Glaciers 
by Peter G. Knight


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.

Back to Glaciers main page


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.

Back to Glaciers main page


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. 

Back to Glaciers main page


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.

Back to Glaciers main page


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.

Back to Glaciers main page
 


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.

Back to Glaciers main page
 
 


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.

Back to Glaciers main page
 
 


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.

Back to Glaciers main page
 
 


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.

Back to Glaciers main page
 
 


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.

Back to Glaciers main page
 
 


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.

Back to Glaciers main page
 


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.

Back to Glaciers main page

 

Glaciers
Peter G. Knight



Knight, P.G. (1999) Glaciers.  261 pages. isbn: 0-7487-4000-7

Published by Nelson Thornes Ltd., Cheltenham, England
 Distributed in the USA and Canada by ISBS, Portland, Oregon.