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Geomorphological processes
S. Brooks
GY2150, 2790150
2011
Undergraduate study in
Economics, Management,
Finance and the Social Sciences
This is an extract from a subject guide for an undergraduate course offered as part of the
University of London International Programmes in Economics, Management, Finance and
the Social Sciences. Materials for these programmes are developed by academics at the
London School of Economics and Political Science (LSE).
For more information, see: www.londoninternational.ac.uk
This guide was prepared for the University of London International Programmes by:
Dr S. Brooks, Senior Lecturer in Physical Geography, School of Geography, Birkbeck, University
of London.
This is one of a series of subject guides published by the University. We regret that due to
pressure of work the author is unable to enter into any correspondence relating to, or arising
from, the guide. If you have any comments on this subject guide, favourable or unfavourable,
please use the form at the back of this guide.
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Published by: University of London
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Reprinted with minor revisions 2011
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Contents
Contents
Chapter 1: The subject guide................................................................................... 1
Aims and objectives........................................................................................................ 2
Learning outcomes......................................................................................................... 2
How to use this subject guide......................................................................................... 2
Structure of the guide..................................................................................................... 3
Essential reading............................................................................................................ 5
Further reading............................................................................................................... 5
Online study resources.................................................................................................... 8
Examination structure..................................................................................................... 9
Examination advice...................................................................................................... 10
Syllabus........................................................................................................................ 11
Chapter 2: Introduction......................................................................................... 13
Aims of the chapter...................................................................................................... 13
Learning outcomes....................................................................................................... 13
Essential reading.......................................................................................................... 13
Further reading............................................................................................................. 13
Physical geography: the biosphere, atmosphere, lithosphere and hydrosphere................ 14
Geomorphology: the realm of the lithosphere................................................................ 15
Geomorphological processes that shape the earth......................................................... 18
The physical regions of the earth................................................................................... 19
Long-term changes in the earth’s relief......................................................................... 20
A reminder of your learning outcomes........................................................................... 21
Sample examination questions...................................................................................... 21
Chapter 3: Denudation: the wearing away of the surface of the earth................ 23
Aims of the chapter...................................................................................................... 23
Learning outcomes....................................................................................................... 23
Essential reading.......................................................................................................... 23
Further reading............................................................................................................. 23
Introduction: why is the ground surface lowering?......................................................... 24
The processes of ground surface lowering..................................................................... 25
The rate of ground surface lowering.............................................................................. 25
The process of isostatic readjustment............................................................................ 26
Climatic control of denudation...................................................................................... 27
Conclusions.................................................................................................................. 28
The work of Dr Vladimir Köppen................................................................................... 28
Global variation in sediment yield................................................................................. 29
Explaining global variations in sediment yield................................................................ 30
Measurement of denudation......................................................................................... 33
A reminder of your learning outcomes........................................................................... 36
Sample examination questions...................................................................................... 36
Chapter 4: Uplift of landsurfaces in the development of
long-term geomorphological models.................................................................... 37
Aims of the chapter...................................................................................................... 37
Learning outcomes....................................................................................................... 37
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150 Geomorphological processes
Essential reading.......................................................................................................... 37
Further reading............................................................................................................. 37
Introduction: long-term change in landsurface elevation................................................ 38
The processes of uplift.................................................................................................. 39
The balance between uplift and denudation.................................................................. 39
The examples of Japan and New Zealand...................................................................... 40
The contribution of geomorphology to studying long-term landscape
development of the earth............................................................................................. 41
A reminder of your learning outcomes........................................................................... 47
Sample examination questions...................................................................................... 47
Chapter 5: Short-term process studies in geomorphology................................... 49
Aims of the chapter...................................................................................................... 49
Learning outcomes....................................................................................................... 49
Essential reading.......................................................................................................... 49
Further reading............................................................................................................. 49
Introduction: materials at the surface of the earth......................................................... 50
The soil profile and its role in surface erosion................................................................ 51
Other reasons why soils are important to geomorphology.............................................. 52
The formation of soil profiles......................................................................................... 53
Hydrological behaviour of soil profiles........................................................................... 54
Temporal change in soil profiles.................................................................................... 56
Conclusions.................................................................................................................. 58
A reminder of your learning outcomes........................................................................... 58
Sample examination questions...................................................................................... 59
Chapter 6: Sediment entrainment and transport on hillslopes:
hillslope erosion.................................................................................................... 61
Aims of the chapter...................................................................................................... 61
Learning outcomes....................................................................................................... 61
Essential reading.......................................................................................................... 61
Further reading............................................................................................................. 61
Introduction: processes that shape hillslopes................................................................. 62
The processes of erosion on hillslopes........................................................................... 63
The balance between precipitation and infiltration........................................................ 64
Soil characteristics important for erosion....................................................................... 67
The processes of erosion and resulting slope forms........................................................ 68
Temporal change in soil profiles.................................................................................... 69
A reminder of your learning outcomes........................................................................... 70
Sample examination questions...................................................................................... 70
Chapter 7: Drainage basins: their role as a sediment transfer system................. 71
Aims of the chapter...................................................................................................... 71
Learning outcomes....................................................................................................... 71
Essential reading.......................................................................................................... 71
Further reading............................................................................................................. 71
Introduction: the study of rivers..................................................................................... 72
Sediment transport within rivers.................................................................................... 75
Channel networks: a morphometric approach to drainage basin analysis....................... 77
The river as a sediment routing system.......................................................................... 81
A reminder of your learning outcomes........................................................................... 81
Sample examination questions...................................................................................... 82
ii
Contents
Chapter 8: Long-term environmental change....................................................... 83
Aims of the chapter...................................................................................................... 83
Learning outcomes....................................................................................................... 83
Essential reading.......................................................................................................... 83
Further reading............................................................................................................. 83
Introduction: long-term change in sediment routing systems.......................................... 84
Environmental change in the Quaternary....................................................................... 85
Causes of environmental change ................................................................................. 86
Geomorphological ideas relating to long-term environmental change............................ 86
Morphogenetic landforms............................................................................................. 88
Conclusion................................................................................................................... 89
A reminder of your learning outcomes........................................................................... 90
Sample examination questions...................................................................................... 90
Chapter 9: Short-term environmental change and geomorphology..................... 91
Aims of the chapter...................................................................................................... 91
Learning outcomes....................................................................................................... 91
Essential reading.......................................................................................................... 91
Further reading............................................................................................................. 91
Introduction: changing time and space scales................................................................ 92
Short-term environmental change over ‘graded’ time..................................................... 93
Vegetation: its role in the geomorphic system................................................................ 93
Selective logging of tropical rain forests and
their changing ............................................................................................................. 95
Soil erosion and vegetation........................................................................................... 97
Vegetation change and slope failure in the Holocene..................................................... 98
A reminder of your learning outcomes......................................................................... 100
Sample examination questions.................................................................................... 100
Appendix 1: Sample examination paper ............................................................ 101
Appendix 2: Guidance on answering the Sample examination paper................ 103
General remarks......................................................................................................... 103
Comments on individual questions.............................................................................. 104
Appendix 3: Additional references useful to supplement the subject guide...... 109
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150 Geomorphological processes
Notes
iv
Chapter 1: The subject guide
Chapter 1: The subject guide
150 Geomorphological processes is a 200 course offered on the
Economics, Management, Finance and the Social Sciences (EMFSS)
suite of programmes. If taken as part of a BSc degree, 147 Physical
geography: fundamentals of the physical environment must be
passed before this course may be attempted.
Much has been written concerning geomorphological concepts. In
particular the issue of scale is of central concern, since different concepts
are applicable to large-scale, intermediate-scale and small-scale landforms.
Throughout all chapters we consider the sensitivity of landforms to
change. We will also discuss the rate at which landforms change and how
quickly they adjust to a change in external forces. Ideas about recovery
are also relevant, as are issues relating to the magnitude and frequency of
applied forces. Hence this guide will try to integrate many of these ideas
concerning landscape change within a broad, generalised conceptual
framework. This framework provides a theoretical scientific platform for
the discipline of geomorphology.
This guide treats the discipline of geomorphology as being centrally
concerned with sediment cycling on and around the earth. The resulting
landforms are varied in type and scale but together they make for a
huge diversity in the form of the earth’s surface. Instead of being static
features, landforms undergo continual change over geological time, and
it is the aim of the geomorphologist to describe and understand this over
timescales where direct observation is impossible. It is a challenge to
achieve this, but research in the last few decades has made considerable
advances, highlighting the importance of geomorphology in assisting our
understanding of the physical world in which we live.
It is a subject which provides insights and understanding of a variety
of processes on many scales. Geomorphology includes the weathering
of material and its organisation into soil profiles. The way soil profiles
influence water movement through them is of central concern. This affects
the hillslope response to rainfall and the way rivers behave following
heavy rain. It also encompasses processes of erosion at the ground surface
and within the soil profile. These affect the way hillslopes change over
time. Landforms are made up of interlinked surfaces (slopes) so the
understanding of slope erosion allows us to understand how landforms
and landscapes change over geological time.
Many students, when they approach this course, think that the earth is
essentially static and unchanging. They often think that the landscape of
today is what has always been there in the past and is what will persist in
the future. They could not be more wrong. Landscapes are in a continual
state of flux, changing continuously, albeit slowly in many cases.
What students take away from this course is an understanding of, and
appreciation for, the great diversity of the earth’s physical environments.
They include the world’s great deserts (cold as well as hot), tropical
rainforests, temperate forests, coral reefs and other marine environments,
as well as dramatic mountains and cliffs.
It is a particularly relevant course for those of you who want to go on
to careers in environmental management or conservation, education or
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150 Geomorphological processes
government policy making about the effects of climate change on physical
environments, to name but a few examples.
I teach a very similar course where it is offered as a third-year course
for the BSc in Environmental Management, Environmental Science and
Environmental Conservation, as well as for the BA in Geography and
Environment. My particular interests are in soil development and its role
in triggering landslides, the relationship between global climate change
and slope erosional processes, as well as the impact of human action
(through vegetation removal) on accelerated erosion. I have worked in the
field in the river valleys of Scotland, the tropical rainforests of Borneo and
the tectonically active regions of New Zealand.
I hope that you enjoy studying this course.
Aims and objectives
The key objective of the course is to introduce you to the main concepts
and theories of the discipline of geomorphology (the study of the form of
the earth).
The course links these topics by presenting an integrated view of the
functioning of earth surface systems under different climate–soil–
vegetation combinations.
At the end of the course, you should have increased your knowledge of the
following aspects of the earth’s natural environment:
• formation and denudation (wearing away) of the earth’s surface
• geology (study of earth interior, tectonic processes, the resulting
constituent rocks and the different physical environments that arise)
• geomorphology (study of landforms and earth surface processes)
• pedology (study of soil formation and evolution)
• hydrology (study of water movement on and within the earth and the
way this shapes the earth)
• global environmental changes in the Holocene and Pleistocene.
Learning outcomes
On completion of the course you should be able to:
• present and understand key aspects of the physics of earth surface
processes
• present and understand key concepts in environmental and earth
sciences
• recognise and describe the main landforms and physical environments
of the earth
• understand the main processes and materials of terrestrial systems
• understand the effects of short-term and long-term change in the earth
system.
How to use this subject guide
The aim of this subject guide is to help you to interpret the syllabus. It
outlines what you are expected to know for each area of the syllabus and
suggests relevant readings to help you to understand the material.
2
Chapter 1: The subject guide
There are only four main set textbooks which you must read for this course;
much of the information you need to learn and understand is contained in
examples and activities within the subject guide itself. The most important
of these set texts is the book by Holden (2005). It is highly relevant to the
material presented here.
I would recommend that you work through the guide in chapter order,
reading the essential text in parallel (i.e. at the same time). Then, when you
have understood the main learning outcomes, try some of the suggested
activities. You should try to supplement your studies by reading further
where possible. There is a comprehensive reading list at the end of the
guide.
Having said this, it is important that you appreciate that different topics are
not self-contained. There is a degree of overlap between them and you are
guided in this respect by the cross-referencing between different chapters.
In terms of studying this subject, the chapters of this guide are designed as
self-contained units of study, but for examination purposes you need to have
an understanding of the subject as a whole.
At the end of each chapter you will find a checklist of your learning
outcomes which is a list of the main points that you should understand, once
you have covered the material in the guide and the associated readings.
Structure of the guide
Chapter 2
This is an introduction to physical geography: the biosphere, atmosphere,
lithosphere and hydrosphere; the dynamic lithosphere; geomorphological
processes that shape the earth; the physical regions of the earth; and longterm changes in the earth’s relief.
Chapter 3
The answers to the questions posed in Chapter 2 will be explored in detail
in Chapter 3. This chapter is concerned mainly with denudation, the
wearing away of the earth’s surface. It will consider the long-term effects
of natural denudation rates, but also the artificially enhanced erosion rates
(accelerated erosion) resulting from human action. To place the human
contribution to denudation in its proper context, geomorphologists can offer
a long-term perspective that takes account of different scales of analysis.
This is the geomorphologist’s major contribution to the understanding of the
earth.
Chapter 4
Denudation and uplift are closely linked through the mechanism of isostatic
equilibrium (or readjustment). Chapter 4 will address the process of isostasy
in detail, by considering the movement of the earth’s plates, the types of
rocks that have formed over the earth’s history and predictions for change in
future.
Chapter 5
As well as knowing about rates of change in the earth’s surface, we need
some detailed knowledge of the processes that lead to the movement and
redeposition of sediment over the earth. The first stage is in the preparation
of sediment for movement. This involves the breakdown of rocks at the
surface of the earth and the formation of soil profiles. This will form the
basis for Chapter 5.
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150 Geomorphological processes
Chapter 6
The resulting land cover (soil and vegetation system) affects the processes
of sediment entrainment (or initiation of movement). The agents of
sediment transportation are water, wind and ice. This relative order of
importance varies from time to time and place to place, but they all have
the effect of shaping and eroding the surface of the earth. Chapter 6 will
consider the effect of these agents, highlighting how sediment is entrained
and removed from the drainage basin, especially under the action of
fluvial (water-based) processes.
Chapter 7
Sediment is deposited when the force provided by the water, wind or
ice that is carrying it becomes insufficient to overcome the strength (or
resistance) of the moving particles. Much sediment is deposited within
drainage basins en route from mountains to oceans. Deltas, alluvial
fans and sand dunes are just a few examples of depositional landforms.
Ultimately most sediment ends up in the ocean basins, collecting in large
sub-marine depressions called geosynclines. Drainage basins and offshore
zones combine to form closed sediment routing systems. Discussion of the
behaviour of these systems will form the focus for Chapter 7.
Chapter 8
Landforms of erosion, transport and deposition make up the diverse
form of the earth. In this guide we will exemplify the changing shape of
the earth resulting from the movement and redistribution of sediment.
Geomorphology involves the study of landforms, and several temporal
and spatial scales are relevant to landform diversity and change. The
world’s landscapes show considerable diversity, the oceans have fluctuated
relative to the land, there have been numerous glacial–interglacial cycles
and pluvial periods have interspersed with interpluvial pluvial periods
in the tropics. Global geomorphology is highly complex. Chapter 8 will
discuss the nature of long-term, large-scale environmental change that
has typified the Quaternary. One important aspect of this chapter is the
way different landforms may reflect ancient processes no longer operating
(relict processes and associated forms). Examples of relict landforms can
be found in many parts of the world. We therefore consider the extent
to which it is possible to use these relict landforms to investigate global
change in the long term.
Chapter 9
Short-term change in landforms and processes is linked to changing
climate in modern times, as well as to direct human interference with both
the climate and the vegetation systems of the world. Chapter 9 considers
such questions as ‘to what extent is human action causing irreversible and
significant changes in global sediment movement and geomorphological
processes?’ or ‘which elements of a highly complex landscape are most
vulnerable to human action and over what timescale?’
4
Chapter 1: The subject guide
Essential reading
The following texts form the essential reading requirement for this course.
They provide vital supplementary information to that contained in the
subject guide. The examination in 150 Geomorphological processes
will require answers that do not simply reproduce information from
this guide, but demonstrate wider knowledge gained from additional
background reading.
Allen, P.A. Earth Surface Processes. (Oxford: Blackwell Science, 1997) [ISBN
0632035072].
Holden, J. (ed.) Physical Geography and the Environment. (Harlow: Pearson
Education Ltd, 2005) [ISBN 0131217615]. You are strongly advised to keep
abreast of the websites listed at the end of chapters in this textbook as they
are relevant to each of the topics in the guide.
Selby, M.J. Hillslope Materials and Processes. (Oxford: Oxford University Press,
1993) second edition [ISBN 0198741839].
Summerfield, M.A. Global Geomorphology. (London: Longman, 1991) [ISBN
0582301564]
A reading list is also provided in the subject guide on a chapter-by-chapter
basis so you can structure reading around the themes introduced in the
guide. I have also provided an additional reference list as an appendix to
the guide in case you wish to pursue topics even further. I hope you find
this helpful.
Finally, it should be noted that this subject builds on previous knowledge
and understanding you will have gained in studying for the prerequisite
courses if you are studying this course as part of a BSc Degree.
Further reading
Please note that as long as you read the Essential reading you are then free
to read around the subject area in any text, paper or online resource. You
will need to support your learning by reading as widely as possible and by
thinking about how these principles apply in the real world. To help you
read extensively, you have free access to the virtual learning environment
(VLE) and University of London Online Library (see below).
Strongly recommended Further reading:
Gilvear, D.J. ‘Fluvial geomorphology and river management’, in Holden, J.
(ed.) An Introduction to Physical Geography and the Environment. (Harlow:
Pearson Education Ltd, 2005) [ISBN 0131217615], Chapter 14.
Grace, J. ‘Humans and environmental change’ in Holden, J. (ed.) An
Introduction to Physical Geography and the Environment. (Harlow: Pearson
Education Ltd, 2005) [ISBN 0131217615] Chapter 21.
*Slaymaker, O. and T. Spencer Physical Geography and Global Environmental
Change. (Harlow: Longman, 1998) [ISBN 0582298296]. * Strongly
recommended additional reading.
Strahler, A. and A. Strahler Physical Geography: science and systems of the
human environment. (New York: Wiley, 2002) second edition [ISBN
0471238007].
Thomas, D.S.G. and A. Goudie (eds) The Dictionary of Physical Geography.
(Oxford: Blackwell, 2000) third edition [ISBN 0631204733].
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150 Geomorphological processes
Full list of Further reading
Journals
Ahnert, F. ‘Functional relationships between denudation, relief and uplift’,
American Journal of Science 268 (1970), pp.243–63.
Ahnert, F. ‘Local relief and the height limits of mountain ranges’, American
Journal of Science 284 (1984), pp.1035–55.
Arnett, R.R. and A.J. Conacher ‘Drainage basin expansion and the 9-unit
landsurface model’, The Australian Geographer 12 (1973), pp.237–49.
Arya, L.M. and J.F. Paris ‘A physicoempirical model to predict the soil moisture
characteristic from particle size and bulk density data’, Soil Science Society
of America Journal 45 (1981), pp.1023–30.
Bockheim, J.G. ‘Solution and use of chronofunctions in studying soil
development’, Geoderma 24 (1980), pp.71–85.
Bonnell, M., S.D. Cassells and D.A. Gilmour ‘Vertical soil water movement in
a tropical rainforest catchment in northeast Queensland’, Earth Surface
Processes and Landforms 8 (1983), pp.253–72.
Brooks, S.M. ‘Slopes and slope processes: research over the past decade’,
Progress in Physical Geography, 3:27 (2003), pp.130–41.
Brooks, S.M., K.S. Richards and M.G. Anderson ‘Approaches to the study
of hillslope development due to mass movement’, Progress in Physical
Geography 17 (1993), pp.32–49.
Brunsden, D. ‘Applicable models of longterm landform evolution’, Zeitschrift für
Geomorphologie SB 36 (1980), pp.16–26.
Brunsden, D. and R.H. Kesel ‘The evolution of a Mississippi river bluff in
historic time’, Journal of Geology 81 (1973), pp.576–97.
Butler, B.E. ‘Periodic phenomena in landscapes as a basis for soil studies’,
CSIRO Australian Soil Publication 14 (1959).
Chorley, R.J. ‘The geomorphic significance of some Oxford soils’, American
Journal of Science 257 (1959), pp.503–15.
Dalrymple, J.B., R.J. Blong and A.J. Conacher ‘A hypothetical 9-unit landsurface
model’, Zeitschrift für Geomorphologie 12 (1968), pp.60–76.
Davis, W.M. ‘The geographical cycle’, Geographical Journal 14 (1899),
pp.481–504.
Douglas, I. ‘Man, vegetation and the sediment yield of rivers’, Nature 215
(1967), pp.925–28.
Dunne, T. and R.D. Black ‘Partial area contributions to storm runoff in a small
New England watershed’, Water Resources Research 6 (1970),
pp.1296–1311.
Flenley, J.R. ‘The Late Quaternary vegetational history of the equatorial
mountains’, Progress in Physical Geography 3 (1979), pp.488–509.
Foster, I.D.L., J.A. Dearing, A. Simpson and A.D. Carter ‘Lake catchment
based studies of erosion and denudation in the Merevale catchment,
Warwickshire’, Earth Surface Processes and Landforms 10 (1985),
pp.45–68.
Gupta, A. ‘The changing geomorphology of the humid tropics’, Geomorphology
7 (1993), pp.165–186.
Harvey, A.M., F. Oldfield, A.F. Baron and G.W. Pearson ‘Dating of postglacial
landforms in the central Howgills’, Earth Surface Processes and Landforms 6
(1981), pp.401–12.
Herwitz, S. R. ‘Interception storage capacities of tropical rainforest canopy
types’, Journal of Hydrology 77 (1985), pp.237–52.
Hewlett, J.D. and A.R. Hibbert ‘Mositure and energy conditions within a
sloping mass during drainage’, Journal of Geographical Research 68 (1963),
pp.1081–87.
Hewlett, J.D. and J.D. Helvey ‘The effects of forest clear-cutting on the storm
hydrograph’, Water Resources Research 6 (1970), pp.768–82.
6
Chapter 1: The subject guide
Horton, R.E. ‘Erosional development of streams and their drainage basins:
hydrophysical approach to quantitative morphology’, Bulletin of the
Geological Society of America 56 (1945), pp.275–370.
Horton, R.E. ‘The role of infiltration in the hydrological cycle’, Transactions of
the American Geophysical Union 14 (1933), pp, 446–60.
King, L.C. ‘Canons of landscape evolution’, Bulletin of the Geological Society of
America 64 (1953), pp.721–52.
Kirkby, M.J. ‘Hillslope runoff processes and models’, Journal of Hydrology 100
(1988), pp.315–39.
Kirkby, M.J. and R.J. Chorley ‘Throughflow, overland flow and erosion’, Bulletin
of the International Association of Scientific Hydrology 12 (1967), pp.5–21.
Kleiss, F.J. ‘Loess distribution along the Illinois soil development sequence’, Soil
Science 115 (1970), pp.194–98.
Langbein, W.B. and S.A. Schumm ‘Yield of sediment in relation to mean
annual precipitation’, American Geophysical Union Transactions 39 (1958),
pp.1076–84.
Ohmori, H. ‘Erosion rates and their relationship to vegetation from the
viewpoint of worldwide distribution’, Bulletin of the Department of
Geography, University of Tokyo 15 (1983), pp.77–91.
O’Loughlin, E.M. ‘Saturation regions in catchments and their relations to soil
and topographic properties’, Journal of Hydrology 53 (1981), pp.229–46.
Page, M.J. and N.A. Trustrum ‘A late Holocene lake sediment record of the
erosion response to landuse change in a steepland catchment, New
Zealand’, Zeitschrift für Geomorphologie 41 (1997), pp.369–92.
Philip, J.R. ‘The theory of infiltration’, Soil Science 83 (1958), pp.345–57.
Ritter, D.F. ‘Landscape analysis and the search for geomorphic unity’, Geological
Society of America Bulletin 100 (1988), pp.160–71.
Saunders, I. and A. Young ‘Rates of surface processes on slopes, slope retreat
and denudation’, Earth Surface Processes and Landforms 8 (1983),
pp.473–501.
Saxton, K.E., W.J. Rawls, J.S. Romberger and R.I. Papendick ‘Estimating
generalised soil-water characteristics from texture’, Soil Science Society of
America Journal 50 (1986), pp.1031–36.
Schumm, S.A. ‘Episodic erosion: a modification of the geographical cycle’ in
Melhorn, W.N. and R.C. Flemal (eds) Theories of landform development.
Proceedings of the Sixth Annual Geomorphology Symposia, 1975, pp.69–
85. (London and Boston: Allen and Unwin, 1975).
Schumm, S.A. ‘The role of creep and rainwash in the retreat of badland slopes’,
American Journal of Science 254 (1956), pp.693–706.
Schumm, S.A. and R.W. Lichty ‘Time, space and causality in geomorphology’,
American Journal of Science 263 (1965), pp, 110–19.
Shreve, R.L. ‘Infinite topologically random networks’, Journal of Geology 75
(1967), pp.178–186.
Stoddart, D.R. ‘Climatic geomorphology: review and reassessment’, Progress in
Geography 1 (1969), pp.159–222.
Van Genuchten, M.Th. ‘A closed-form equation for predicting the hydraulic
properties of unsaturated soils’, Soil Science Society of America Journal 44
(1980), pp.892–98.
Walker, P.H. ‘Postglacial environments in relation to landscapes and soils’, Iowa
Agricultural Experimental Station Research Bulletin 549 (1966).
Wilson, L. ‘Variations in mean annual sediment yield as a function of mean
annual precipitation’, American Journal of Science 273 (1973), pp.335–49.
Wolman, M.G. and J.P. Miller ‘Magnitude and frequency of forces in
geomorphic processes’, Journal of Geology 68 (1960), pp.54–74.
Wolman, M.G. and R. Gerson ‘Relative scales of time and effectiveness
of climate in watershed geomorphology’, Earth Surface Processes and
Landforms 3 (1978), pp.189–208.
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150 Geomorphological processes
Books
Ahnert, F. ‘Modelling landform change’ in Anderson, M.G. (ed.) Modelling
Geomorphological Systems. (Chichester: Wiley, 1988) [ISBN 0471918008].
Anderson, M.G. and T.P. Burt ‘Subsurface runoff’ in Anderson, M.G. and T.P.
Burt (eds) Process Studies in Hillslope Hydrology. (Chichester: Wiley, 1990)
[ISBN 0471927147], Chapter 11.
Brooks, S.M., K.S. Richards and M.G. Anderson ‘Shallow failure mechanisms
during the Holocene: utilisation of a coupled soil hydrology-slope stability
model’ in Thomas, D.S.G. and R.J. Allison (eds) Landscape Sensitivity.
(Chichester: Wiley, 1993) [ISBN 0471936367], Chapter 10.
Ellis, S. and K.S. Richards ‘Pedogenic and geotechnical aspects of Late
Flandrian slope instability in Ulvadalen, West Central Norway’ in Richards,
K.S., R.R. Arnett and S. Ellis (eds) Geomorphology and Soils. (London, Allen
and Unwin, 1985) [ISBN 0045510938].
Jenny, H. Factors of soil formation. (London: Constable, 1994; first published
New York, 1941) [ISBN 0486681289].
Kirkby, M.J. ‘Infiltration, throughflow and overland flow’ in Chorley, R.J. (ed.)
Introduction to Physical Hydrology. (London: Methuen, 1977) [ISBN 04166
88101], Chapter 5.
Penck, W. Morphological analysis of landforms. Translated by H. Czech and K.C.
Boswell. (London: Macmillan, 1971; first published 1953)
[ISBN 0028501306].
Roberts, N. The Holocene: an environmental history. (Oxford: Blackwell, 1998)
second edition [ISBN 0631186387].
Strahler, A.N. ‘Quantitative geomorphology of drainage basins and channel
networks’ in Chow, V.T. (ed) Handbook of Applied Hydrology. (New York:
McGraw Hill, 1964) [ISBN 0070107742].
Street-Perrott, F.A., N. Roberts and S. Metcalfe ‘Geomorphic implications of late
Quaternary hydrological and climatic changes in the northern hemisphere
tropics’ in Douglas, I. and T. Spencer (eds) Environmental Change and
Tropical Geomorphology. (London: Allen and Unwin, 1985) [ISBN
0045510741] pp.165–83.
Walling, D.E. and B.W. Webb ‘Patterns of sediment yield’ in Gregory, K.J. (ed)
Background to Palaeohydrology. (Chichester: Wiley, 1983)
[ISBN 0471901792], pp.69–100.
Young, A. and I. Saunders ‘Rates of surface processes and denudation’ in
Abrahams, A.D. (ed.) Hillslope Processes. (Boston and London: Allen and
Unwin, 1986) [ISBN 0045511020], pp.3–27.
Online study resources
In addition to the subject guide and the Essential reading, it is crucial that
you take advantage of the study resources that are available online for this
course, including the VLE and the Online Library.
You can access the VLE, the Online Library and your University of London
email account via the Student Portal at:
http://my.londoninternational.ac.uk
You should receive your login details in your study pack. If you have not,
or you have forgotten your login details, please email uolia.support@
london.ac.uk quoting your student number.
The VLE
The VLE, which complements this subject guide, has been designed to
enhance your learning experience, providing additional support and a
sense of community. It forms an important part of your study experience
with the University of London and you should access it regularly.
8
Chapter 1: The subject guide
The VLE provides a range of resources for EMFSS courses:
• Self-testing activities: Doing these allows you to test your own
understanding of subject material.
• Electronic study materials: The printed materials that you receive from
the University of London are available to download, including updated
reading lists and references.
• Past examination papers and Examiners’ commentaries: These provide
advice on how each examination question might best be answered.
• A student discussion forum: This is an open space for you to discuss
interests and experiences, seek support from your peers, work
collaboratively to solve problems and discuss subject material.
• Videos: There are recorded academic introductions to the subject,
interviews and debates and, for some courses, audio-visual tutorials
and conclusions.
• Recorded lectures: For some courses, where appropriate, the sessions
from previous years’ Study Weekends have been recorded and made
available.
• Study skills: Expert advice on preparing for examinations and
developing your digital literacy skills.
• Feedback forms.
Some of these resources are available for certain courses only, but we
are expanding our provision all the time and you should check the VLE
regularly for updates.
Making use of the Online Library
The Online Library contains a huge array of journal articles and other
resources to help you read widely and extensively.
To access the majority of resources via the Online Library you will either
need to use your University of London Student Portal login details, or you
will be required to register and use an Athens login:
http://tinyurl.com/ollathens
The easiest way to locate relevant content and journal articles in the
Online Library is to use the Summon search engine.
If you are having trouble finding an article listed in a reading list, try
removing any punctuation from the title, such as single quotation marks,
question marks and colons.
For further advice, please see the online help pages:
www.external.shl.lon.ac.uk/summon/about.php
Examination structure
Important: the information and advice given here are based on the
examination structure used at the time this guide was written. Please
note that subject guides may be used for several years. Because of this
we strongly advise you to always check both the current Regulations for
relevant information about the examination, and the VLE where you
should be advised of any forthcoming changes. You should also carefully
check the rubric/instructions on the paper you actually sit and follow
those instructions.
9
150 Geomorphological processes
Remember, it is important to check the VLE for:
• up-to-date information on examination and assessment arrangements
for this course
• where available, past examination papers and Examiners’ commentaries
for the course which give advice on how each question might best be
answered.
The examination paper for this course is three hours in duration and you
are expected to answer three questions, from a choice of seven. The first
part of the examination offers a choice of short answer questions, which
should take approximately 15–20 minutes each. You are expected to select
three short answers from a total of six possibilities. The second part of
the examination offers you a choice of two essay questions chosen from
six possibilities. You must attempt to answer two essay questions as fully
as possible, using diagrams where appropriate, allowing approximately
50–60 minutes for each answer. Equal weighting is given to each essay
question, and the three short answers together will be weighted the
same as the essay questions. The examiner attempts to ensure that all
of the topics covered in the syllabus and subject guide are examined.
Some questions could cover more than one topic from the syllabus since
the different topics are not self-contained. A sample examination paper
appears as an appendix to this guide, along with a sample Examiners’
commentaries.
The Examiners’ commentaries contain valuable information about how to
approach the examination and so you are strongly advised to read them
carefully. Past examination papers and the associated reports are valuable
resources when preparing for the examination.
You should ensure that all three questions are answered fully, allowing an
approximately equal amount of time for each question and attempting all
parts or aspects of a question.
Examination advice
In approaching this examination the most important thing to remember
is that even if you know and fully understand the material, if you cannot
clearly convey this to the examiner then you will not gain the marks. You
must ensure that you answer the question that is actually set and not
simply write everything you know about the topic. Read all the questions
carefully before making your choice. Then re-read the ones you have
selected to answer, underlining the key words and considering carefully
what the answer should contain. Include a short introductory paragraph
that leaves the examiner in no doubt that you have understood the
question. Then develop your main arguments (and counter-arguments) in
the main body of the essay. Write a concluding paragraph that summarises
what you have discussed and again answers the question.
Diagrams can be very helpful in conveying ideas. They also save you
time as they can include a lot of information if drawn carefully, but make
sure that they are clear and concise. Do not clutter them with irrelevant
material. Label diagrams clearly and make sure that you refer to them
where appropriate in the main part of your text.
Keep to the point and let your arguments flow. Don’t be afraid to express
opinions or develop arguments of your own. Provided they are justified
by appropriate facts, your own opinions are often more likely to gain you
marks than simply repetition of notes taken from the subject guide. At this
stage in your studies, you should be developing your own ideas based on a
10
Chapter 1: The subject guide
range of material written by a range of authors. Try to include the breadth
of material you have studied so you can gain credit for the extra hard work
you have put in over the year.
Syllabus
Denudation and erosion of the earth: operation denudational processes,
variability in denudation rates, factors causing variation in denudation,
pattern of global denudation, measuring denudation, the relationship
complex between denudation and climate.
Tectonic processes: the lithosphere and its long-term, dynamic nature,
processes of isostatic readjustment, the relationship between uplift and
denudation, long-term landscape development, the geomorphological
models proposed by Davis, Penck, King and Schumm.
Weathering and soil development: soil profiles and their fundamental role
in the operation of hydrological and erosional processes, soil profiles
development over time (vertical and lateral water movement), the study of
chronosequences.
Links between soils and slopes: hydrology and erosion at the ground surface
and within the soil profile, links between slope process and form.
Fluvial hillslope processes including mass movement: runoff production and
hillslope erosion, processes of rainsplash and surface wash, shallow and
deep-seated mass movement.
Drainage basins and sediment routing systems: the role of river action in
shaping landscapes, different approaches to the study of drainage systems.
The impact of Quaternary environmental change on landscapes and
geomorphological processes: Quaternary events involving frequent ice
ages linked to the changing orbit of the earth around the sun, climate
change due to lowering of sea level (ocean circulation), vegetation change
(altered albedo), variations in solar output and changes in atmospheric
gases, vegetation change in the Holocene.
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150 Geomorphological processes
Notes
12
Chapter 2: Introduction
Chapter 2: Introduction
Aims of the chapter
The aims of this chapter are:
• to introduce you to the diversity of the earth
• to enable you to learn about some of the main processes that shape the
earth
• to show how the lithosphere is dynamic over geological time
• to consider how the lithosphere acts as a platform for the earth’s
physical regions.
Learning outcomes
By the end of this chapter, and having completed the Essential readings
and activities, you should be able to:
• explain the sub-disciplines covered within physical geography
• understand the relationship between geomorphology and physical
geography
• describe what is meant by the lithosphere and be able to appreciate its
dynamic nature
• list some of the main processes that shape the earth’s surface
• explain why it is important to include both long and short timescales in
any explanation of the earth’s evolution
• offer a brief explanation of why the earth has different physical regions.
Essential reading
Holden, J. Physical Geography and the Environment. (Harlow: Pearson
Education Ltd, 2005) [ISBN 0131217615], Chapter 1.
Summerfield, M.A. Global Geomorphology. (London: Longman, 1991) [ISBN
0582301564], Chapter 2.
Further reading
Chorley, R.J., S.A. Schumm and D.E. Sugden Geomorphology. (London:
Methuen, 1984) [ISBN 0416325904]
Schumm, S.A. and R.W. Lichty ‘Time, space and causality in geomorphology’,
American Journal of Science 263 (1965), pp.110–19.
Slaymaker, O. and T. Spencer Physical Geography and Global Environmental
Change. (Harlow: Longman, 1998) [ISBN 0582298296], Chapter 1.
Strahler, A. and A. Strahler, Physical Geography: science and systems of the
human environment. (New York: Wiley, 2002) second edition [ISBN
0471238007], Chapter 2.
Wolman, M.G. and J.P. Miller ‘Magnitude and frequency of forces in
geomorphic processes’, Journal of Geology 68 (1960), pp.54–74.
Wolman, M.G. and R. Gerson ‘Relative scales of time and effectiveness
of climate in watershed geomorphology’, Earth Surface Processes and
Landforms 3 (1978), pp.189–208.
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150 Geomorphological processes
Physical geography: the biosphere, atmosphere,
lithosphere and hydrosphere
To begin with we can suggest a few pertinent questions that might help
you clarify in your minds some of the main issues for the geomorphologist.
These questions provide a good starting point for you, so try to answer
them briefly and clearly:
• How old is the earth?
• How high are the highest mountains on earth (or beneath the oceans)?
• What processes are responsible for the erosion of the earth?
• How fast is the surface of the earth being lowered where erosion rates
are highest on the earth?
• How do we measure erosion rates?
• Why is it so difficult to come up with a reliable estimate of the rate of
earth surface lowering?
• How long might it take to remove the highest mountains of the earth if
current rates of erosion continue unchanged?
• Consider the term isostatic readjustment. What do you understand this
to be?
• How do ground surface lowering and isostatic readjustment act
together?
• So why isn’t the earth gradually disappearing through the erosion
processes you have identified above?
From your studies so far you should have become familiar with the
discipline of ‘Physical Geography’. Briefly, you should recall that the study
of physical geography involves four great realms and their associated
processes. The realms are as follows:
1. the biosphere
2. the atmosphere
3. the lithosphere
4. the hydrosphere.
These realms are clearly identified in all the main physical geography
textbooks you are likely to come across, including those listed above
(Holden, 2005; Slaymaker and Spencer, 1998; Strahler and Strahler, 2002;
Summerfield, 1991).
These four realms have associated sub-disciplines through which the
main processes are studied and synthesised. Hence the biosphere is
associated with the sub-discipline of biogeography, ecology and pedology
(soil processes). Likewise the atmosphere is associated with the subdiscipline of climatology, the lithosphere with geomorphology and, finally,
the hydrosphere with hydrology (Slaymaker and Spencer, 1998). These
four crucial fields of science are interlinked through fluxes of mass and
energy. Hence, one of the pioneering publications for physical geography
remains that of Chorley and Kennedy (1971) ‘Physical Geography: a
systems approach’, since it provides the first real integration of these
apparently separate fields. Despite being published over 30 years ago,
Slaymaker and Spencer (1998) still lament the fact that one of the main
failings in the teaching of modern physical geography is in the continual
separation of these four main components of biosphere, atmosphere,
lithosphere and hydrosphere. This subject guide, largely focused on the
14
Chapter 2: Introduction
third sub-discipline of geomorphology (and the processes operating to
erode and shape the lithosphere), aims to emphasise the links with the
other three sub-disciplines, while broadening your specialist knowledge
of geomorphological processes. It is impossible to understand how the
lithosphere is shaped without considering the role of the hydrosphere in
delivering the water that carries out the erosion. Likewise the significance
of environmental change cannot be understood without considering
atmospheric processes and their changes during the Quaternary. Finally,
soil profiles and vegetation cover provide protection to the lithosphere
and moderate the surface processes. These are fundamental and will
be discussed throughout this subject guide as we explore the realm of
geomorphology.
In the twenty-first century there is growing concern for the state of the
earth, especially in terms of the sustainability of resources and possible
irreversible effects of human intervention in physical systems. One of the
main contributions geomorphologists can make to the debate is through
examining both long-term and short-term effects of human action on the
operation of geomorphological processes. Clearly, this requires knowledge
and understanding of all of the above four fields, and a sensible
appreciation of their interlinkages. They cannot be seen as separate
entities but require full appreciation of the linkage and interaction. This
needs to be done at different scales in space and time. Geomorphological
processes acting in the present day are, in part, influenced by processes
that have acted in the past. These provide the present-day landscape
upon which the processes act. Hence, a major contribution from the
geomorphologist to the understanding of current earth surface processes is
through scale linkage, integrating the past with the present.
Geomorphology: the realm of the lithosphere
It is clear from the distinctions made above between the four main subdisciplines of physical geography that geomorphology may be equated
with the lithosphere. A useful starting point in defining the business of the
geomorphologist is to consider exactly what is meant by lithosphere.
According to Strahler and Strahler (2002), the lithosphere is the solid
earth, forming a stable platform for the life layer (or biosphere). It bears
a shallow layer of soil, providing nutrients for organisms, in which plants
take root. The authors add the significant point that the lithosphere is
sculpted into surface landforms that provide diverse and complex habitats
for life. Therefore one central concern for geomorphologists is the current
shape of the solid portion of the earth. In a way we are looking at a snapshot in time in an attempt to understand how life is supported by the
lithosphere and how the rich diversity of habitat is maintained and how it
functions. However, there is more!
The dynamic lithosphere
The view expressed above could be considered a static view. The
lithosphere is, in fact, highly dynamic, providing a major challenge for
geomorphologists. You can probably recall from your foundation course in
Physical geography that the earth is composed of a thin, solid crust and a
deeper, more fluid mantle. The crust is made up of several plates, which
move by convection currents in the mantle. Where the plates meet, intense
tectonic activity occurs, either as earthquake activity, volcano formation,
the uplifting of great mountain chains or the rifting apart of sections of
the earth. At the same time, solid portions of the crust disappear into the
mantle replaced by new land. The material that makes up the lithosphere
15
150 Geomorphological processes
is in continual flux: forming, disappearing and reforming continually over
geological time.
This whole idea of the earth being divided into crust and mantle (each
having separate behaviour) is a little outdated now as research pushes
back the frontiers of understanding. Newer models of the dynamic earth
tend to emphasise the distinction between the lithosphere (the traditional
domain of geomorphology) and the underlying asthenosphere, rather than
between crust and mantle.
In the Dictionary of Physical Geography (third edition, 2000) we find
a definition that begins to appreciate and emphasise this dynamic
aspect of the lithosphere in more detail. The newer definition of the
lithosphere involves ‘the earth’s crust and a portion of the upper mantle,
which together constitute a layer of strength relative to the more easily
deformable asthenosphere below’. So the boundary is thought to be diffuse
rather than abrupt and the upper mantle acts with the crust in forming a
solid platform traditionally known as the lithosphere. Things are a little
more complex than a simple crust comprising the lithosphere that the
geomorphologist needs to study. As well as introducing the idea that the
lithosphere is difficult to define precisely, recent research shows that it is
dynamic. Such a complex definition implies the need for geomorphologists
to acknowledge long-term time and space scales and not simply see their
role as understanding the processes that shape the earth’s crust in the
present day. The issue of scale is very important and will be discussed
further throughout this guide.
A more complex definition of the lithosphere
Summerfield (1991) provides an even more comprehensive description
of the lithosphere. This involves its division into zones at the surface of
the earth, with the lithosphere sandwiched between the crust and the
asthenosphere. To all intents and purposes we can take the lithosphere
to include this thin solid crust, as well as part of the more readily
deformable mantle below. The solid crust over the earth on which the
main geomorphological processes act is comparatively thin and easily
deformable under the influence of varying loads (consider the effect of ice
advance or of continual stripping of solid material from the surface). The
crust (solid portion) is around 35 km over the continents but just 5–10 km
over the oceans. The total lithosphere (which includes some of the mantle
below the crust) is of variable thickness, with a maximum depth of 350–
400 km. Like the crust alone, the whole lithosphere is thinner under the
oceans. Finally, we should emphasise the fact that the boundary with the
asthenosphere lying below the lithosphere is diffuse and difficult to define
precisely.
Conclusion
It is clear from this discussion that the lithosphere is far more complex
than just the earth’s solid crust, as it also incorporates the upper part of
the mantle. The main distinction between the asthenosphere and the
lithosphere involves their respective behaviour. The lithosphere moves
by elastic deformation in response to changes in loading at the surface of
the earth. The asthenosphere, on the other hand, moves more gradually,
only responding over long time periods, and may be responsible for the
lateral (sideways) movement of the sections of the lithosphere in the
form of plates. Research has yet to resolve the complex issue of how
exactly plates move and whether it is just the asthenosphere or the whole
mantle involved in the process. What is clear is that the lithosphere may
16
Chapter 2: Introduction
be reincorporated into the asthenosphere over very long periods of time,
making for a highly dynamic earth in which the solid platform for life is
continually renewed and changing in character. This provides a central
focus and challenge for the study of geomorphological processes.
Activity 2.1
Chapter 2 in Summerfield’s book is particularly helpful for this activity.
The table below provides data giving the approximate area of the earth’s surface
occupying different elevation bands. The first column gives the elevation bands (above
and below sea level), the second column gives the total area within each band and the
third column gives the cumulative area.
Elevation with respect to
sea level (km)
% of earth’s surface at
this elevation
Cumulative % of earth’s
surface at this elevation
8–9
0.2
0.2
7–8
0.2
0.4
6–7
0.2
0.6
5–6
0.2
0.8
4–5
0.5
1.3
3–4
1.0
2.3
2–3
2.0
4.3
1–2
5.0
9.3
0–1
20.0
29.3
0–1
8.0
37.3
1–2
3.0
40.3
2–3
6.0
46.3
3–4
14.0
60.3
4–5
24.0
84.3
5–6
14.0
98.3
6–7
1.1
99.4
7–8
0.15
99.55
8–9
0.15
99.70
9–10
0.15
99.85
10–11
0.15
100.00
Above sea level
Below sea level
Plot two graphs:
•• showing elevation (x-axis) versus surface area (y-axis) as a bar graph
•• showing cumulative surface at a particular elevation (x-axis) versus elevation
(y-axis) as a line graph.
What is the maximum height of the earth’s surface?
What is the minimum elevation of the earth’s surface?
Why do the two graphs show distinct elevation groups?
Why is the earth’s elevation distributed as it is?
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150 Geomorphological processes
Geomorphological processes that shape the earth
The preceding section has set the context for us to define the scope of
geomorphology. We now know that geomorphology is concerned with the
understanding of the processes that shape the earth’s crust, operating on
a variety of timescales. Hence over longer time periods geomorphology
crucially embraces the processes which cause the surface of the earth’s
crust to change shape. Some argue that this only involves the land
surface (terrestrial processes), while others would extend this definition
to include sub-marine features as well. There is also a growing call for
geomorphology to concern itself with the surface features evident on
other planets in the solar system, and this has given rise to the study
of planetary geomorphology. Furthermore, processes taking place over
even greater timescales cause the deeper lithosphere to change. Due to
its elastic properties, the lithosphere can stretch and bend (lithospheric
flexuring), causing rifts in the solid crust. Portions of the solid crust can
change their global position (with consequent climate change) and the
crust can be reincorporated into the lithosphere over very long periods of
time. Such processes, effective over millions of years, also fall within the
scope of geomorphology.
Processes that shape the earth’s surface
The processes that cause the earth to change making up the discipline of
geomorphology are immense in scope and scale. Some processes act upon
the visible surface of the earth and others take place within the mantle, as
the lithosphere becomes reincorporated into the asthenosphere. Processes
acting in the short term, such as catastrophic landslides or accelerated soil
erosion are familiar headline-makers and form the basis for much recent
research in geomorphology. However, long-term change in the lithosphere
related to plate movement and global tectonics is also significant and
must be included in the study of the earth’s surface. These processes act
over millions of years, making them more difficult to envisage, study and
measure. Somewhere in between these extremes lie processes that shape
the landscape on an intermediate scale. A good example is glacial erosion,
which has left its mark on the landscape in many parts of the world, but
which is not operating in many of these areas in the present day.
Chorley et al. (1984) describe the physical landscapes studied by
geomorphologists as forming palimpsests of forms. This idea has found
favour among geomorphologists as it provides a useful platform to begin
to unravel the many processes that have contributed to the formation of
a given landscape. Chorley et al. (1984) argue that all landscapes have a
basic platform of large-scale features, such as drainage divides, formed by
processes deep within the lithosphere, operating over millions of years.
These define the landscape at a large spatial scale and respond very slowly
to change.
Superimposed onto this platform are smaller-scale features, nested within
the larger structure. Examples are valley-side slopes, containing relict
(ancient) landslide scars or the imprint of glacial processes. The features
have resulted from processes that have operated in the past but are more
recent than those that formed the overall structure in the first place.
Hence, they are related to shorter timescales and involve smaller-scale
features.
Nested within this are yet smaller features, such as individual drainage
channels (ephemeral or perennial) comprising rills, gullies and permanent
river channels, which have been formed over shorter timescales still. They
18
Chapter 2: Introduction
are smaller in scale and respond quickly to events that are occurring in the
present day. For example, changes in rainfall totals have an impact on the
drainage channels through changes to the amount of water flowing within
them.
Finally, there are even smaller features, such as short stretches of alluvial
channel, that are highly sensitive to change, but also have relatively
high recovery rates from present-day events. Within any given landscape
there is a hierarchy of features, with many such small-scale features,
which respond rapidly to current-day processes, but smaller numbers
of large-scale features taking longer to be formed and eradicated. This
phenomenon is frequently referred to as a ‘nested hierarchy’ of forms, all
resulting from and responding to geomorphological processes operating
at different scales. There have been many publications on this subject, as
well as on the subjects of landscape sensitivity (propensity to change),
magnitude of force required to alter different features (threshold for
change), landscape recovery and whether features are in equilibrium
with present processes (Wolman and Miller, 1960; Schumm and Lichty,
1965; Wolman and Gerson, 1978). These concepts, embedded within
geomorphology, will be discussed more fully in Chapters 8 and 9.
Activity 2.2
Think about the different events that shape the earth, such as present-day rainstorms or
periods of glacial activity in the past. Try to list as many as you can. Then place them in an
approximate rank order, starting with those that occur daily and finishing with rare events
that may happen once or twice over geological time.
Which events do the most work when they occur?
What sorts of recovery times might relate to the impact of each event on the earth’s
surface?
How do these different events (and resulting landscape features) combine to mould the
way the earth evolves over geological time: include both small but frequent events as well
as large, infrequent events and all those in between (remember there is no right or wrong
answer here)?
The physical regions of the earth
Geomorphology is about the physical world, and the processes that shape
and change the lithosphere. To begin with, it is helpful to look at the
world’s major physical regions as they exist in the current day (see Figure
2.4 in Summerfield). The earth is approximately 4.5 billion (million
million) years old. The oldest rocks, dating from the Palaeozoic, form
highly eroded remnants of former active plate boundaries. The continental
lowland interiors, situated away from currently active plate boundaries,
were uplifted in the past when they were located at the plate boundaries.
They have since been eroded and reduced in size. They tend to be made
up from secondary rocks (sedimentary rocks) dating from the Mesozoic
and Cenozoic periods. These secondary rocks are formed when the
primary (igneous) rocks are eroded from the landsurface and redeposited,
often beneath the sea. They are exposed as sea level falls, when they
become new land.
The most active parts of the world in the present day are located at the
plate boundaries, where collision and rifting zones occur. The highest
mountain chains in the world, the Andes, the Alps and the Himalayas, date
from the more recent period of uplift during the Tertiary (or Cenozoic).
These have high relief and are actively being uplifted and eroded in the
present day.
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150 Geomorphological processes
Briefly, the earth can be divided into three zones: the oldest, inactive
mountain remnants, the eroded continental interiors of low relief and the
highest mountains dating from the most recent period of uplift. Of course,
this is a broad generalisation concerning the form of the earth and more
detail will be added later in the guide.
Long-term changes in the earth’s relief
As Geomorphology is about the processes that change the surface
of the earth, we need to consider how erosion (land reduction) and
uplift (land elevation) act together in the long term. This is a complex
area. Erosional processes gradually strip the earth’s surface of its solid
cover, causing large-scale sediment redistribution. This means that the
weight distribution over the earth will change in the long term. The
effect of this on the earth’s relief is complex. One of the most important
processes to understand if you are a geomorphologist is the process of
isostatic equilibrium or readjustment. We have seen how the lithosphere
is more rigid and less dense than the underlying asthenosphere. In
effect the surface layers of the lithosphere ‘float’ on the more mobile
asthenosphere. With added mass, blocks of lithosphere can depress the
asthenosphere and sink into it to some extent. If the lithospheric blocks
are relieved of their mass (possibly by a process of unloading through
erosional processes stripping mass from the surface), they will regain
height. Hence, any loading or unloading at the surface of the earth will
produce a compensation in the depth to which the lithosphere sinks into
the asthenosphere. The vertical movement of these lithospheric blocks is
termed isostatic adjustment. Summerfield (1991) provides an excellent
discussion of this process.
One of the best, and most easily pictured, examples of this process comes
from the recent period of the Quaternary. During this 1.65 million-year
period the earth has been subjected to repeated advances and retreats
of the world’s major ice caps and mountain glaciers. The effect of an
advancing ice mass is to depress the lithosphere, through loading, while a
retreating ice mass will cause isostatic rebound. This gives rise to variable
uplift rates in time and space, according to the movement and thickness
of the ice at its maximum extent. The earth as a whole is still responding
to the effect of the removal of ice following the last Ice Age (which ended
10,000 years ago). There was greater change (and more ice added) in high
latitudes, so the response is greatest there. Over even longer timescales,
the same process occurs in response to the redistribution of sediment over
the surface of the earth. Although responses may be less extensive and
more gradual, they have affected the earth for such a long time that the
net effect is at least as important.
The balance between denudation (the removal of material from the earth’s
surface) and uplift is therefore a central concern for the geomorphologist,
as this is the fundamental control on the changing form of the earth’s
surface. Currently, high denudation rates are found where relief is highest.
These are the tectonically active, more recent mountain chains. High
denudation rates are also linked to climate, as well as to human action in
modifying the landsurface. These issues will be discussed fully in Chapter
3, forming a fundamental starting point for your study of the discipline of
geomorphology.
20
Chapter 2: Introduction
Activity 2.3
Turn again to the questions listed at the start of this chapter. You should be able to
answer some of these questions by now. More information is available in Chapter 3,
but for now see if you can write a sentence or two in answer to these questions before
moving on. If you are stuck, read through Chapter 3 and then return to this activity once
more.
A reminder of your learning outcomes
Having completed this chapter, and the Essential readings and activities,
you should be able to:
• describe the sub-disciplines covered within physical geography
• explain the relationship between geomorphology and physical
geography
• describe what is meant by the lithosphere and be able to appreciate its
dynamic nature
• list some of the main processes that shape the earth’s surface
• explain why it is important to include both long and short timescales in
any explanation of the earth’s evolution
• offer a brief explanation of why the earth has different physical regions.
Sample examination questions
1. Describe and account for the great diversity in the earth’s physical
landscapes.
2. What are the main processes associated with each of the four realms of
the atmosphere, the lithosphere, the hydrosphere and the biosphere?
3. In what ways and for what reasons has the earth changed over the long
term, since its birth about 4.5 billion years ago?
21
150 Geomorphological processes
Notes
22
Chapter 3: Denudation: the wearing away of the surface of the earth
Chapter 3: Denudation: the wearing
away of the surface of the earth
Aims of the chapter
The aims of this chapter are:
• to enable you to understand what denudation is all about
• to show you how denudation rates are highly variable around the earth
• to introduce the complex relationship between climate and denudation.
Learning outcomes
By the end of this chapter, and having completed the Essential readings
and activities, you should be able to:
• define what is meant by denudation
• explain that different processes give rise to denudation but that rates
are highly variable
• describe and account for global variations in denudation rates
• know some different methods for estimating denudation in the long
term
• appreciate the complex relationship between denudation and climate
and why different research has reached different conclusions about this
relationship.
Essential reading
Selby, M.J. Hillslope Materials and Processes. (Oxford: Oxford University Press,
1993) second edition [ISBN 0198741839], Chapter 13.
Summerfield, M.A. Global Geomorphology. (London: Longman, 1991)
[ISBN 0582301564], Chapters 1 and 15.
Further reading
Brunsden, D. and R.H. Kesel ‘The evolution of a Mississippi river bluff in
historic time’, Journal of Geology 81 (1973), pp.576–97.
Douglas, I. ‘Man, vegetation and the sediment yield of rivers’, Nature 215
(1967), pp.925–28.
Foster, I.D.L., J.A. Dearing, A. Simpson and A.D. Carter ‘Lake catchment
based studies of erosion and denudation in the Merevale catchment,
Warwickshire’, Earth Surface Processes and Landforms 10 (1985), pp.45–68.
Langbein, W.B. and S.A. Schumm ‘Yield of sediment in relation to mean
annual precipitation’, American Geophysical Union Transactions 39 (1958),
pp.1076–84.
Ohmori, H. ‘Erosion rates and their relationship to vegetation from the
viewpoint of worldwide distribution’, Bulletin of the Department of
Geography, University of Tokyo 15 (1983), pp.77–91.
Page, M.J. and N.A. Trustrum ‘A late Holocene lake sediment record of the
erosion response to landuse change in a steepland catchment, New
Zealand’, Zeitschrift für Geomorphologie 41 (1997), pp.369–92.
Saunders, I. and A. Young ‘Rates of surface processes on slopes, slope retreat
and denudation’, Earth Surface Processes and Landforms 8 (1983),
pp.473–501.
23
150 Geomorphological processes
Schumm, S.A. ‘The role of creep and rainwash in the retreat of badland slopes’,
American Journal of Science 254 (1956), pp.693–706.
Strahler, A. and A. Strahler Physical Geography: science and systems of the
human environment. (New York: Wiley, 2002) second edition [ISBN
0471238007], Chapter 13.
Walling, D.E. and B.W. Webb ‘Patterns of sediment yield’ in Gregory, K.J. (ed.)
Background to Palaeohydrology. (Chichester: Wiley, 1983)
[ISBN 0471901792], pp.69–100.
Willgoose, G. and S. Riley ‘The long term stability of engineered life forms
of the Ranger Uranium Mine, Northern Territory, Australia: application
of a catchment evolution model’, Earth Surface Processes and Landforms 3
(1998), pp.237–59.
Wilson, L. ‘Variations in mean annual sediment yield as a function of mean
annual precipitation’, American Journal of Science 273 (1973), pp.335–49.
Young, A. and I. Saunders ‘Rates of surface processes and denudation’ in
Abrahams, A.D. (ed.) Hillslope Processes. (Boston and London: Allen and
Unwin, 1986) [ISBN 0045511020], pp.3–27.
Introduction: why is the ground surface lowering?
One of the key questions from Chapter 2 was how fast the surface of the
earth is being lowered. There is no single answer to this question, as great
variability in denudation rates exist from time to time and place to place.
Denudation is the term used to describe the wearing away of the earth’s
surface. It involves the stripping of material at the surface of the earth,
often revealing the underlying rock. Denudation includes two main
processes. First, there is weathering and second, there is erosion of
the weathered fragments of rock. Denudation therefore includes the
preparation of sediment for movement, which takes place when rock is
weathered into soil and the actual movement brought about by the main
agents of denudation. The first phenomenon happens in situ (without
removal) and the second involves both upslope and downslope movement
of material.
There are three ‘agents’ of denudation. The main denudational agent is
water, since it is comprehensively involved in many weathering processes,
as well as in the removal of sediment through erosion. The other agents
of erosion are ice and wind. Although water is the most effective agent of
denudation, the relative importance of the three agents varies from time
to time and place to place. Wind, for example, is most important under
arid conditions (in the cold and hot deserts of the world). Ice is important
in glacial landscapes, so has been more effective in the past than it is in
the present day. Several factors influence the effectiveness, or intensity,
of these agents of denudation, meaning that denudation rates are highly
variable in time and space. The main factors that influence the rate of
denudation are relief, climate, vegetation and human action.
24
Chapter 3: Denudation: the wearing away of the surface of the earth
Activity 3.1
Having read Chapter 2 and completed some preliminary reading, try to answer the
following questions:
How fast is the surface of the earth being lowered?
Can you give a very approximate, average figure?
How might you try to estimate the amount of denudation taking place on the hillslopes
near where you live (there is more than one way)?
The processes of ground surface lowering
Several processes related to the action of water, wind and ice are
responsible for the denudation of the earth. Some occur directly at the
ground surface, such as the combination of rainsplash and surface runoff
that removes splash particles, while others take place within the soil
cover. Examples of this include gradual soil creep and more rapid,
shallow mass movement of soil downslope (shallow landslides). These
geomorphological processes act together to gradually lower the ground
surface, but each operates in a somewhat different manner with a different
effect on both the rate of lowering and the shape of the resulting landform.
For example, from a series of field experiments, Schumm (1956) has
argued that slopes being lowered by surface wash (runoff) maintain a
rectilinear (straight) shape, with a constant gradient through time, while
slopes being lowered by soil creep or shallow mass movement undergo
a progressive decline in gradient through time, and exhibit a convexoconcave shape.
The rate of ground surface lowering
Denudation rates are normally expressed as a rate of ground surface
lowering in millimetres per thousand years (mm ka–1). However, you will
also see rates expressed as metres per million years (m Ma–1) or even
millimetres per year (mm a–1). The unit chosen reflects the timescale of
the study and the actual rate of denudation. One other unit used is the
Bubnoff unit B. A Bubnoff unit is equivalent to one millimetre of
ground surface lowering per thousand years, or one metre per
million years. As Summerfield (1991) states, this does seem to represent
a needless proliferation of units, but they are included in this guide as you
will encounter them in your further reading.
It is very difficult to estimate exactly how fast the ground surface is
being lowered, mainly because it happens so slowly and sporadically.
Young and Saunders (1986) made one of the earliest attempts to compile
statistics for ground surface lowering from different physical regions of the
world, taking place under different geomorphological processes. Of
course, this is a highly challenging exercise. As we shall see, denudation
is extremely difficult to measure, especially over the long term, which
partly accounts for the great variability in the data presented by Young
and Saunders. Also, more research has taken place in certain parts of
the world than in others, resulting in some gaps and uncertainties in the
record.
25
150 Geomorphological processes
Activity 3.2
Study Figure 7.17 in Summerfield (1991) which shows the results of the work of Young and
Saunders. Note that the preferred units in this study are Bubnoff units. Answer the following
questions:
How does climate appear to affect denudation rates?
What processes are responsible for the greatest amount of denudation?
Why is there more denudation on steep than on gentle slopes?
Despite the great variability in rates and incomplete data sets, some significant
issues emerge from the graphs in Summerfield (1991). First, not all the
major global climatic zones have the same denudation rates. Climate is a
major factor influencing the intensity of denudation. It is just possible to see
from the graphs that the regions having the highest denudation rates are the
semi-arid/Mediterranean regions. Second, possibly obviously, rates of ground
surface lowering from soil creep are higher for steep slopes than for gentle
slopes. Typical rates for steep slopes are 2–200 B, while for gentle slopes these
rates range from 1 to 20 B, an order of magnitude lower. Hence slope gradient
(or relief) is another major factor affecting denudation rate. Third, there
are clear differences between denudation rates resulting from gradual soil
creep compared with those from rapid mass movement (shallow landslides).
The latter process, although confined to the steeper slopes, produces rates
of ground surface lowering which are an order of magnitude higher. In the
case of marine cliff erosion, rates are 3–4 orders of magnitude higher. Large
landslides tend to occur only on steep slopes, so these high rates only relate to
a small proportion of the landscape. Soil creep, although having a smaller rate
of denudation associated with it, is far more widespread so may, in fact, be a
more significant process in the lowering of the earth’s surface.
Activity 3.3
From the available rates for denudation, we can begin to consider how fast the earth is being
eroded in the long term.
A mountain chain having an initial elevation of 3,000 m might have 2,000 m of its height
denuded at a rate of around 100 B, equivalent to mass movement on steep slopes. As 1 B is
equivalent to a ground surface lowering of 1 mm per 1,000 years, or 1 m per million years,
then 100 B is equivalent to 100 m per million years. Hence the first 2,000 m of elevation will
be removed in 20 million years.
Add to this the remaining 1,000 m, eroding at the lower rate of 10 B, or 10 m per million
years. This will then take 100 million years to be removed.
Hence the complete removal of the mountain will take approximately 120 million years. This
is a very crude calculation and you will be able to think of many factors which complicate
this idea. However, by extrapolating current measurements of denudation to the long term
we discover that, provided current conditions have been maintained over this period, large
mountains could be removed over comparatively short periods of the earth’s history.
We know that the earth is approximately 4.5 billion (million million) years old, so there is
sufficient time for mountains to be removed and reformed approximately 100 times.
The process of isostatic readjustment
The process of isostatic readjustment is fundamental to the operation of
geomorphological processes. We will further discuss this phenomenon later
in the guide, but let’s first consider the process of isostatic equilibrium and
how this is coupled with surface erosion. This interaction between isostatic
equilibrium and surface erosion complicates the argument presented above.
26
Chapter 3: Denudation: the wearing away of the surface of the earth
Consider what might happen when 100 m of sediment, having a density of
around 1.8 g/cm3 is stripped from a large mountain chain and redeposited
in the nearest ocean basin. The mountain chain itself is relieved of a
considerable weight over a large area, but the area of deposition gains
that weight, possibly over a somewhat more restricted area. How would
isostatic readjustment operate under such circumstances? What impact
would this have on the total amount of ground surface lowering that
actually takes place? Would there be 100 m of ground surface lowering
within the mountain chain or would the actual figure be higher or lower
than this? It is important to envisage the interaction between erosion and
redepositon, and its impact on isostaic readjustment. This is discussed
in considerable detail, with comprehensive examples, in Chapter 15 of
Summerfield (1991).
Climatic control of denudation
We have just shown how isostatic processes need to be considered further
in explanations of the long-term geomorphology of the earth. This will be
explored further in Chapter 4. Considering climate, another major factor
responsible for determining the rate at which denudation takes place, we
know that this is not constant in time or space. Different parts of the world
have entirely different rainfall, thermal and seasonal regimes. In particular,
denudation rates have a strong link with precipitation. Individual rainfall
events can carry out considerable erosion, but most research tries to
establish a link between mean annual rainfall and denudation rate. Not
all research reaches the same conclusions, however. The following table
presents data from two research papers that discuss the relationship
between climate (expressed as mean annual precipitation) and denudation
rate. Although the denudation rates are shown in mm/ka this is equivalent
to Bubnoff units, as used by Young and Saunders and introduced above.
Langbein and Schumm (1958)
Ohmori (1983)
Mean annual
precipitation
(mm)
Denudation rate
(mm ka–1)
Mean annual
precipitation
(mm)
Denudation rate
(mm ka–1)
20
10
200
10
40
30
220
25
100
70
250
50
250
100
400
45
400
75
450
20
600
50
650
15
800
45
800
40
1000
42
900
90
1250
42
1100
150
1300
200
1500
300
Activity 3.4
Plot these relationships on graph paper or in Excel, using the mean annual precipitation
as the independent variable (x-axis) and the denudation rate as the dependent variable
(y-axis).
Describe each line and what it tells us about the relationship between denudation and
precipitation.
27
150 Geomorphological processes
Describe any apparent similarities and differences between the two relationships.
Why do you think there is a peak in denudation (shown on both plots) at an annual
precipitation of about 250 mm?
Why do you think the two data sets show differences in what happens at high mean
annual precipitation?
Conclusions
What your graphs should show are regions of agreement between the
two research findings, but also a large area of disagreement. Both show
a strong positive relationship between precipitation and denudation,
where precipitation is lower than about 400 mm per year. Above 400
mm per year of precipitation, denudation starts to decline again as mean
annual precipitation increases, until about 700 mm of precipitation. It is
at mean annual precipitation totals in excess of 700 mm that the main
disagreement occurs. One data set shows a slight decline in denudation
rates as annual precipitation increases, while the other shows a
considerable increase in denudation. We should try to seek explanations
for this.
The work of Dr Vladimir Köppen
Dr Vladimir Köppen, a climatologist and plant geographer, devised a
system of climatic classification (in 1918) that has found widespread
significance and use. Köppen based his divisions of the world into different
climatic zones on the observed divisions between major vegetation types.
This classification can help us to interpret the relationship between mean
annual precipitation and denudation described in the previous section.
Of particular relevance is the division between desert climates and steppe
(semi-arid grassland) climates, which occurs at an annual precipitation
of around 400 mm. This approximately coincides with the peak on both
graphs you have plotted. It provides a clear indication that the role of
vegetation is a highly significant influence on the form of the relationship
between precipitation and denudation. At very low levels of annual
precipitation little opportunity exists for vegetation cover to become
established, but also there is insufficient precipitation to carry out much
weathering and erosion. As annual precipitation increases, weathering and
erosion also increase, but there is not an equivalent rise in opportunity
for vegetation cover to develop as precipitation totals are still too low.
Hence, with rising rainfall totals in the absence of significant increases in
vegetation cover, erosion rates increase dramatically. However, with yet
higher levels of precipitation, vegetation begins to become denser, taller
and more varied in the way it modifies incoming precipitation. This affects
the amount of water received at the ground surface, its power to erode
surface material and the amount of water that is transferred through
the soil. There comes a point, around just above 400 mm mean annual
precipitation, where the vegetation cover becomes sufficiently dense to
provide better protection to the soil. Although denudation is still high, at
annual precipitation totals above this, vegetation becomes increasingly
dense and denudation rates fall. Of course, hidden within this are the
seasonal variations that are increasingly important with latitude. These do
not show up when data relate only to annual averages.
As annual precipitation increases still further, we find totals typical of the
humid climatic zones (tropical and temperate). Langbein and Schumm
(1958) have argued that the vegetation cover continues to increase in
height and density, offering ever-increasing levels of protection to the
28
Chapter 3: Denudation: the wearing away of the surface of the earth
ground surface. In contrast to this, Ohmori (1983) indicates a clear
secondary increase in denudation with further annual precipitation
increases in these humid temperate and tropical zones. Again,
explanations can be found in the role of vegetation. While Langbein and
Schumm argue for continued increases in vegetation height and density,
Ohmori suggests that there comes a point where vegetation provides
maximum cover, and any further increases in height and density do not
add further protection. Furthermore, the role of vegetation becomes more
complex as tall trees provide greater fall heights for raindrops, enabling
raindrops dripping from leaves to reach their terminal velocity. This means
that their kinetic energy is increased with increases in canopy height. Tree
cover is also spatially variable, and there are often exposed areas of soil
between trees which are especially prone to erosion. By contrast, grass
and other low-growing species provide a more complete cover in which
there is virtually no opportunity for intercepted rain to regain momentum.
This debate about the precise effects of vegetation cover on erosion is very
complex, and will be discussed further in Chapter 9 of this subject guide.
There we will consider the alteration of the humid rain forests (temperate
and tropical) to grassland at the hand of anthropogenic (human) activity.
It is clear however, that vegetation is a major factor in the operation of
denudation processes, offering a great deal of protection to the ground,
but also involving other processes which have a more complex relationship
with denudation. It is inextricably linked with climate as climate
determines the type of vegetation development that takes place in any
given region of the earth.
Global variation in sediment yield
Many researchers have considered denudation in terms of rates of
ground surface lowering in order that they can be compared with rates
of uplift (see Chapter 4). From the resulting data we aim to work out the
net change in ground surface elevation over the long term (positive or
negative). However, we are often interested in the amount of sediment
that is being moved around the earth, so we also find the effect of
weathering and erosion presented in terms of tonnes of sediment removed
per km2 per year.
The average sediment yield is usually obtained from repeated
measurements of the suspended load of rivers. Figure 15.9 in Summerfield
(1991) shows how a world map of sediment yield looks, based on the
research of Walling and Webb (1983). We will consider how these data
have been obtained and how this map has been produced in a moment,
but for now we can identify world regions having high sediment yields
and those having low sediment yields. First, in the group of high-yielding
areas there is a major band running from the west coast of Canada, down
through the western part of the USA, central America and along the
Andes mountains of South America. Within South America, parts of the
north and north-east also have high yields of sediment. Note, too, a ring
of islands in the western Pacific that appear to continue from a region of
high sediment yields within south-east Asia. Finally, we see areas of high
sediment yield in equatorial Africa, as well as to the south-east of the
African Continent. Low sediment yields are found over the Continental
interiors, in particular the Eurasian and Canadian Shields, as well as over
the world’s great deserts. The Sahara and Kalahari Deserts in northern
Africa, the Gobi Desert in Western China and the Great Australian Desert
stand out particularly clearly. The reasons for these variations in sediment
yield are important and will be examined in the next section.
29
150 Geomorphological processes
Explaining global variations in sediment yield
Compare Figure 15.9 with Figure 2.4 (Summerfield, 1991), which was
introduced in Chapter 2. This allows you to see how sediment yields
are related to the world’s main physical regions. Major areas of high
denudation rates coincide with the most recent (Tertiary) mountain belts
in America and Eurasia. Also, the East African Rift System is associated
with high sediment yields. The lowlands and continental platforms
show the lowest sediment yield. Hence the most influential factor that
explains the pattern of denudation on a global scale is the current level of
tectonic activity and the resulting relief. Relief is the difference between
the highest and lowest points in a region, and where it is high there is a
great deal of energy for sediment movement. Tectonically active mountain
belts generate very high degrees of relief, and this is positively related
to denudation rate. By contrast, areas of low relief, such as continental
shields and lowlands which have been eroded for a very long time,
exhibit low levels of denudation. We will discuss this complex relationship
between denudation and uplift again in Chapter 4.
From the map of global sediment yields, we can also see areas of high
sediment yield that are not apparently tectonically active. These areas are
somewhat smaller in scale, such as parts of South-East Asia, Africa and
South America. These areas are not associated with Tertiary mountain
chains and current high rates of uplift. These are all regions of high annual
precipitation, including monsoonal climates, where denudation is high
despite the possibility that the vegetation cover is dense. The research of
Ohmori (1983) discussed and plotted in the previous section, is supported
by the evidence shown in these maps and diagrams. Other research,
such as that by Douglas and Wilson, supports this still further. Areas of
high annual precipitation do tend to be associated with high rates of
denudation. However, it is clear that this relationship relates to smaller
scales than the relationship between tectonic processes and denudation
rates.
A major issue that does not show clearly in global-scale maps and
diagrams of denudation rates or sediment yield relates to vegetation
removal, especially in areas of high annual precipitation, due to
anthropogenic (human-induced) activity. Although this is largely
anthropogenic, bear in mind that natural factors which cause the
vegetation to be depleted, such as lightning strikes, landslides or
intense dry periods during the Pleistocene glacials (see Chapter 8)
are very important as well. Where vegetation is so depleted that there
is no protection for the underlying soil, sediment yields can increase
by several orders of magnitude. However, the area over which this
happens is normally small on a global scale, so does not readily show
up on maps and diagrams that depict global patterns, such as Figure 2.5
(Summerfield, 1991). Hence, by looking at Figure 2.5, we would not be
able to pick out such factors operating on comparatively small time and
space scales. The above discussion highlights one of the major challenges
for geomorphologists seeking explanations for different phenomena. The
explanation that is most appropriate to explore further is often dependent
on the scale at which the investigation is carried out. So when trying to
explain global sediment yields, the importance of tectonic processes must
be emphasised at a global scale. However, at this scale, the effect of human
action, also important, is totally obscured. Climate shows up to an extent,
being of intermediate scale, but is not apparently the most important
factor. So the explanation that is sought, and the one which offers greatest
30
Chapter 3: Denudation: the wearing away of the surface of the earth
insight into the problem, is scale-dependent. Geomorphologists develop
the skills needed to offer multi-faceted explanations of environmental
phenomena on a range of scales and can bridge the links between them.
Activity 3.4: Denudation rates around the world
Objectives of the activity:
a. to gain an idea of average denudation rates for a selection of the world’s drainage systems;
b. to evaluate some of the factors which influence denudation rates;
c. to consider the problems inherent in the data relating to world denudation rates;
d. to appreciate the need for logarithmic graphs to display and analyse data.
World denudation rates:
The following table provides information about average denudation rates and related
factors for some of the world’s major drainage basins. Study it carefully.
River
Amazon
D (mm/ka) Basin
length (m)
69
3310
Basin
height (m)
6768
Basin
Relief
2.045
Precipitation Mean flow Runoff
(mm/y)
rate (m3/s) Coefficient
1490
200000
0.67
Brahmaputra 316
1270
7736
6.091
2661
19300
0.38
Colorado
87
1300
4730
3.638
310
32
Columbia
8
1200
3748
3.123
568
7930
0.62
Danube
32
1250
3087
2.470
960
6660
0.29
Elbe
2
705
1603
2.274
694
690
0.22
Ganges
198
1560
8848
5.672
1573
11600
0.25
Indus
96
1610
8611
5.348
543
7610
0.49
Irrawaddy
235
1420
5881
4.142
1878
13600
0.56
Limpopo
28
840
2322
2.764
520
160
0.02
Loire
5
540
1885
3.491
795
Mekong
73
2950
6000
2.034
1800
14900
0.33
Mississippi
44
2220
4400
1.982
612
18400
0.26
Niger
11
1950
2918
1.496
937
6020
0.18
Nile
17
3600
5110
1.419
832
317
0.005
Orange
33
1285
3482
2.710
415
2890
0.22
Orinoco
59
1550
5493
3.544
1300
34900
0.9
Parana
16
2175
6720
3.090
1027
18000
0.21
Rio Grande
17
1725
4295
2.490
160
95
0.01
São Francisco 3
1510
1800
1.192
1145
3080
0.13
Seine
5
370
902
2.438
711
685
0.39
Senegal
1
900
1000
1.111
665
761
0.08
St Lawrence
1
1650
1917
1.162
794
14300
0.48
Ural
5
1020
1000
0.980
364
301
0.11
Volga
7
1640
1638
0.999
489
8400
0.42
Yellow
45
2070
5500
2.657
484
1550
0.1
Zaire
3
2020
4507
2.231
1586
40900
0.22
Zambezi
13
2040
2606
1.277
957
6980
0.16
31
150 Geomorphological processes
Denudation is highly variable. It depends on many factors, some of which are included in
the table. Hence, in this activity, denudation is a dependent variable. The other factors are
independent variables.
Scattergraphs provide a straightforward and effective means of presenting data prior to
further analysis. They can be extremely clear, provided they are appropriately labelled,
in displaying relationships between variables. It is conventional to plot the dependent
variable as the y-axis (vertical) and the independent variable as the x-axis (horizontal).
Most scattergraphs you are used to plotting and using will probably have a linear form.
The raw data (actual numbers) will have been plotted using straightforward linear axes
for both independent and dependent variables. However, there can be problems with this.
Look at the table again and see whether you can identify what those problems might be
(start with the column about mean flow rate, as this gives the clearest indication of the
problems that you would face if you tried to plot this as a linear graph).
In many of the scientific papers that you will read, especially in geomorphology where
data are highly variable and wide-ranging, you will find logarithmic or log-linear (semilogarithmic) graphs. For this activity, where we are investigating factors of significance
to world denudation, logarithmic graphs are particularly useful as they enable us to
display data that range widely in magnitude. Denudation itself, as well as basin relief and
precipitation, show a range of three orders of magnitude. Flow rate and runoff coefficient
show a range of five orders of magnitude. We therefore need to use either three- or fivecycle logarithmic graphs to show these data.
You can plot such data very easily using a standard spreadsheet package, such as Excel.
First, select an independent variable and look at the range in the data. Then set these
x-axes up very carefully, using the numbers on the axis as a guide. Then look at the range
in values for denudation rate and label the y-axis accordingly. You can now plot the
relationship between denudation and one of the following variables as a series of points
on the graph:
a. basin relief
b.precipitation
c. flow rate
d. runoff coefficient.
What is the relationship between logarithms and linear data? You will notice that the
major divisions of the axes are the same. The distance along the scale is the same
between 1, 10, 100, 1,000, etc. However, this is not the case for the minor divisions.
Logarithmic graphs generally use base 10, although in theory any number could be used
as a base for logarithmic graphs and data analysis.
The (base-10) logarithm of a number is the power by which 10 needs to be raised in
order to obtain the original number. For example, if the number is 100, the (base-10)
logarithm is 2, since 10 raised to the power 2 (102) gives 100. The logarithm of 1,000
is 3 (103), of 10,000 is 4 (104) and so on. In this way, we can show a very large range
of data on a simple set of axes. This technique can also reveal relationships in data that
would otherwise not be clear from simple linear plots. Logarithmic graphs are generally
used .when a data set is composed of:
a. a large range in values; and
b. many instances of low numbers with just a few instances of high numbers.
Case (b) is very common in nature, and in geographical data sets. We often find a lot of
small values and just a few large values (e.g. sizes of settlements, discharge of rives, daily
rainfall totals, etc.). You should be able to think of other examples.
32
Chapter 3: Denudation: the wearing away of the surface of the earth
Measurement of denudation
Introduction
To understand the rate at which the earth is changing, data about
denudation rates are invaluable. However, we must question the accuracy
and general applicability of the rates found from the various studies that
measure denudation rate. As you can imagine, it is virtually impossible
to produce a global-scale map that gives a complete and precise set of
denudation measurements for every place in the present day. Several
assumptions, involving spatial and temporal extrapolation of thinly spread
point measurements, have to be made to generate a map such as that of
Figure 2.5 in Summerfield (1991). The issues are even more complex
when we try to produce estimates of denudation in the past, especially
over timescales where direct observation is impossible.
Ground survey methods
Perhaps the most obvious way to measure denudation rates is the direct
method of ground survey, comparing surface elevations at a range
of locations for two different points in time. The ground elevation is
measured at some point in time, and then returned to for remeasurement
at exactly the same location. By superimposing the two sets of
measurements the amount of denudation can be estimated. This method of
resurveying can work well if ground surface elevation is changing rapidly,
such as for a rapidly accreting or eroding salt marsh, or by surveying
before and after a landslide has taken place. For the more gradual rates
that are commonplace, survey equipment has to be extremely accurate, the
equipment has to be operated by a skilled researcher and measurements
need to be separated by large time intervals for anything meaningful to
show up. There is also the problem that the method of ground survey
gives elevation change for individual points (which have to be accurately
relocated), rather than over continuous areas. Not surprisingly, there are
very few studies of denudation that employ direct observation. Alternative
methods are used.
Measuring river sediment load
The sediment load carried by rivers reflects the amount of denudation that
is taking place within their catchment area. You will learn more about river
sediment loads later in this guide. Hence, an alternative method to ground
survey involves measuring the sediment load being carried by rivers.
Mainly this involves the suspended portion of the sediment load, and
occasionally the bedload, but rarely is the dissolved load also taken into
account. By measuring the relationship between sediment concentration
and discharge, and comparing this with a record of discharge over a
period of time, it is possible to provide an estimate of the total amount
of sediment being removed from a catchment. This method has been
commonly used in much of the earliest research on global denudation
(Corbel, 1959; Fournier, 1960). There are several problems associated
with this method, not least because it is extremely difficult to produce
a relationship between discharge and sediment load in the first place.
Rates of ground surface lowering can only be calculated by assuming
that sediment is being shed uniformly from all parts of the catchment.
Furthermore, there is no way of accounting for sediment being shed from
one part of the catchment but being redeposited and stored before it ever
gets to the river. Many catchments contain large areas of storage, such as
alluvial fans, resulting from large denudation rates in localised parts of the
33
150 Geomorphological processes
catchment. These are not evident from measurements of river sediment.
It is also important to note that the most effective way sediment is moved
via rivers is at high discharges. Large events can shift a very large amount
of sediment, so these events are the ones that need to be monitored most
closely. However, the largest events are the hardest to monitor as they
require equipment that is ‘up to the job’ and we need to know when such
events will take place so they can be monitored. It is somewhat ironic that
the largest-scale events that we are most interested in are the ones which
are the most difficult to study in the field.
Sediment accumulation in reservoirs and lakes
Over time denudation rates fluctuate in response to changes in the
controlling factors identified above (i.e. relief, climate, human activity
and vegetation change). For example, in the Holocene (last 10,000 years),
denudation rates have varied in response to climate change or to human
interference with the vegetation. As a quick method providing a crude
average for denudation rates over time, researchers often measure the net
accumulation of sediment in reservoirs. Given that the time of reservoir
construction is known, the depth of sediment can be used to estimate the
average rate of accumulation since the reservoir was built. However, even
this method can only involve comparatively short timescales.
Several researchers have used a variant of this technique, involving
natural lakes, to look at denudation over even longer periods of time. In
this type of research, a more sophisticated approach is taken, whereby
sediment cores are extracted and analysed in detail. Buried organic
material provides radiocarbon dates, giving the approximate time of
burial. This allows study of thousand-year timescales, provided the
correct material can be located and dated. Pollen analysis can then show
whether the vegetation changed markedly over the period, and particle
size analysis can help to ascertain the type of conditions in which sediment
was transported and deposited. The pollen and sediment record can
be matched to the radiocarbon dates to provide a reasonably accurate
time frame for the processes. From such research we can gain a picture
of changing rates of sedimentation over time with possible concurrent
environmental changes that might have been responsible (see Foster et
al., 1985). Sediment cores taken from lakes in North Island, New Zealand,
provide evidence for increasing sedimentation rates in the Holocene,
explained by deforestation (see Page and Trustrum, 1997, and Chapter
9 of the subject guide). A final problem involves the difficulty of using
these sedimentation rates to provide an average denudation rate for
the catchment as a whole. A high-resolution temporal record does not
necessarily translate into a high-resolution spatial picture.
Measurement methods for longer periods
To address even longer periods of time, and to overcome problems
inherent in using sediment records from cores and rivers, several other
methods for estimating denudation over time have been employed. The
first of these involves the principle of ergodic reasoning, in which space is
substituted for time. One of the classic examples of this type of research is
provided by Brunsden and Kesel (1973), involving a study of a Mississippi
river bluff. As the Mississippi river has meandered from side to side across
its floodplain, different parts of the bluff have been undercut at different
times. Brunsden and Kesel used aerial photographs to work out which
parts of the bluff were being eroded at different time periods and, more
importantly, when this basal erosion ceased. By comparing profiles of
bluffs being undercut at present with those having increasingly longer
34
Chapter 3: Denudation: the wearing away of the surface of the earth
periods since basal erosion ceased, it is possible to estimate the rate at
which the bluffs have been degrading.
Activity 3.5
The data in the table below are based on the work of Brunsden and Kesel. They give the
elevation of four bluffs each having a different date for the cessation of basal erosion.
Table of data based on the work of Brunsden and Kesel
Profile A
Profile B
Profile C
Profile D
Height
Distance
Height
Distance
Height
Distance
Height
Distance
(m)
(m)
(m)
(m)
(m)
(m)
(m)
(m)
30
0
23
0
20
0
18
0
30
20
22
15
19
15
16
10
30
40
21
30
18
22
14
15
25
41
20
38
15
24
10
20
20
43
15
42
10
29
6
25
15
48
10
45
8
33
4
30
10
56
5
51
5
42
2
40
5
64
2
54
2
54
0.5
60
0
70
0
70
0
70
0
70
a. Using an ordinary linear graph plot all four profiles on the same graph and label them clearly.
b. Describe briefly how the shape of the profiles changes from A through to D.
c. Taking A as the original profile, estimate approximately how much sediment has been removed from A to B, A to C and A to D.
d. Profile A has an age of zero, it is approximately 5 years since location B was cut off from regular undercutting, C has not been undercut for 100 years and D for 200 years. Plot the amount of sediment removed since basal erosion stopped against the estimated age of the slope.
e. What can you say about the changes in denudation rates over time?
f. What are the problems with this approach?
From the graph, we can see that denudation is rapid initially, with lots of sediment being
removed, but that it decreases over time as the relief of the slope is reduced. As we
saw above, relief is one of the main factors governing denudation rates and this will be
explored further in Chapter 4 of the subject guide.
Use of computer models
A more recent research method used to estimate long-term rates of
denudation involves computer models. Computational methods have
increasingly been used throughout the 1970s, 1980s and 1990s in efforts
to understand the way the surface of the earth has changed through time
(Willgoose and Riley, 1998). Digital Elevation Models (DEMs) provide the
central input and output to these exercises, as they show how elevation
changes in response to various processes acting on the surface and within
the slope. The main requirement is a series of mathematical equations
that describe the processes thought to be carrying out the erosion (see
Chapter 6). These equations are used to calculate sediment redistribution
and, extrapolated over long time periods, can show how surface elevation
changes over time periods that cannot be directly observed. Often
35
150 Geomorphological processes
assumptions have to be made about the initial form of the landscape
because it cannot be observed or measured. While modelling has enabled
us to learn about the changing shape of catchments, the accuracy and
long-term applicability of the governing equations is critical. Obviously,
there will continue to be major advances in this area of research, driven
by increased computer power, but problems of accuracy and process
representation remain. The best way forward in understanding more
about short-term and long-term denudation rates, and their links, is
through a combination of field measurement, laboratory monitoring and
mathematical modelling.
A reminder of your learning outcomes
Having completed this chapter, and the Essential readings and activities,
you should be able to:
• define what is meant by denudation
• explain that different processes give rise to denudation but that rates
are highly variable
• describe and account for global variations in denudation rates
• know some different methods for estimating denudation in the
long term
• appreciate the complex relationship between denudation and climate
and why different research has reached different conclusions about this
relationship.
Sample examination questions
1. Give a reasoned account of global variations in denudation rates in the
present day.
2. Discuss the main changes to rates of environmental processes over the
past 2 million years.
3. How is the concept of ergodic reasoning helpful in understanding
changes in landforms over periods where direct observation is not
possible?
36