PHYSICAL GEOGRAPHY Physical Geography: The Key Concepts is a thought-provoking and up-to-date introduction to the central ideas and debates within the field. It provides extended definitions of terms that are fundamental to physical geography and its many branches, covering topics such as: • biogeography • ecology • climatology • meteorology • geomorphology • hydrology • pedology. Complete with informative tables, diagrams, and suggestions for further reading, this is a highly accessible guide for those studying physical geography and related courses. Richard Huggett is a Reader in Physical Geography at the Uni versity of Manchester. His publications include Fundamentals of Biogeography, Fundamentals of Geomorphology and The Natural History of the Earth, all published by Routledge. ALSO AVAILABLE FROM ROUTLEDGE The Complete Guide to Climate Change Brian Dawson and Matt Spannagle 978–0–415–47790–1 Companion Encyclopedia of Geography Ian Douglas, Richard Huggett and Chris Perkins 978–0–415–43169–9 (2 vols) Fundamentals of the Physical Environment (4th edn) Peter Smithson, Ken Addison, Ken Atkinson 978–0–415–39516–8 Fifty Key Thinkers on the Environment Joy Palmer, David Cooper and Peter Blaze Corcoran 978–0–415–14699–9 Fifty Key Thinkers on Development David Simon 978–0–415–33790–8 PHYSICAL GEOGRAPHY The Key Concepts Richard Huggett First published 2010 by Routledge 2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN Simultaneously published in the USA and Canada by Routledge 270 Madison Ave, New York, NY 10016 Routledge is an imprint of the Taylor & Francis Group, an informa business This edition published in the Taylor & Francis e-Library, 2009. To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk. © 2010 Richard Huggett All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data Huggett, Richard J. Physical geography : the key concepts / Richard Huggett. p. cm.—(Routledge key guides) Includes bibliographical references and index. 1. Physical geography. I. Title. GB54.5.H84 2009 910Ј.02–dc22 2008054417 ISBN 0-203-87567-2 Master e-book ISBN ISBN10: 0–415–45207–4 (hbk) ISBN10: 0–415–45208–2 (pbk) ISBN10: 0–203–87567–2 (ebk) ISBN13: 978–0–415–45207–6 (hbk) ISBN13: 978–0–415–45208–3 (pbk) ISBN13: 978–0–203–87567–4 (ebk) For my family CONTENTS Preface ix Acknowledgements xi List of Key Concepts xii KEY CONCEPTS 1 Bibliography 180 Index 206 vii PREFACE Physical geography is a diverse discipline. I doubt that few of its practitioners today would care to describe themselves as physical geographers except when wishing to distinguish themselves from human geographers. Most specialize in a branch of physical geography – biogeography, climatology and meteorology, ecology, geomor- phology, hydrology, or pedology. The chief aim of this book is to provide extended definitions of concepts and terms that are central to discourse within physical geography and its many branches, and that are helpful for undergraduate students and lay readers. All entries will include a clear and full definition of the concept. Some of the entries for more controversial topics, such as ‘uniformitarianism’, will also include a short critical appraisal of the concept itself. In selecting terms, I have used three criteria: first, they are germane to physical geography as a whole (e.g. ‘energy’, ‘equilibrium’, ‘feed- back’); second, they are central to a branch of physical geography (e.g. ‘dispersal’, ‘etchplanation’, ‘natural selection’); third, they are important concepts from other disciplines that play a starring role in some aspect of physical geography (e.g. ‘plate tectonics’). It proved exceedingly difficult to select a hundred or so concepts that I felt were ‘key’ to the discipline, and I am acutely aware of concepts that, for want of wordage, are excluded. Indeed, I suspect that my physical geographical peers will not think all my chosen concepts are key, and that they can offer other concepts that they would regard as key. Perhaps that says something about the rich diversity of physical geography and its practitioners. Nonetheless, I trust that all readers will find something of interest in the discussions on offer, and will have as much pleasure in reading the material as I had in researching and writing it. I should like to thank those people who have made the completion of this book possible: Nick Scarle for drawing the diagrams; Ian Douglas for kindly commenting on my original list of key concepts; Andrea Harthill, formerly of Routledge, for asking me to write the ix PREFACE book; and David Avital and Katherine Ong of Routledge, for help during the later stages of writing. As always, special thanks go to my wife and to my two youngest children for letting me use my PC occasionally. Richard Huggett Poynton November 2008 x ACKNOWLEDGEMENTS The author would like to thank Taylor and Francis Books UK for granting permission to reproduce material in this work: Figure 5 from R. J. Huggett (2007) Drivers of global change, in I. Douglas, R. Huggett, and C. Perkins (eds) Companion Encyclopedia of Geography: From Local to Global, pp. 75–91, Abingdon: Routledge, Figure 6.1 (p. 83); Figures 15, 18, 38, and 39 from R. J. Huggett (2007) Fundamentals of Geomorphology, 2nd edn, London: Routledge, Figures 1.7 (p. 16), 1.8 (p. 17), 1.9 (p. 18), and 2.2 (p. 37); Figures 16 and 35 from R. J. Huggett (2007) Climate, in I. Douglas, R. Huggett, and C. Perkins (eds) Companion Encyclopedia of Geography: From Local to Global, pp. 109–28, Abingdon: Routledge, Figures 8.1 (p. 111) and 8.4 (p. 121); Figures 32 and 36 from R. J. Huggett (2004) Fundamentals of Biogeography, 2nd edn, London: Routledge, Figures 2.3 (p. 19) and 10.7 (p. 171). xi LIST OF KEY CONCEPTS active and passive margins actualism/non-actualism adaptation adaptive radiation advection aridity astronomical (orbital) forcing bioaccumulation and biomagnification bioclimate biodiversity and biodiversity loss biogeochemical cycles bombardment carrying capacity catastrophism catena chronosequence climate change climax community community change complexity continental drift convection cyclicity/periodicity desertification directionalism dispersal disturbance drainage basin xii LIST OF KEY CONCEPTS ecological niche ecoregion ecosystem ecotone endogenic (internal) forces energy/energy flow environment environmental change equifinality equilibrium ergodicity (space–time or location–time substitution) etchplanation eustasy evolution evolutionary geomorphology evolutionary pedology exogenic (external) forces extinction feedback functional–factorial approach Gaia hypothesis general circulation of the atmosphere general circulation of the oceans geochronology geodiversity geographical cycle geological cycle global warming gradualism habitat habitat loss and habitat fragmentation homeostasis/homeorhesis hydrological cycle invasive species island biogeography, theory of isostasy land degradation xiii LIST OF KEY CONCEPTS landscape ecology limiting factors and tolerance range local climate (topoclimate) magnitude and frequency mass balance microclimate natural selection no-analogue communities pedogenesis plate tectonics plume tectonics populations/metapopulations refugia region resilience scale sea-level change soil soil–landscapes solar forcing speciation succession sustainability systems taxonomy tectonics and neotectonics teleconnections thresholds time topography transport processes uniformitarianism vicariance zonality xiv PHYSICAL GEOGRAPHY The Key Concepts ACTIVE AND PASSIVE MARGINS ACTIVE AND PASSIVE MARGINS Active margins occur where tectonic plates converge (to form sub - duction zones in which one plate sinks below another) or where they slide by one another (transform zones) (Figure 1). They are common around the rim of the Pacific Ocean, which gives them the alternative name of Pacific-type margins. The coast of the Pacific northwest of North America and the west coast of South America are active margins with a subduction zone; central and southern California is an active margin with a transform zone. Tectonic activity (earthquakes, volcanoes, uplift, and the formation of new igneous rock) is character- istic of active margins. Passive margins form where two continental plates split (Figure 1). They are not the sites of plate boundaries, and although continental crust abuts oceanic crust, they are part of the same tectonic plate and subduction does not take place. The east coast of the Americas and the west coast of Africa and Europe are examples of passive (or Atlantic-type) continental margins. Tectonic activity at passive margins is negligible as no plate collision or subduction occurs. The distinction between active and passive margins has proved an influential concept in understanding many aspects of continental geomorphology. Active margins characteristically involve mountains (or island arcs), with short rivers and little or no continental shelf that plunges steeply into an offshore subduction trench. Passive margins have generally low relief (although mountains do occur), long rivers (such