Contents

List of plates, figures, and tables ix

Preface xiii

Acknowledgements xv

1 Introducing debates 1

Debates and the geosphere 1 Debates and the biosphere 6

2 Building the Earth 11

Plate tectonics 11 Plume tectonics 20

Contraction and expansion tectonics 25

3 Bombarding the Earth 32

The bombardment hypothesis 32 Cosmic missiles 37 Impact craters 43 Periodic bombardment? 47

Freezing the Earth 50

The eventful ice age 50 What caused the last ice age? 58 Snowball or slushball Earth? 60 Hothouses and icehouses 68

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