The Sea Floor

The Sea Floor

Eugen Seibold

Wolfgang Berger

An Introduction to Marine Geology

Fourth Edition

Springer Textbooks in Earth Sciences,

Geography and Environment

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Eugen Seibold • Wolfgang Berger

The Sea Floor

An Introduction to Marine Geology

Fourth Edition

Eugen Seibold

Freiburg, Germany

Wolfgang Berger

Geosciences Research Division

Scripps Institution of Oceanography Geosciences

Research Division

La Jolla, California

USA

ISSN 2510-1307 ISSN 2510-1315 (electronic)

Springer Textbooks in Earth Sciences, Geography and Environment

ISBN 978-3-319-51411-6 ISBN 978-3-319-51412-3 (eBook)

DOI 10.1007/978-3-319-51412-3

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v

Man’s understanding of how this planet is put together and how it evolved has changed

radically during the last 30 years. This great revolution in geology – now usually subsumed

under the concept of Plate Tectonics – brought the realization that convection within the Earth

is responsible for the origin of today’s ocean basins and continents, and that the grand features

of the Earth’s surface are the product of ongoing large-scale horizontal motions. Some of these

notions were put forward earlier in this century (by A. Wegener, in 1912, and by A. Holmes, in

1929), but most of the new ideas were an outgrowth of the study of the ocean floor after World

War II. In its impact on the earth sciences, the plate tectonics revolution is comparable to the

upheaval wrought by the ideas of Charles Darwin (1809–1882), which started the intense

discussion on the evolution of the biosphere that has recently heated up again. Darwin drew his

inspiration from observations on island life made during the voyage of the Beagle

(1831–1836), and his work gave strong impetus to the first global oceanographic expedition,

the voyage of HMS Challenger (1872–1876). Ever since, oceanographic research has been

intimately associated with fundamental advances in the knowledge of Earth. This should come

as no surprise. After all, our planet’s surface is mostly ocean.

This book is the result of our conviction that to study introductory geology and oceanogra￾phy and environmental sciences, one needs a summary of the tectonics and morphology of the

sea floor, of the geologic processes active in the deep sea and in shelf seas, and of the climatic

record in deep-sea sediments.

Our aim is to give a brief survey of these topics. We have endeavored to write for all who

might be interested in the subject, including those with but little background in the natural

sciences. The decade of the 1980s was characterized by an increasing awareness of man’s

dependency on natural resources, including the ocean as a weather machine, a waste bin, and a

source of energy and minerals. This trend, we believe, will persist as resources become ever more

scarce and as the impact of human activities on natural cycles escalates in the coming decades.

An important part of this awareness will be an appreciation for the elementary facts and concepts

of marine geology, especially as they apply to processes within hydrosphere and atmosphere.

In what follows, we shall first give a brief overview of the effects of endogenic forces on

the morphology of the sea floor. Several excellent summaries for the general reader are

available for this topic, which is closely linked to the theory of continental drift, and has been

a focus of geologic discussion for the last three decades. For the rest, we shall emphasize the

exogenic processes, which determine the physical, chemical, and biological environment on

the sea floor, and which are especially relevant to the intelligent use of the ocean and to an

understanding of its role in the evolution of climate and life.

The results and ideas we report on are the product of the arduous labors of many dedicated

marine geologists. We introduce some particularly distinguished scientists by portrait (Fig. 0.1).

Of course, there are many more, and most of them are alive today. We have occasionally men￾tioned the authors of important contributions. However, we did not find it possible in a book

like this to give credit systematically where it is due. We sincerely apologize to our colleagues

for this unscholarly attitude, citing necessity in defense. For those who wish to pursue the

subjects discussed in greater depth, we append suggested readings at the end of each chapter,

as well as a list of key references.

Preface to the Third Edition

vi

For this second edition, we have extensively rewritten those parts of the first edition where

substantial and fundamental progress has occurred in the fields of interest. Also, we have

incorporated many of the suggestions for improvements that were communicated to us by

several colleagues and reviewers. There are, however, limitations to the scope of subjects that

can be treated in a short introduction such as this: we attempted neither a balanced nor an

encyclopedic survey of all of marine geology with its many ramifications. We tried to keep

highly technical information to a minimum, relegating certain necessary details to the

Appendix.

Both authors wish to express their profound gratitude to collaborators and students who,

over the years, have shared the excitement of discovery and the toil of research on numerous

expeditions and in the laboratory. We also owe special thanks to the colleagues who helped us

put this book together, by sending reprints and figures, or by offering advice.

Freiburg, Germany E. Seibold

La Jolla, CA, USA W.H. Berger

Spring 1993

Preface to the Third Edition

vii

This book is the fourth edition of the Seibold-Berger text on elementary marine geology mainly

based on introductory lectures to students in Kiel and in La Jolla. W. Berger added materials

concerning new developments in the field, some 30 years after E. Seibold determined the

nature and range of subjects discussed in the second and third editions of the text. There are

several things that set this text apart from many similar ones. Eugen Seibold (1918–2013),

distinguished pioneer of marine geology (Fig. 1), emphasized observation of modern marine

environments and the relationships between ongoing ocean processes and ancient marine

rocks. His interest in ancient rocks and in sedimentation on Atlantic-type margins is reflected

throughout in the book.

E. Seibold emphasized open questions, that is, the fact that much remains unknown in the

(historically very young) fields of geology and especially of marine geology, notably at the

cutting edge of exploration. As a consequence, he emphasized elementary findings that have

proven their worth. He favored simple conceptualization, as in his classic paper on sediments

in shelf seas (Fig. 2; Sect. 9.5.1). He clearly preferred concepts based on observation to nomen￾clature and to speculation. The term “new” did not carry special weight with him. On the

contrary, if a newly introduced concept had not run a decade-long gauntlet of critique and

survived, he remained doubtful of its viability. His basic philosophy is evident in all editions

of The Sea Floor, including the present one. Also, it governs his book The Memory of the Sea

(in German), and it emerged strongly in discussions, official or private. Also, it helped guide

the synthesis reports of Leg 41 of the Deep Sea Drilling Project (off NW Africa), for which he

was co-chief (together with the marine geologist Yves Lancelot).

Since the time of the early editions of this book, emphasis has grown in “earth system sci￾ence,” with forays into geophysics, geochemistry, oceanography, and indeed all of the

Preface to the Fourth Edition of “The

Seafloor”

Fig. 1 Eugen Seibold, pioneer of marine geology (Photo courtesy of

Dr. Ilse Seibold, Freiburg)

viii

climate-related sciences including ecology (Figs. 3 and 4). It is an approach that Eugen Seibold

urged and fostered. In his acceptance speech of the Blue Planet Prize (in 1994), he said this:

“What is a marine geologist? A marine geologist investigates the present situation of the sea￾floor and the processes which shape it. Furthermore, he tries to learn from the layers beneath

the seafloor, i.e. he tries to learn from the past. With this knowledge from the present and the

past, he has a responsibility to comment also on future developments if he is able to do so with

scientific reasoning …” Evidently, he saw a marine geologist as a scientist who takes the ocean

and climate change seriously. In this fourth edition, I have emphasized this approach. Space

requirements calling for trade-off in space resulted in some cutting back of important items,

notably the celebrating of contributions of some important pioneers he had identified.

Time scale matters in all of history. We now do have an excellent scale for the entire

Cenozoic (i.e., the last 65 million years) largely through the untiring efforts of the Woods Hole

biostratigrapher W. A. Berggren and his colleagues. A reliable scale is necessary to put rates of

change in evolution and items of geologic history into perspective (Fig. 5). The established

geologic time scale for pre-Cretaceous time is from reliable and traditional sources, being

fundamental in geologic work (Fig. A3.1, in the Appendix).

warm layer

evaporation dominates

winter sinking

of salty water

mixed layer

cool, salty

oxygen-rich

nutrient-poor

oxygen-poor

nutrient-rich

Deep Tethys

warm layer

mixed layer

cool, salty

Deep Tethys

TROPICS

TROPICS MONSOON RAINS / RIVERS

shelf sea collecting carbonate

shelf sea collecting organic carbon

MAKING PURE LIMESTONE

MAKING BLACK SHALE

SUBTROPICS

freshwater

influx

rising of nutrient￾rich water

upwelling

3000 m

300 m

Fig. 2 Eugen Seibold’s scheme of making carbonate

or organic-rich shale in shelf seas, depending upon

the contrast in circulation in arid and humid regions.

Upper panel (a): arid conditions, schematic; compare

Persian Gulf. Lower panel (b): humid conditions,

schematic; compare Baltic Sea. Inset photos:

Mesozoic shelf deposits, marine carbonate rocks

(arid) and black shale (humid) in southeastern France

(for oceanography, see Sect. 9.5) (After ideas of

E. Seibold, published in 1971)

Preface to the Fourth Edition of “The Seafloor”

ix

0

10

20

30

40

50

60

70

3.0 2.0 1.0 0 -1.0 -2.0

ATLANTIC

BENTHIC

FORAMINIFERS

PLIO￾PLEISTOCENE OLIGOCENE EOCENE MIOCENE

PALEOCENE

CRETACEOUS

AGE (MYBP)

ICE SHEETS POSSIBLY ICE FREE

0 4

4 1 8 2

8 “modern” (δw=-0.28o/oo)

“ice free” (δw=-1.2 o/oo)

δ

18O (o/oo; PDB)

T°C

Fig. 3 A chief result from deep-sea drilling: the

cooling of the planet since the early Eocene as seen in

a temperature proxy on the deep seafloor (oxygen iso￾topes of benthic foraminifers; red, warm; yellowish

green, intermediate; blue, cold (ice age)) (After K. G.

Miller, R. G. Fairbanks, and G. S. Mountain, who

compiled data from (Atlantic) DSDP sites (1987;

Paleoceanography 2:1))

SYSTEM

INPUT OUTPUT

RESPONSE

Fig. 4 The concept of earth system science. Input is

exogenic and endogenic forcing; output is the

recorded climate change and sedimentation (After

J. Imbrie et al., 1982, in W. H. Berger and J. C.

Crowell, (eds.) Climate and Earth History. Studies in

Geophysics)

Preface to the Fourth Edition of “The Seafloor”

x

I am indebted to Eugen Seibold for many discussions and also to many other colleagues (includ￾ing my mentors D. L. Eicher, Colorado, F. B. Phleger and F. L. Parker (La Jolla), and Gerold Wefer

at the Marum Institute, University of Bremen) for advice or for offering (or reviewing) illustrations

of important geological concepts. Authors are acknowledged in the appropriate figure captions.

Many others, including pioneers in the field, contributed important ideas. Trying to mention them

all here would run the risk of leaving off many important contributors. Many or most are listed in

“suggested readings.” In any case, it is well to realize that the selection of “pioneers” is quite arbi￾trary. Older pioneers of marine geology (starting in the nineteenth century) tend to be underrepre￾sented, and the reverse is true for teachers and colleagues of the authors of this book.

Concerning this or any other textbook, it may be well to keep in mind what the famous

Californian physicist Richard Feynman (1918–1988) said; that is, science begins with doubting

traditional textbook assertions. Feynman made an interesting observation, but actually marine

geology is too young a science to have a long list of textbooks for testing his statement. It seems

that this particular field advanced not so much by raising doubts about what was being taught by

the professionals but mainly by making new observations and measurements, commonly by using

new methods, and by integrating with results from other disciplines (including physics). In this

actual history of scientific research, much new information was delivered by geophysics (“physics

applied to geology” in the words of erstwhile S.I.O. director Fred Spiess) and by deep-ocean

0

2

5

23 23

34

56

66

QUATERNARY

PLIOCENE

MIOCENE

OLIGOCENE

EOCENE

PALEOCENE

NEOGENE PALEOGENE

CRETACEOUS

CENOZOIC

AGE (MILLIONS OF YEARS)

Fig. 5 Cenozoic time scale, simplified. Time scale of

“the Age of Mammals,” that is, the time since the

Cretaceous, following the extinction of ammonites

and dinosaurs. Numbers are published estimates of

ages of stratigraphic boundaries in millions of years,

mainly from the most recent ODP volumes. Note the

great lengths of the Eocene and the Miocene, epochs

that dominate the Paleogene and the Neogene peri￾ods, respectively. In turn, Cenozoic sediments and

events dominate the marine geologic history of the

modern ocean. Cretaceous deposits are found below

somewhat less than one half of the seafloor

Preface to the Fourth Edition of “The Seafloor”

xi

coring and drilling (i.e., by engineering feats) in the second half of the last century. The ensuing

results have changed our understanding of all aspects of seafloor lore. And yes, the advances did

make old geology texts obsolete while building on established concepts that remained useful.

General Background and References

H.U. Sverdrup, M.W.Johnson, and R.H. Fleming, 1942. The Oceans- Their Physics, Chemistry

and General Biology. Prentice Hall, Englewood Cliffs, N.J.

E. Seibold and W.H. Berger, 1996. The Sea Floor, an Introduction to Marine Geology, 3rd ed.,

Springer,Berlin Heidelberg New York.

E. Seibold, 1991. Das Gedächtnis des Meeres. Boden Wasser Leben Klima. [The memory of

the ocean; seafloor, water, life, climate] Piper, Mϋnchen.

D. Seidov, B.J. Haupt, M. Maslin (eds.) 2001. The Oceans and Rapid Climate Change – Past,

Present and Future. Am. Geophys. Union, Geophysical Monograph 126

J.H. Steele, K.K. Turekian, and S.A. Thorpe (eds.) 2001. Encyclopedia of Ocean Sciences

(6 vols.).Academic Press, San Diego.

V. Gornitz, V. (ed.) 2009. Encyclopedia of Paleoclimatology and Ancient Environments.

Springer, Dordrecht.

F.T. MacKenzie, 2011. Our Changing Planet, 3rd ed., Pearson Education, Boston.

Eugen Seibold (1918–2013)

Seibold was born in Stuttgart. He studied geology in Bonn and in Tübingen. Subsequently, he

taught at the University of Tübingen but moved to Kiel in 1958 to study modern marine sedimen￾tation and manage the Geological-Palaeontological Institut of the university as its director. From

1980, he accepted positions as the president of the DFG (the German National Science

Foundation), as the vice-president of the European Science Foundation, and as the president of

the International Union of Geological Sciences. He was president of the European Science

Foundation from 1984 to 1990 and a member of various academies, including the Leopoldina

(Akademie der Naturforscher, Halle, in Saxony-Anhalt) and the Académie des Sciences in Paris.

Seibold’s many contributions to geology were well recognized – he was a recipient of inter￾nationally known awards (e.g., the Gustav Steinmann Medal, the Hans Stille Medal, the

Leopold von Buch Plakette), as well as the Walter Kertz Medal in geophysics and the Blue

Planet Prize of the Asahi Foundation. The Asahi Foundation’s prize especially recognizes con￾tributions of relevance to society. The prize was used, in part, to fund the Eugen and Ilse

Seibold Prize, an award furthering Japanese-German scientific interaction.

Among outstanding paradigms within Seibold’s many contributions (including geologic

education), one might emphasize his insights regarding the role of exchange between marginal

basins and the open sea in determining the deposits accumulating in shelf basins. He assigned

an estuarine-type exchange to black shale sedimentation and an anti-estuarine type to carbon￾ate deposits. Both types of sediment are prominent in the geologic record (and are conspicuous

in the Jurassic of southern Germany, his original training ground). Significantly, black shales

are commonly a source for hydrocarbon products, while carbonates often serve as reservoir

rocks. Obviously, both rock types help define our time in human history. It is typical for Seibold

that he thought we should know about their origin.

[Source of information: largely the Deutsche Forschungsgemeinschaft, DFG]

La Jolla, CA, USA W.H. Berger

December 2015

Preface to the Fourth Edition of “The Seafloor”

xiii

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Origin and Morphology of Ocean Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Origin and Morphology of Ocean Margins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Sources and Composition of Marine Sediments. . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5 Effects of Waves and Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6 Sea-Level Processes and Effects of Sea-Level Change . . . . . . . . . . . . . . . . . . . . . . 75

7 Productivity of the Ocean and Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

8 Benthic Organisms and Environmental Reconstruction . . . . . . . . . . . . . . . . . . . . 105

9 Imprint of Climate Zonation on Marine Sediments . . . . . . . . . . . . . . . . . . . . . . . 119

10 Deep-Sea Sediments: Patterns and Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

11 Geologic History of the Sea: The Ice-Age Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . 153

12 Cenozoic History from Deep-Ocean Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

13 Cretaceous Environments and Deep-Ocean Drilling . . . . . . . . . . . . . . . . . . . . . . . 187

14 Resources from the Ocean Floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

15 Problems Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

Contents

© Springer International Publishing AG 2017 1

E. Seibold, W. Berger, The Sea Floor, Springer Textbooks in Earth Sciences, Geography and Environment,

DOI 10.1007/978-3-319-51412-3_1

Introduction

1.1 The Great Geologic Revolutions

of the Twentieth Century

1.1.1 General Background Information

There were an enormous number of striking geologic discov￾eries made and geologic theories created in the twentieth

century. All of these bear importantly on marine geology.

Four findings stand out: (1) plate tectonics (linked to conti￾nental drift and based largely on the geomorphology of the

seafloor, geomagnetism surveys, heat flow patterns, and

earthquakes at sea), (2) Orbital Ice Age Theory (informed by

solar system astronomy and confirmed by the study of deep￾sea sediment), (3) stepwise Cenozoic cooling (based on

results from deep-sea drilling), and (4) confirmation of the

impact theory for the end of the Mesozoic (clinched by stra￾tigraphy of pelagic sediments on land). The respective

widely recognized pioneers are (1) a number of largely US

American and British geologists, geophysicists, and geo￾magnetists (e.g., Lamont’s marine geologist Bruce Heezen

(1924–1977), the US Navy’s Robert Dietz (1914–1995) and

Harry Hess (1906–1969), the UK geophysicist Fred Vine

(Ph.D. 1965, Cambridge)), and also the German meteorolo￾gist Alfred Wegener (1880–1930); (2) Milutin Milankovitch

(1879–1958), Serbian astronomer and civil engineer, and

two astronomers (John Stockwell and Urbain Leverrier)

delivering input to his calculations (The leading contempora￾neous proponent of orbital forcing is André Berger, Belgian

astronomer and climatologist.); (3) contemporaneous pio￾neers are the NZ-US marine geologist James P. Kennett

(Ph.D. 1965, Wellington), the British geophysicist Nick

Shackleton (1937–2006), the isotope chemist Sam Savin

(Ph.D. 1967, Pasadena), and the geologist Robert Douglas

(Ph.D. 1966, U.C. Los Angeles); and (4) the impact pioneers

are the German-Swiss geologist and paleontologist Hanspeter

Luterbacher, the Italian geologist Isabella Premoli Silva, and

the Californian physicist Luis Alvarez (1911–1988) and his

geologist son Walter (Berkeley) and their associates F. Asaro

(1927–2014) and H. Michel (Berkeley). The crucial papers

and books were published (1) in the 1920s and 1960s

(Continental Drift and Plate Tectonics), (2) in the 1920s and

1980s (Orbital Ice Age Theory, proposed and verified), (3) in

the 1970s (microfossils and oxygen isotopes), and (4) in the

1960s and 1980s (sudden end-of-Cretaceous mass extinction

documented in pelagic sediments on land surveyed and irid￾ium maximum found, respectively).

What, if anything, do the four great revolutions have in

common?

They emphasize the control of geologic history by outside

forcing, either “endogenic” (processes driven by mantle con￾vection) or “astronomic” ones controlled by solar system

phenomena and a call on “positive feedback” for amplifica￾tion (“negative feedback” stabilizes).

The discovery of the great importance of outside forcing

(including the unpredictable and highly variable factors of

earthquakes, volcanism, and collisions in space) has resulted

in some discrimination against regular surficial Earth pro￾cesses, which have considerably damaged the perception

that the pronouncements of the British lawyer Charles Lyell

(1797–1875) regarding uniformitarianism (a long bastion of

textbook geology) hold water. His central concept, that the

present is the key to the past, became suspect as a dominating

rule in the interpretation of the geologic record, especially in

an ice age. The same is true for the reverse assertion that the

past is the key to the present (or to the future). “Endogenic

forces” are difficult to observe, being generated in the mantle

of the planet. While regular astronomic forcing can be calcu￾lated with great precision, it is not necessarily well under￾stood in its applications to a complicated Earth response.

In system analysis (which started as a concept in engi￾neering), “negative feedback” drives a system toward origi￾nal conditions during episodes of change. Negative feedback

favors the status quo. The idea of negative feedback of a

chemical system is the backbone of “Le Chatelier’s

Principle,” proclaimed many decades ago by the French pio￾neer chemist Henry Louis Le Chatelier (1850–1936) and

1

2

soon after elaborated for the Earth system by the Russian￾French geochemist Vladimir Vernadsky (1863–1945). The

“daisyworld” model of the UK chemist James Lovelock

(Ph.D. 1948, London) beautifully illustrates the negative

feedback concept. “Positive feedback,” in contrast, increases

any change and eventually leads to blowout if not checked.

As a corollary, when positive (“non-Gaian,” K.J. Hsϋ) feed￾back is at work, relatively modest forcing can result in enor￾mously large changes. In a very general way, negative

feedback supports Lyell and Gaia and traditional geologic

thinking going back almost two centuries, while positive

feedback does not: it produces unexpected results and abrupt

change and tends to be rather hazardous, therefore.

1.1.2 Plate Tectonics and Endogenic Forcing

A little more than half a century ago, it was still possible to

think that sediments on the deep seafloor offer information

for the entire Phanerozoic, that is, the last half billion years.

In fact, some geologists thought the deep-sea record might

lead us back even into the Precambrian, into times a billion

years ago, or more. Today, it is no longer possible to harbor

such thoughts. The deep seafloor is young, geologically

speaking. The most ancient sediments recovered are about

150 million years old, that is, less than 5% of the age

accorded to ancient rocks on land. Deep-sea deposits older

than Jurassic presumably existed at some point, but it is

thought that they vanished, entering the mantle by subduc￾tion in trenches.

The main relevant activity providing for the forcing of plate

tectonics is in the mantle of Earth (Fig. 1.1). It is not known

just how the mantle operates in the context of plate tectonics.

“Seafloor spreading” is responsible for the world￾encircling mid-ocean ridge and for the Atlantic. Volcanic

activity associated with subduction (Fig. 1.2, lower panel) is

rampant all around the Pacific Basin (hence, the label “Ring

of Fire” for the Pacific margin). It is especially evident in

South America (Fig. 1.3) but also on the northern West Coast

of the USA, in Alaska, and in Japan. Earthquakes generated

in the subduction belts can and do produce “tsunamis,” that is,

waves that travel on the surface of the ocean at the speed of jet

aircraft and that grow to enormous size in shallow water.

1.1.3 Northern Ice-Age Cycles

Unlike his predecessors, the ice-age pioneer Milutin

Milankovitch supposed that it is forced melting of ice in high

northern latitudes that holds the answer to the cycles and not

the pulsed making of ice (which is the question that was tra￾ditionally addressed). According to Milankovitch, what mat￾ters is whether the sun’s warmth is strong in northern summer

or not. The influx of solar heat is controlled by the changing

tilt of the Earth’s axis (the “obliquity”) studied quantitatively

by the French astronomer Pierre-Simon de Laplace (1749–

1827) and by the changing Earth-Sun distance, a topic tack￾led by Johannes Kepler (1571–1630) some four centuries

ago. Milankovitch’s proposition, worked out mathemati￾cally, is now known as “Milankovitch Theory,” reflecting its

Oceanic crust

Continental crust

Rift valley

Solid core

Liquid core

Subduction

Mantle

Lithosphere

(crust and upper mantle)

broken into tectonic plates

divergence

ridge crest

Fig. 1.1 The main elements

of the planet’s structure, not

to scale and hypothetical

regarding mantle motion.

Roughly one half of the Earth

is mantle; crust is negligible

by volume, in comparison.

The mantle convects, largely

in unknown ways.

Alternatively to the picture

shown, fast convection may

be restricted to an upper

layer, while convection in the

lower mantle is much slower.

The lithosphere (the stiff

uppermost part of the mantle)

is roughly 100 km thick.

Together with the crust, it

defines the “plates”

(Background graph courtesy

of S.I.O. Aquarium Education

Program, here modified)

1 Introduction