The Physics of Glaciers

486 Pages·1994·7.512 MB·English

by  W. S. B. Paterson (Auth.)

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The Physics of Glaciers
Third Edition
by
W. S. B. PATERSON
PERGAMON
U.K. Elsevier Science Ltd, The Boulevard, Langford Lane,
Kidlington, Oxford OX5 1GB, England
U.S.A. Elsevier Science Inc., 660 White Plains Road,
Tarrytown, New York 10591-5153, U.S.A.
JAPAN Elsevier Science Japan, Tsunashima Building Annex,
3-20-12 Yushima, Bunkyo-ku, Tokyo 113, Japan
Copyright © 1994 Elsevier Science Ltd
All Rights Reserved. No part of this publication may be reproduced,
stored in a retrieval system or transmitted in any form or by any means:
electronic, electrostatic, magnetic tape, mechanical, photocopying,
recording or otherwise, without permission in writing from
the publishers.
First edition 1969
Reprinted 1972, 1975
Second edition 1981
Reprinted with corrections 1983, 1993
Third edition 1994
Library of Congress Cataloging-in-Publication Data
Paterson, W. S. B.
The physics of glaciers/W.S.B. Paterson. - 3rd ed.
p. cm.
Includes index
1. Glaciers. I. Title.
GB2403.5.P37 1994
551.3' 12-dc20 94-30918
ISBN 0-08-037945 1 Hardcover
ISBN 0-08-037944 3 Flexicover
In order to make this volume available as economically and as
rapidly as possible, the author's typescript has been reproduced in
its original form. This method unfortunately has its typographical
limitations but it is hoped that they in no way distract the reader.
Printed and Bound in Great Britain by Redwood Books, Trowbridge
Preface to Third Edition
The aim of this book and the level of treatment remains unchanged. The
text has, however, been completely revised in order to keep pace with the
extensive developments since the second edition was published. Changes
in structure include a new chapter about the deformation of subglacial till,
a previously neglected topic that is now of major interest, and reorgani-
zation of the chapters about flow in the different types of ice mass. Lack
of space has forced the elimination of the chapter on measurement tech-
niques and is also the reason for the continued absence of any discussion
of glacial erosion and sedimentation. The chapters are now arranged in
what is perhaps a more logical order; however, I have tried to make each
reasonably self-contained.
I am most grateful to Richard Alley, Ian Allison, Roger Braithwaite,
Nancy Brown, Garry Clarke, Kurt Cuffey, Dorthe Dahl-Jensen, Chris
Doake, Paul Duval, Tony Gow, Bernard Hallet, William Harrison, Richard
Hindmarsh, Almut Iken, Sigfus Johnsen, Anne Letréguilly, Atsumu Oh-
mura, Charles Raymond, Niels Reeh, Hans Röthlisberger, Joe Wälder,
and Chris Wilson for reviewing individual chapters. Their comments have
resulted in many improvements; however, the responsibility for the final
form and contents is mine alone. Word-processing was in the capable
hands of Shawn Marshall, Susan and Jeffrey Schmok, and Dan Stone.
T. S. Eliot remarked somewhere "The years between fifty and seventy
are the hardest. You are continually asked to do things but are still not
decrepit enough to refuse." I shall be well beyond these limits by the time
another edition is needed. This is probably fortunate. The vast expansion
of the literature in recent years is making it increasingly difficult for one
person to write a comprehensive account of the physics of glaciers.
Copenhagen, Denmark W. S. B. PATERSON
June 1994
v
Preface to Second Edition
Developments in the 12 years since the first edition went to press have
made necessary a complete revision of the text. Extensive new field data
have shown that, although the basic concepts developed in the 1950s still
stand, many of them are over-simplified. As a result, theories have become
more complicated and, in addition, computer modelling has added a new
dimension to glacier studies. Nevertheless, the aim of the book remains
unchanged. I hope that this is also true of the level of treatment, in spite
of the increased complexity of the subject.
New chapters on ice core studies and glacier hydrology deal with top-
ics that are now of major importance but on which little work had been
done when the first edition was written. A new chapter on structures and
fabrics in glaciers and ice sheets treats a subject to which I perhaps paid
too little attention in the original book. The chapter entitled "Heat Bud-
get and Climatology of Glaciers" amalgamates two closely-related topics
that were previously discussed separately, while the new Chapter 3 is de-
voted to the mechanism of ice deformation and the flow law. Almost all
the other chapters have been extensively rewritten.
The major part of this edition was written while I was enjoying the
hospitality of the Department of Geophysics and Astronomy at the Uni-
versity of British Columbia (R. D. Russell, head). Numerous colleagues
kindly devoted time to reviewing parts of the manuscript; comments by
B. T. Alt, R. G. Barry, C. S. Benson, G. K. C. Clarke, W. Dansgaard,
D. A. Fisher, A. J. Gow, M. J. Hambrey, W. D. Harrison, B. Holmgren,
P. J. Hudleston, S. J. Jones, R. M. Koerner, J. F. Nye, C. F. Raymond,
G. de Q. Robin, R. H. Thomas, and J. Weertman have resulted in sig-
nificant improvements. However, the responsibility for the final version
remains my own. R. C. Rumley did the final typing. Last, but by no
means least, my wife Lyn typed all the drafts, helped with references, in-
dexing, and proof-reading, and provided continuous encouragement in the
often disheartening task of writing this book.
Ottawa, Canada W. S. B. PATERSON
May 1980
vi
Preface to First Edition
The aim of this book is to explain the physical principles underlying the
behaviour of glaciers and ice sheets, as far as these are understood at the
present time.
Glaciers have been studied scientifically for more than a century. Dur-
ing this period, interest in glaciers has, like the glaciers themselves, waxed
and waned. Periods of activity and advance have alternated with periods
of stagnation and even of retrogression when erroneous ideas have become
part of conventional wisdom. The past 20 years, however, have seen a ma-
jor advance in our knowledge. Theories have been developed which have
explained many facts previously obscure; improved observational tech-
niques have enabled these theories to be tested and have produced new
results still to be explained.
This seems an appropriate time to review these recent developments.
At present there is, to my knowledge, no book in English which does this.
The present book is a modest attempt to fill the gap. To cover the whole
field in a short book is impossible. I have tried to select those topics
which I feel to be of most significance, but there is undoubtedly some bias
towards my own particular interests.
While this book is intended primarily for those starting research in
the subject, I hope that established workers in glacier studies, and in
related fields, will find it useful. The treatment is at about the graduate
student level. The standard varies, however, and most chapters should be
intelligible to senior undergraduates.
I am much indebted to Dr. J. F. Nye for reading the whole manuscript
and making many helpful suggestions. I am grateful to Drs. S. J. Jones,
G. de Q. Robin and J. Weertman for reviewing individual chapters. I
should also like to thank Drs. J. A. Jacobs and J. Tuzo Wilson for general
comments and encouragement. The responsibility for the final form and
contents of the book of course remains my own.
Ottawa, Canada W. S. B. PATERSON
March 1968
vii
1
Introduction
"There is nothing new except what is forgotten."
Anonymous
Glacier ice covers some 10 per cent of the earth's land surface at the
present time and covered about three times as much during the ice ages.
However, at present, all but about one per cent of this ice is in areas
remote from normal human activities, the great ice sheets of Greenland
and Antarctica. Thus it is not surprising that the relatively small glaciers
on mountain areas were the first to attract attention.
Descriptions of glaciers can be found in 11th century Icelandic litera-
ture, but the fact that they move doesn't appear to have been noticed, or
at any rate recorded, until some 500 years later. Since that time, the prob-
lem of how large, apparently solid mass of ice can flow has been studied
and debated by many eminent scientists. Altmann, in 1751, correctly rec-
ognized that gravity was the cause of glacier motion, but he thought that
movement consisted entirely of the ice sliding over its bed. Many glaciers
do slide in this way but, in addition, the ice itself can flow, somewhat
like a very viscous fluid, as Bordier suggested in the late 18th century. In
1849 Thomson demonstrated ice flow in the laboratory, though the inter-
pretation of his experiment later caused some confusion. Forbes asserted
that glacier movement was viscous flow, but Tyndall opposed this view.
He thought that motion resulted from the formation of numerous small
fractures that were subsequently healed by pressure melting and refreez-
ing. Forbes' view prevailed, though only after much heated controversy.
A proper understanding of the mechanism of glacier flow has been reached
only in the past 40 years, by the application of modern ideas in solid state
physics and metallurgy. This followed the realization that, because ice is
1
2 THE PHYSICS OF GLACIERS
a crystalline solid, it should deform like other crystalline solids such as
metals, at temperatures near their melting points.
The two mechanisms that enable ice to move past bedrock bumps
were identified by Deeley and Parr in 1914. Water is also important in
sliding; observations, dating back to Forbes, showed increases in velocity
after heavy rain. Elaborate mathematical theories of sliding have been
constructed in recent years, but the extent to which they apply to the
motion of real glaciers is problematical. These theories are based on highly
simplified assumptions: a rigid and impermeable glacier bed, for example.
In fact, glacial sediments cover 80 per cent of the area formerly occupied
by the Eurasian and North American ice sheets. Although geologists have
long known that such sediments, when saturated with water, can deform,
glaciologists have, until very recently, ignored this fact. Bed deformation
is a third mechanism of glacier motion.
Studies of the flow and storage of water in glaciers, which date back
to dye-tracing experiments by Forel at the turn of the century, contribute
to the understanding of sliding and bed deformation. They also have
practical applications. Hydro-electric plants in several European countries
derive much of their water from glacier-fed rivers. Tunnels have even
been drilled beneath glaciers to tap subglacial streams. And the sudden
drainage of ice-dammed lakes or of water stored within glaciers has caused
extensive damage in Iceland, Canada, and elsewhere.
Systematic measurements of glacier flow were begun about 1830 in
the Alps. The aim of most early work was to find out how movement
varied from place to place on a glacier. Agassiz showed that the velocity is
greatest in the central part and decreases progressively towards each side.
He also found that a glacier moves more slowly near its head and terminus
than elsewhere. Reid, in 1897, showed that the velocity vectors are not
parallel to the glacier surface. They are inclined slightly downwards in the
higher parts of the glacier, where snow accumulates, and slightly upwards
in the lower reaches to compensate for ice lost by melting. Figure 1.1
shows this pattern. Note that "upwards" and "downwards" are relative
to the plane of the surface, not the horizontal.
Ice movement at depth was long the subject of debate. "Extrusion
flow" had its proponents, even in the 1930's. This hypothesis, that glaciers
flow more rapidly at depth than at the surface, wets based on the mistaken
belief that ice deforms more readily when the hydrostatic pressure is high.
In the early 1900's, Bliimcke and Hess used a thermal drill in a glacier in
the Tyrol and attained bedrock in eleven holes, one of them more than
INTRODUCTION 3
FIG. 1.1. Velocity vectors in idealized glacier.
200 m deep. Rods left in the holes gradually tilted downhill. This sug-
gested that the surface ice moves more rapidly than the ice at depth, a
fact confirmed by recent, more sophisticated, borehole measurements.
Other developments about the turn of the century were the observa-
tion of Vallot of what would now be called a kinematic wave moving down
the Mer de Glace and the development of mathematical models of glacier
flow by Finsterwalder and others. Reid, for instance, analyzed the time
lag between the advance of the terminus and the increase in snowfall that
produced it. Finsterwalder also pioneered photogrammetric methods of
mapping glaciers.
That glaciers advance and retreat in response to changes in climate is
common knowledge, but the relationship is more complex than is usually
assumed. Ahlmann, between 1920 and 1940, carried out classic investiga-
tions on this subject on glaciers in Scandinavia, Spitsbergen, Iceland and
Greenland. Complementary studies of how a glacier surface receives heat
during the melt season were begun by Sverdrup in 1934. But an under-
standing of the meteorological problems is not enough; the flow charac-
teristics of each particular glacier determine how it reacts to a climatic
change. The past 30 years have seen impressive theoretical developments
in this aspect of the problem. It is now possible to predict the future
behaviour of a glacier, given sufficient data about its present state and
some assumptions about future climate. Such predictions are important
when roads, pipelines, mines and hydro-electric plants are being built near
glaciers.
In recent years, emphasis has shifted from mountain glaciers to the ice
sheets of Greenland and Antarctica. These ice sheets are able to maintain
themselves, although precipitation in their central areas is as low as in
4 THE PHYSICS OF GLACIERS
deserts. How will global warming change their thickness and extent and,
as a consequence, world sea level? How old is the ice at different depths?
What can chemical analyses of such ice, and the air trapped within it,
tell us about climate at the time it was deposited as snow? Study of the
dynamics of existing ice sheets helps us to understand the behaviour of
ice-age ice sheets. It also helps us to interpret the deposits they left behind
and to assess the theories that have been proposed to account for the ice
ages.
Noteworthy early work in Greenland is Koch and Wegener's study
of snow stratigraphy during their crossing of the ice sheet in 1913. They
also measured temperatures in the ice, in one instance down to a depth of
24 m. Wegener's Greenland Expedition of 1930-31, which wintered in the
central part of the ice sheet, studied the way in which snow is transformed
to ice. They also made seismic measurements of ice thickness, a method
first tried a few years earlier in the Alps.
Modern study of the Antarctic Ice Sheet began with the Norwegian-
British-Swedish Antarctic Expedition of 1949-52. The continent can be
divided into three parts (Fig. 1.2): the East and West Antarctic Ice Sheets,
separated by the Transantarctic Mountains, and the Antarctic Peninsula,
characterized by relatively small ice caps and valley glaciers. The maxi-
mum surface elevation in East Antarctica is slightly over 4000 m. Although
the greatest ice thickness is nearly 4800 m, most of the bedrock is above
sea level. Much of the base of the West Antarctic Ice Sheet, in contrast, is
well below sea level and would remain so after isostatic rebound following
removal of the ice. This is the basis for suggestions that this ice sheet may
be unstable.
Near the coast, most of the outflow of ice is channelled into outlet
glaciers that cut through the mountains, or into fast-moving ice streams
that may owe their existence to deformable subglacial sediments. Most
of the glaciers and ice streams flow into floating ice shelves that surround
much of the continent.
Calving of icebergs accounts for about 85 per cent of the mass loss
from Antarctica. Thus the ice extent is controlled not directly by climate,
but by sea level which, in turn, depends largely on the size of the ice
sheets in the northern hemisphere. Surface ablation occurs at a few places
where the outflow of ice is blocked by mountains. Fresh snow is blown
away and ice at the surface sublimates. Several hundred meteorites have
been collected from these "blue ice" areas. They are carried by the ice
from where they fell, accumulate where the flow is blocked, and are then
exposed at the surface by ablation.