close to their snouts. It is
therefore wiser to refer to the different types of glacier by using labels
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which describe their location in terms of their latitude. Quite
commonly nowadays glaciologists talk about temperate, sub-polar
and polar glaciers; these terms tell us something about the
temperature characteristics of the glacier ice involved in each case,
and they are less open to misinterpretation.
Another, quite different, way of referring to glaciers is through
the use of descriptive terms whch tell us somethng about their shape
or size. Using these criteria the simplest scheme is one in which we
recognize only two different types of glacier: those w h c h assume
their forms and dimensions largely independently of the shape of the
underlying bedrock topography, and those which are controlled or
constrained by topography.
Under the first heading we may include ice caps and ice
sheets, whch build themselves up to massive ice domes and which
may completely submerge hills and valleys and even complete
mountain ranges. Ice caps are usually less than 50,000 sq km in area.
Ice sheets are quite different in scale, and the only two surviving at the
present day are in Greenland and Antarctica. The highest point on
the Antarctic ice sheet is more than 4200 m above sea-level, and there
Radio-echo sounding profile of part of the Antarctic ice sheet. The
profile shows a complex landscape of mountains, ridges and valleys,
with all but the highest summits buried beneath the ice. (Source:
Scott Polar Research Institute)
are vast areas where the ice is more than 3000 m thck. The so-called
East Antarctic section of the ice sheet measures some 4500 km x 2500
km, and the area of the Antarctic ice sheet as a whole is about 12.5
million sq km. Ice sheets and ice caps tend to build themselves up
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to a characteristic cross-sectional form w h c h is called an equilibrium
profile. The profile has been calculated from theoretical work on the
physical properties of ice, and it is matched very closely by the
surveyed surface profiles of the Antarctic and Greenland ice sheets
and many smaller ice caps. The main features of an equilibrium
profile are similar to those of a parabola, where the gradient is steep
at the glacier edge and lessens progressively with distance from the
margin towards the centre.
Glaciers which are constrained by the relief of the land are
different in many respects. They are generally much smaller than ice
sheets and ice caps, and they d o not have dome-shaped upper
surfaces. They occupy bedrock depressions in the mountains, and
these act as glacier nourishment areas called icefields. Sometimes
these icefields d o submerge bedrock ridges, but always the direction
of ice movement is controlled by the slope of the bedrock surface in
the valleys between the ridges.
A highland icefield w i t h valley glaciers and piedmont lobes, north of
Surprise Fjord, Axel Heiberg Island, Canada. (Source: Canadian
Dept of Energy, Mines and Resources)
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Highland icefields and valley glaciers near the east coast of
Greenland. The iceljields "feed" ice via steep and crevassed icefalls
into the valley glaciers, which in turn flow towards the lowlands.
(Source: Geodetic Institute, Denmark)
T w i n valley glaciers on the Blosseville Kyst, Greenland. Strictly
these should be called "outlet glaciers" since they are fed from a part
of the Greenland ice sheet. (Source: Geodetic Institute, Denmark)
Page 13
The glaciers which leave the icefields are called valley glaciers.
Exceptionally they may be over 100 km in length, but more commonly
they are less than 30 km long. Regardless of their length, they are
generally joined by tributary glaciers from upland cirques or ice caps
on either side of the glacial trough. Cirque glaciers are small ice
masses which occupy armchair-shaped hollows in the mountains.
These glaciers, if they are very active, may flow downhill for
considerable distances, thus becoming indistinguishable from normal
valley glaciers. However, if the glacier snout does not extend beyond
its rock basin it can be referred to as a true cirque glacier which is
typically very broad in proportion to its length. These small glaciers
are found in many mid-latitude upland areas in distinct groups; by
their presence they indicate that climatic and other conditions are
only just adequate for the continuing existence of glacier ice.
GROWTH IN THE LOWLANDS
Most glaciers grow in the uplands which give them birth, because here
snowfall totals are hghest and summer temperatures are low enough
for glacier ice to survive. Those glaciers which reach the lowlands are
usually subject to the processes of ablation, which tend to reduce both
glacier volumes and dimensions. We will look at these processes in a
moment, but before doing so we should consider briefly some
exceptional circumstances which can lead to glacier growth at low
altitudes and in "unexpected"situations.
One peculiar type of glacier is the piedmont lobe, in which a
glacier spreads out as a broad, flattish sheet of ice as soon as it is
freed from the constraints of its upland valley. Such glaciers are
common in Antarctica, North Greenland and Arctic Canada, where
they occur in broad valleys which are otherwise free of ice, or else on
coastal lowlands. Piedmont lobes in relatively arid environments
demonstrate the amazingly viscous character of sub-polar and polar
ice. One author has likened the behaviour of the ice to white
molasses poured over the landscape, and many glaciology students
have enjoyed making models of piedmont glacier lobes by pouring
plaster-of-paris down narrow wooden chutes. The Commonwealth
Glacier in the McMurdo area of Antarctica is a famous example of a
Page 1 4
glacier of this type, and there are many other examples in North
Greenland and on Ellesmere Island. Perhaps the most famous of all
piedmont lobes is the vast Malaspina Glacier in Alaska. After
leaving the mountains the glacier spreads laterally into a lobe about
40 km across, with a surface area of 2400 sq. km. With its great
extent and its characteristic pattern of striped moraines it is a
prominent feature on satellite photographs oi the Alaskan coast.
Temperate ice is even more viscous or deformable than polar
and sub-polar ice, but because temperate glacier snouts tend to exist
in areas of relatively high melting, spectacular piedmont lobes have
little chance of survival.
An idea which has caused much controversy among
glaciologists and climatologists is the theory that ice sheets and ice
caps may sometimes grow outside the more normal process of
spreading outwards and downwards from collecting-grounds in the
mountains.
Long ago it was suggested that the Greenland,
Scandinavian and Laurentide (North American) ice sheets grew
gradually from glaciers which originated in the mountains and then
filled the lowlands, allowing the ice surface to build up over many
thousands of years of climatic cooling. The newer theory, loosely and
spectacularly labelled the "snowblitz theory", involves the growth of
ice sheets or ice caps in the lowlands. The suggested mechanism
involves a sequence of cold winters during which the ground surface
becomes snow-covered to a considerable thickness. If large expanses
of snow survive from one winter to the next, the warming effect of the
summer sun will be reduced because large amounts of solar energy will
be radiated back into space by the bright snow surface. Thus, over
time, the summer melting seasons will become less and less effective
and the winter snowfalls more and more effective. This is a situation
referred to as "positive feedback". Eventually, after but a few
centuries, so much of the surface will be snow-covered that a point of
no return is reached, and an ice cap or ice sheet will develop.
This is a highly speculative and somewhat alarmist theory,
and it is doubtful whether a snowblitz of this type could occur in the
middle latitudes. It could, however, occur in high latitude areas such
as the Canadian Arctic, and there were signs during the early 1970s of
an increased snowcover in certain areas which had the apparent
effect of lowering local summer temperatures. Also, it has been
suggested that the Laurentide ice sheet grew by the coalescence of a
Page 1 5
number of broad snow domes or ice caps which developed on the
plateau areas of Baffin Island and Labrador-Ungava. It may well be
that if climatic conditions are sensitive enough, glaciers can be born
and nurtured in both lowland and upland situations, greatly
speeding up the onset of a glacial episode.
THE BALANCE EQUATION
So far we have only examined the birth and growth of glaciers. We
have looked at the ways in whch they form and the situations in
which they can best exist. But glaciers cannot increase in size
indefinitely, for as they grow bigger they trespass into areas which are
environmentally unsuitable. Once this happens they melt away, and
the rate of melting conditions glacier size just as much as the rate of
snow and ice accumulation. If a glacier maintains its dimensions over
the years it is said to be in balance; this means that the amount of
material being added to the glacier is balanced by the amount of
material being lost.
Most of the material which builds up the glacier falls onto its
surface on its highest parts in a zone called the accumulation zone.
Conversely, most of the material which leaves the glacier is removed
from its lower parts. The area of the glacier involved is called the
ablation zone. Sometimes the two zones occupy more or less equal
areas of the glacier surface, but it is more common for the
accumulation zone to be the larger of the two.
The figure on the next page explains, in very simplified form,
the relations between the two parts of a glacier.
Generally
accumulation is at its greatest near the head of a glacier and falls off
gradually towards the snout. Conversely ablation (or melting) is at
its greatest near the snout, and decreases gradually up-glacier. Hence
somewhere in the middle of a glacier we should expect a line where
accumulation more or less balances ablation. This imaginary line,
much beloved of glaciologists, is called the equilibriumline, for here
the glacier surface is neither being raised by accumulation nor lowered
by ablation. As we can see from the diagram, a glacier whch is more
or less in a state of balance receives a "wedge" of snow each year in its
accumulation zone and loses a "wedge" of ice each year in its ablation
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A model glacier. This diagram shows in a very simplified form how
a small glacier gains material in its accumulation zone and loses
material in its ablation zone. Because a glacier moves, i t can
achieve an approximate state of balance between "input" and
"output".
zone. The state of balance between the two zones is maintained by
the mechanism of ice movement. As ice moves down glacier from the
accumulation zone to the ablation zone it ensures that the glacier
surface maintains a more or less constant altitude from one year to
the next. If a glacier is not in equilibrium because it is gaining more ice
than is lost through ablation, it will advance. On the other hand, if
the rate of ablation is faster than the rate of accumulation, the glacier
must retreat. The rate of advance or retreat will vary according to
how strongly negative or positive its annual budget happens to be.
In the real world it is often extremely difficult to determine the
budget or mass balance of a glacier. On small glaciers the budget
may be positive one year and negative the next, but because glaciers
take a long time to respond to climatic events the behaviour of the
snout is sometimes apparently independent of the short-term changes
in snowfall and melting rates. For example, a glacier snout may
continue to retreat during several years of positive budget, for the
surplus snow and ice whch it receives will take time before it works
its way through to the glacier snout.
Paae 1 7
GENERAL
CLIMATE
__+
LOCAL GLACIER
CLIMATE
GLACIER MASS
A N D ENERGY
EXCHANGE
N E T MASS
BALANCE
GLACIER
RESPONSE
ADVANCE
OR
RETREAT
A "flow diagram" showing the relationships between climate and
glacier snout behaviour. The "response time" which elapses between
an environmental change and a snout advance or retreat varies
according to the size and "dynamism" o f a glacier.
HOW GLACIERS DIE
The removal of material from the ablation zone of a glacier takes
place by a variety of different mechanisms. If the lower part of a
glacier is located in an area of high summer air temperatures, the
effect will be to remove ice from the surface by direct melting.
Generally no more than a few metres are removed each year, but some
Norwegian, Icelandic and Alaskan glaciers lose up to 15 m of ice fro~n
the vicinity of their snouts each summer. Mostly the product of this
melting is meltwater, and the work which it can do is often
spectacular. In cold and windy environments glaciers may lose a
great deal of surface snow by deflation, and on the Antarctic ice sheet
snow is constantly shifted from one area to another by high winds,
giving rise to characteristic wind-eroded forms called sastrugi. These
may be more than a metre in height, and they are notorious for the
problems wluch they present to ice sheet travellers.
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A "honeycomb" texture on the lower part of a glacier surface. The
pits are caused by enhanced ablation around dark-coloured rock
fragments and even around grains of sand. If the air is dry the
melted ice may be evaporated directly into the atmosphere.
If a polar environment is very arid, glacier surfaces may be
lowered by evaporation - that is, by the direct conversion of ice into
water-vapour. In some areas this mechanism may account for up to
5 per cent of the total annual loss of ice from the ablation zone. Of
much greater importance in h g h latitudes is the mechanism of calving.
This occurs only at the snouts or on the edges of glaciers which are in
contact with lakes or the sea; the most efficient calving of all occurs
where glaciers are actually afloat, as in some of the fjords of
Greenland and around parts of the Antarctic continent. Most of the
ablation around Antarctica is achieved by calving, leading to the
presence of vast quantities of icebergs and smaller ice fragments in the
surrounding seas. Some of the calving glaciers of Greenland produce
more than 100 million tons of icebergs per day.
Another type of ablation which is important beneath
temperate glaciers is basal melting. Because of the heat transmitted
from the centre of the earth the temperature on a glacier bed is raised
as a result of the contact between ice and rock. Certain types of rock
Paae 1 9
Large "tabular" icebergs being calved from the snout of a n outlet
glacier in North- West Greenland. In the photograph w e can also see
large amounts of "brash iceff, some old icebergs, and both old and
new sea ice. The floating glacier snout is a t the bottom of the
photograph. (Source: Geodetic Institute, Copenhagen)
Paoe 2 0
transmit more heat than others; for example glacier beds in volcanic
districts receive about three times as much heat as glacier beds on the
old Precambrian Sheld areas of the world. When t h s heat is added
to the heat created by a glacier sliding on its bed, and when we
consider that thick glaciers may have their bottom layers close to the
pressure melting point in any case, it is not difficult to understand
that bottom melting can occur. Beneath some glaciers several
centimetres can be removed from the bottom ice layers each year by
t h s type of ablation.
Chaotic wastage or melting of a glacier snout, Svartisen, Norway.
Here the ice i s breaking up into detached blocks, and much of the
"dead ice" is covered w i t h rock debris. (Source: M. Alexander)
Generally glacier wastage close to the snout is not particularly
spectacular, but sometimes catastrophic wastage can occur. If the
climate around a glacier snout becomes warmer, or if a glacier
advances strongly into an area where it cannot possibly survive, very
rapid melting may affect vast areas of the glacier surface. The ice of
the ablation zone may be broken u p into separate blocks, deep
channels will be cut by meltwater streams, and the glacier surface may
be converted into a wilderness of ponds, masses of rock debris,
Paae 2 1
The debris-covered lower portion o f t h e Khumbu Glacier, notfar from
Mount Everest. Here the disintegrating glacier i s bounded by huge
ridges of lateral moraine. (Source: E. Schneider)
Rapid melting on the snout of a glacier on Spitzbergen. A s the ice
melts, debris slides down the glacier face, creating a striped effect.
(Source: E. Kosiba)
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hummocks and hollows, lakes and quagmires. This type of
catastrophic melting is not by any means restricted to small glaciers,
for it is known to have occurred during the decline and death of the
Laurentide ice sheet in North America. The mid-latitude ice sheets
seem to behave in a consistent way; they grow to vast size,
eventually pushing so far southwards that they trespass into areas
where the climate is too warm for their continued survival. At the
same time they become so large that the snow-bearing winds which
nourish them can no longer carry their loads into the ice sheet
interiors; in other words, their source of supply is cut off or
drastically reduced. When there is a high rate of melting near the ice
sheet edge and an inadequate renewal of ice from the interior, ice
sheet decline is only a matter of time. Catastrophic melting sets in,
and the ice sheet simply disintegrates.
ACKNOWLEDGEMENTS
I am grateful to the original publishers of "The Ice Age" for
allowing the use of a chapter from the book in a new format. I
thank the members of the Editorial Board of the Norwegian
Glacier Centre for their constructive comments and suggestions
during the preparation of this text. I also thank the copyright
owners of the illustrations for their original consents. I have been
unable to trace the owners of some of the illustrations, and will be
grateful for any information which will enable me to make due
acknowledgement in any future editions of this booklet.
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