Glaciers
by Peter
G. Knight |
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Extracts from the text
1.4 Conclusion
Glaciers come in a huge range of shapes and sizes. Different
glaciers, and even different parts of the same glacier, can have a variety
of different thermal, hydrological and dynamic characteristics. Glaciers
occur in locations ranging from the poles to the equator, and most parts
of the world have experienced the direct effects of glaciation at some
time in the past. Glaciers currently occupy less of the planet than they
have done in geological history, but nevertheless exert a profound influence
on the global environment. Our developing understanding of glaciers will
play an important role in our understanding of the global environmental
system.
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2.5 Conclusion
Glaciers are an important part of a linked global system involving
global energy sources and sinks, the hydrological cycle, the atmospheric
and oceanic circulation, climate, crustal rheology and sea level.
The system is internally complex and poorly understood, rich in feedback
loops and non-linearities. Glaciers both drive and are driven by elements
of this system, and can by their characteristics and behaviour give us
insight into the dynamics, history and possible future of the physical
environment. Successful modelling of the global environment requires an
understanding of the role of glaciers in the environmental system,
of glacier dynamics and of the glacial response to environmental inputs.
The remainder of this book explores the properties and characteristics
of glaciers as far as they are known.
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3.8 Conclusion
Glaciers can be viewed as a system of input, throughput and output
of mass. The mass balance of a glacier is largely driven by environmental
(climatic) controls, and has a major influence on the glacier’s characteristics
and behaviour. Measurements of mass balance can be used as an indicator
of a glacier’s “health”, and as a proxy measure of climate change, but
oversimplification of the complexity of the controls on mass balance can
lead to misinterpretation of the evidence they present. Even major and
sustained periods of negative mass balance can arise from cyclical dynamic
instabilities, such as surging or ice-shelf calving, that do not necessarily
reflect climate change. Mass balance is intimately linked with glacier
dynamics as well as with climate, and mass balance offers a convenient
conceptual link between the broad environmental context of glaciation and
the details of glacier behaviour.
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4.9 Conclusion
The characteristics and behaviour of glaciers are determined
to a large extent by the properties of the material from which they are
made, so an understanding of the properties of ice is the necessary basis
of a sound understanding of glaciers. However, glaciers comprise more than
ice alone. Huge, long-lasting masses of polythermal, polycrystalline glacier
ice, incorporating particulate debris, gas, solutes, and water under self-generated
thermal and pressure gradients are complex physical and chemical systems.
The physical, chemical and thermal properties of glaciers do exert a control
on glacier behaviour, but are also controlled by it. Glacier dynamics,
thermal regime, hydrology, chemistry and rheology are all complexly interrelated.
The interrelationship between the physical, chemical and dynamic properties
of ice and of glaciers remains a major area in which our understanding
and modelling of glacier behaviour is imperfect.
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5.9 Conclusion
Glaciers can be characterised in terms of their morphology and
their structure. Gross glacier morphology is largely controlled by glacier
dynamics and, except in the case of large ice sheets, topography. Glacier
structure is a function of the way in which glaciers in general are formed
and behave. Most glaciers therefore have broad structural characteristics
in common, including their principal stratigraphic components: the supraglacial,
englacial and basal layers. It is convenient to discuss glaciers in terms
of these common features. Differences between glaciers, which are the crucial
clues for understanding how glaciers relate to their different environmental
and dynamic settings, can then be readily identified. The subdivision of
glaciers into areas where different processes dominate and where different
controlling variables play key roles is also a convenient device for organising
descriptions and understanding of glaciers and glacial processes.
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6.8 Conclusion
An understanding of glacier hydrology is central to an understanding
of glacier behaviour. Glacier hydrology controls many of the major glacier
dynamic and glacial geologic processes. The behaviour of water in glaciers
also reveals the structure of the ice and of the glacier at a variety of
scales, and indicates how this structure changes through time in response
to seasonal and longer term changes in the glacier and its environment.
Major issues in modern glaciology, including surges, ice streams, and deforming
beds, hinge on the role of water at the glacier bed. The development of
our understanding of traditional glaciological problems such as sliding
has relied on a growing knowledge of the effect of water within the glacier.
However, substantial portions of our understanding of subglacial drainage
are based on theoretical modelling in the absence of access to direct observation.
Observations of englacial and subglacial hydrology rely on remote sensing
techniques such as dye tracing, water-pressure monitoring and chemical
analysis of meltwater which, in turn, lack a reliable theoretical basis
for their interpretation. New developments in chemical analysis of meltwater
have been put forward as promising avenues for progress in understanding
glacier hydrology, but chemical analysis of meltwater is still limited
by the problems that beset all forms of remote sensing of the englacial
zone. The problems of studying parts of glaciers that we cannot practically
access are discussed further in chapter 12.
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7.7 Conclusion
Glacier movement should be placed at the heart of any comprehensive
conceptual map of glacial phenomena. Movement is an inevitable consequence
of the physical characteristics and environmental context of glaciation,
and it is a direct cause of most of the major consequences of glaciation.
If glaciers did not move, there would be no glacial geomorphology and a
constipated hydrological cycle. The nature of glacier movement is a valuable
indicator of englacial and subglacial conditions, but one that we are not
yet fully able to interpret. Movement is conditioned by driving stresses,
ice rheology and substrate conditions, and especially by water pressure
at and beneath the bed. Intra-annual variations in motion reflect seasonally
evolving subglacial conditions, and long-term periodicities such as surging
reflect long-term changes in subglacial conditions that are as yet incompletely
understood. Elucidation of the mechanical background to the differences
between “fast” and “normal” flow states remains a major goal of glaciology,
as does the elaboration of a realistic and comprehensive sliding theory
for different bed conditions and the establishment of flow laws for
realistic glacier ice and bed conditions. These goals can be approached
from a variety of routes including laboratory experimentation, theoretical
modelling, and the interpretation of geomorphic and geologic evidence.
This is a huge and exciting area of ignorance in glaciology, the exploration
of which may hold the keys to unravelling outstanding problems in all aspects
of the subject.
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8.4 Conclusion
Glacier fluctuations occur primarily in response to changes in
mass balance. The principal driving forces are thus usually climatic, but
ice-dynamic and geographical controls strongly influence the observed terminus
response to environmental forcing. Responses may be delayed, and may be
non-linear with respect to the magnitude of driving forces. In addition,
mass balance at the terminus is affected by non-climatic factors such as
calving dynamics and ice-divide migration. The positions of glacier margins,
and the record of former margin positions, thus reflect a composite effect
of several controlling parameters, and the use of former margin positions
to reconstruct climate change is fraught with difficulty. Likewise, contemporary
advances and retreats of glacier margins cannot always be interpreted as
simple indicators of climate change. Especially for large ice sheets and
floating glacier tongues, margin fluctuations observed at the present time
may have little to do with contemporary climate change. A major consequence
of glacier fluctuations is the repeated glaciation and deglaciation of
areas of the Earth’s surface, and, hence, the origin of glacial geomorphology.
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9.4 Conclusion
Glacial geomorphology arises from, and can therefore be used
as an indicator of, glacier processes. Many students and researchers approach
glaciology largely as an aid to the understanding of glacial geomorphology.
It is perhaps a shame that more glaciologists do not entertain the study
of glacial geomorphology as an aid to the understanding of glaciology.
Key glaciological issues such as glacier sliding, the development of basal
ice, and deforming bed motion are intimately connected to geomorphic processes,
and landforms are the calling cards of glacier-bed processes. Palaeoglaciological
reconstructions such as that of Kleman et. al. (1997) point the way to
collaborations between glaciology and geomorphology that could be of enormous
value. The sediment transfer that constitutes the heart of glacial geomorphology
is also significant at a much broader scale than that of individual landforms
or even landform regions. The sedimentological concomitants of glaciation
and deglaciation induce continental-scale tectonics and associated sea-level
change, as well as changes in the chemical composition of oceans
and atmosphere as a result of changes in global chemical cycles that are
associated with the glacial and deglacial sediment weathering processes.
Sugden and John (1976) called for a more glaciological type of geomorphology
and a more geomorphological type of glaciology. The former, a glacial geomorphology
with a sound glaciological basis, is now becoming well established, but
the latter, a glaciology that exploits the potential of geomorphological
evidence, is only showing the first signs of emerging.
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10.5 Conclusion
Glaciers have a significant impact on human activity, for both
good and bad, both locally and globally. Glacier hazards are tragically
familiar in glaciated areas, and glacier resources of various kinds have
been exploited in the context of different technological perceptions
and capabilities. Although technology has played a role in the mitigation
of glacier hazards, hazard mitigation usually relies heavily on hazard
prediction, which the theoretical status of glaciology is not yet able
to provide at the level of sophistication that it one day almost certainly
will. The exploitation of resources depends on a perceived need as well
as a technological capability, and, hitherto, the perceived potential value
of glaciers has been relatively limited. It is exciting to speculate what
glaciers might offer if any effort was expended on exploring their full
potential. Glaci-electric power stations driven by the slow but hugely
forceful motion of ice streams, or by the phase change of billions of tonnes
of ice at the melting point, are not yet, as far as I know, even science
fiction. However, the future of applied glaciology, like the future of
theoretical glaciology, will be filled with things that we have not yet
even thought of.
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11.5 Conclusion
Paterson (1981) famously argued that a handful of mathematical
physicists who may never have set foot on a glacier had contributed more
to glaciology than had all the measurers of ablation stakes and terminus
positions. It is tempting to see remote sensers and ice-core drillers as
the inheritors of the ablation-stake tradition. Ice cores provide data
that can be used to calibrate models, to test hypotheses, and to inspire
new ones. Following Paterson’s argument, it is especially important in
the case of ice-coring programs that the practical program of data collection
is carefully attuned to the theoretical research context. Ice cores give
us one of the few windows that we have into what is actually down there
beneath the ice surface. Unfortunately it is a window somewhat narrower
than the sole of my boot, and it needs to be carefully placed.
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12.7 Conclusion
“Glaciology is a small profession.” Hughes (1985, p.39)
The boundaries of glaciology are a little fuzzy. From the inside,
looking out, the limits are hard to define and the discipline is huge.
From the outside, the fuzzy little blob that is glaciology, with tentacles
wriggling out into neighbouring fields, is easier to delimit. I am
often tempted to suggest to my students, usually in the context of an examination,
that “when you’ve seen one glacier you’ve seen ’em all (discuss).”
The goal of one sort of glaciological research seems to be to achieve a
universally applicable set of principals by which the glacier condition
can be defined. Each specific instance, a surge here, a jökulhlaup
there, will then be recognisable as a variant on some established and understood
theme. Glaciology is enriched, however, by a sense of variety and surprise
in the natural phenomena that it considers. Partly this may be because
glaciers themselves are places of variety and surprise, not to say wonder,
and the study of them in the field is associated with a whole range of
experiences beyond the purely scientific. Partly, however, the surprise
element in the study of glaciers arises because there is still so much
about them that we do not know.
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