1

Historical Background

1.1 Introduction

The origin of mountain ranges, volcanoes, earthquakes, the ocean basins, the continents, andthe very nature of the Earth’s interior are questions as old as science itself. In the developmentofmodern science, virtually every famousNatural Philosopher has conjectured on the state ofthe deep interior and its relationship to the Earth’s surface. And every one of these thinkershas come to essentially the same general conclusion: despite the obvious solidity of theEarth beneath our feet, the interior must have flowed, in order to create the complex surfacegeology we see today. Although we can trace this idea as far back as written scientificrecord permits, it nevertheless remained a strictly qualitative hypothesis until the early partof the twentieth century. Then several timely developments in physics, fluid mechanics,geophysics, and geology finally established a true physical paradigm for the Earth’s interior,mantle convection.

In reviewing the development of the concept ofmantle convection, we find it is impossibleto identify one particular time or event, or one particular individual, as being decisive in eitherits construction or its acceptance. Instead, the subject’s progress has followed a meanderingcourse, assisted along by the contributions of many. Still, there are a few scientific pioneerswhose insights were crucial at certain times. These insights deserve special recognitionand, when put together, provide some historical context with which future progress can bemeasured.

The idea of flow in the Earth’s interior was popular among the early Natural Philosophers,as it was commonly assumed that only the outermost portion of the Earthwas solid. Descartesimagined the Earth to consist of essentially sedimentary rocks (the crust) lying over a shell ofdenser rocks (the mantle) with a metallic center (the core). Leibniz proposed that the Earthcooled from an initially molten state and that the deep interior remained molten, a relic ofits formation. Edmond Halley argued that the flow of liquids in a network of subsurfacechannels would explain his discovery of the secular variation of the geomagnetic field. BothNewton and Laplace interpreted the equatorial bulge of the Earth to be a consequence of afluid-like response to its rotation. The idea that the Earth’s interior included fluid channelsand extensive molten regions, as expressed in the artist’s drawing in Figure 1.1, was thedominant one until the late nineteenth century, when developments in the theory of elasticityand G. H. Darwin’s (1898) investigation of the tides indicated that the Earth was not onlysolid to great depths, but also “more rigid than steel.”

In the eighteenth and early nineteenth centuries, resolution of an old controversy ledgeologists to accept the idea of a hot and mobile Earth interior. This controversy centered on

1

2 Historical Background

Figure 1.1. An early depiction of the Earth’s interior, showing channels of fluid connected to a molten centralcore. This was the prevailing view prior to the nineteenth century.

the origin of rocks and pitted the so-called Neptunists, led byAbrahamWerner, who thoughtall rocks were derived by precipitation from a primeval ocean, against others who held thatigneous rocks crystallized from melts and were to be distinguished from sedimentary rocksformed by surficial processes. Theirmost influentialmemberwas an amateur scientist, JamesHutton (Figure 1.2), who advanced the concept of uniformitarianism, that the processesevident today were those that shaped the Earth in the past. He also held to the idea of amolten, flowing interior, exerting forces on the solid crust to form mountain ranges, close tothe modern view based on mantle convection. Ultimately Hutton’s view prevailed, and withit, an emphasis on the idea that the fundamental physical process behind all major geologicalevents is heat transfer from the deep interior to the surface. Thus, geologists were receptiveto the idea of a hot, mobile Earth interior. However, most of the geological and geophysicalevidence obtained from the continental crust seemed to demand vertical motions, ratherthan horizontal motions. For example, in the mid-nineteenth century it was discovered thatmountain ranges did not have the expected positive gravity anomaly. This was explainedby low-density continental roots, which floated on the denser mantle like blocks of woodin water, according to the principle of hydrostatic equilibrium. This implied, in turn, thatthe mantle behaved like a fluid, allowing vertical adjustment. However, the notion that thecrust experiences far larger horizontal displacements was less well supported by evidenceand was not widely held.

1.1 Introduction 3

Figure 1.2. James Hutton (1726–1797), thefather of Geology and proponent of internalheat as the driving force for Earth’s evolution.

The idea that flow in the Earth’s interior is a form of thermal convection developedslowly. Recognition of the significance of thermal convection as a primary fluid mechanicalphenomenon in nature came from physicists. Count Rumford is usually given credit forrecognizing the phenomenon around 1797 (Brown, 1957), although the term convection(derived from convectio, to carry) was first used by Prout (1834) to distinguish it from theother known heat transfer mechanisms, conduction and radiation. Subcrustal convection inthe Earth was first suggested byW.Hopkins in 1839 and the first interpretations of geologicalobservations using convectionweremade byOsmond Fisher (1881). Both of these presumeda fluid interior, so when the solidity of the mantle was established, these ideas fell out offavor.

The earliest experiments on convection in a layer of fluid heated from below and cooledfrom above were reported by J. Thompson (1882), who observed a “tesselated structure” inthe liquid when its excess temperature, compared to that of the overlying air, was sufficientlylarge. But the name most closely associated with convection is Henri Bénard (Figure 1.3).Bénard (1900, 1901) reported the first quantitative experiments on the onset of convection,including the role of viscosity, the cellular planform, and the relationship between cell sizeand fluid layer depth. Bénard produced striking photographs of the convective planform inthin layers of viscous fluids heated from below (Figure 1.4). The regular, periodic, hexagonalcells in his photographs are still referred to as Bénard cells. Since Bénard used fluid layers incontact with air, surface tension effects were surely present in his experiments, as he himselfrecognized. It has since been shown that Bénard’s cells were driven as much by surfacetension gradients as by gradients in buoyancy. Still, he correctly identified the essentials ofthermal convection, and in doing so, opened a whole new field of fluidmechanics. Motivatedby the “interesting results obtained by Bénard’s careful and skillful experiments,” LordRayleigh (1916; Figure 1.5) developed the linear stability theory for the onset of convection

4 Historical Background

Figure 1.3. Henri Bénard (1880–1939) (on the left) madethe first quantitative experiments on cellular convection inviscous liquids. The picture was taken in Paris about 1920with Reabouchansky on the right.

Figure 1.4. Photograph of hexagonal con-vection cells in a viscous fluid layer heatedfrom below, taken by Bénard (1901).

in a horizontally infinite fluid layer between parallel surfaces heated uniformly from belowand cooled uniformly from above, and isolated the governing dimensionless parameter thatnow bears his name. It was unfortunate that these developments in fluid mechanics were notfollowed more widely in Earth Science, for they might have removed a stumbling block toacceptance of the milestone concept of continental drift.

1.2 Continental Drift 5

Figure 1.5. Lord Rayleigh (1842–1919) developed thetheory of convective instability in fluids heated frombelow.

1.2 Continental Drift

The earliest arguments for continental drift were largely based on the fit of the continents.Ever since the first reliable maps were available, the remarkable fit between the east coastof South America and the west coast of Africa has been noted (e.g., Carey, 1955). Indeed,the fit was pointed out as early as 1620 by Francis Bacon (Bacon, 1620). North America,Greenland, and Europe also fit as illustrated in Figure 1.6 (Bullard et al., 1965).

Geological mapping in the southern hemisphere during the nineteenth century revealedthat the fit between these continents extends beyond coastline geometry. Mountain belts inSouth America match mountain belts in Africa; similar rock types, rock ages, and fossilspecies are found on the two sides of the Atlantic Ocean. Thus the southern hemispheregeologists were generally more receptive to the idea of continental drift than their northernhemisphere colleagues, where the geologic evidence was far less conclusive.

Further evidence for continental drift came from studies of ancient climates. Geologistsrecognized that tropical climates had existed in polar regions at the same times that arc-tic climates had existed in equatorial regions. Also, the evolution and dispersion of plantand animal species was best explained in terms of ancient land bridges, suggesting directconnections between now widely separated continents.

As previously indicated, most geologists and geophysicists in the early twentieth centuryassumed that relative motions on the Earth’s surface, including motions of the continentsrelative to the oceans, were mainly vertical and generally quite small – a few kilometersin extreme cases. The first serious advocates for large horizontal displacements were twovisionaries, F. B. Taylor and Alfred Wegener (Figure 1.7). Continental drift was not widelydiscussed until the publication ofWegener’s famous book (Wegener, 1915; see alsoWegener,1924), but Taylor deserves to share the credit for his independent and somewhat earlieraccount (Taylor, 1910). Wegener’s book includes his highly original picture of the breakup

6 Historical Background

Figure 1.6. The remarkable “fit” between the continental margins of North and SouthAmerica and Greenland,Europe, and Africa (from Bullard et al., 1965). This fit was one of the primary early arguments for continentaldrift.

and subsequent drift of the continents, and his recognition of the supercontinent Pangaea (allEarth). (Later it was argued (duToit, 1937) that there had formerly been a northern continent,Laurasia, and a southern continent, Gondwanaland, separated by theTethys ocean.)Wegenerassembled a formidable array of facts and conjecture to support his case, includingmuch thatwas subsequently discredited. This partially explains the hostile reception his book initiallyreceived. However, the most damaging criticisms came from prominent geophysicists suchas H. Jeffreys in England and W. Bowie in the U.S., who dismissed the idea because thedriving forces for continental drift proposed by Taylor and Wegener (tidal and differentialcentrifugal forces, respectively) were physically inadequate. (Wegener was a meteorologistand recognized that the Earth’s rotation dominated atmospheric flows. He proposed that these

1.2 Continental Drift 7

Figure 1.7. Alfred Wegener (1880–1930), the father ofcontinental drift.

Figure 1.8. Harold Jeffreys (1891–1989), themost influential theorist in the early debate overcontinental drift and mantle convection.

rotational forces were also responsible for driving the mantle flows resulting in continentaldrift.) At the same time, seismologists were exploring the Earth’s deep interior, and wereimpressed by the high elastic rigidity of the mantle. In his influential book, The Earth(Jeffreys, 1929), Sir Harold Jeffreys (Figure 1.8) referred to themantle as the “shell,” arguingthat this term better characterized its elastic strength. Paradoxically, Jeffreys was at the same

8 Historical Background

time making fundamental contributions to the theory of convection in fluids. For example,he showed (Jeffreys, 1930) that convection in a compressible fluid involved the differencebetween the actual temperature gradient and the adiabatic temperature gradient. This resultwould later figure prominently in the development of the theory of whole mantle convection.But throughout his illustrious career, Jeffreys maintained that the idea of thermal convectionin the highly rigidmantlewas implausible onmechanical grounds. The realization that a solidcould exhibit both elastic and viscous properties simultaneously was just emerging from thestudy of materials, and evidently had not yet come fully into the minds of geophysicists.

The failure of rotational and tidal forcesmeant that some othermechanism had to be foundto drive the motion of the continents with sufficient power to account for the observed defor-mation of the continental crust, seismicity, and volcanism. In addition, such a mechanismhad to operate in the solid, crystalline mantle.

Question 1.1: What is the source of energy for the tectonics and volcanism ofthe solid Earth?

Question 1.2: How is this energy converted into the tectonic and volcanicphenomena we are familiar with?

The mechanism is thermal convection in the solid mantle, also referred to as subsolidusmantle convection. A fluid layer heated from below and cooled from above will convect ina gravitational field due to thermal expansion and contraction. The hot fluid at the base ofthe layer is less dense than the cold fluid at the top of the layer; this results in gravitationalinstability. The light fluid at the base of the layer ascends and the dense fluid at the topof the layer descends. The resulting motion, called thermal convection, is the fundamentalprocess in the Earth’s tectonics and volcanism and is the subject of this book. We will seethat the energy to drive subsolidus convection in the mantle and its attendant geologicalconsequences (plate tectonics, mountain building, volcanic eruptions, earthquakes) derivesfrom both the secular cooling of the Earth’s hot interior and the heat produced by the decayof radioactive elements in the rocks of the mantle.

The original proposal for subsolidus convection in the mantle is somewhat obscure. Bull(1921, 1931) suggested that convection in the solid mantle was responsible for continentaldrift, but he did not provide quantitative arguments in support of his contention. About thesame time, Wegener came to realize that his own proposed mechanism was inadequate forcontinental drift. He apparently considered the possibility of mantle convection, and madepassing reference to it as a plausible driving force in the final edition of his book (Wegener,1929). It was during this era that the importance of convection was first being recognized inhis own field of meteorology. ButWegener chose not to promote it as the cause of continentaldrift, and the idea languished once again.

1.3 The Concept of Subsolidus Mantle Convection

Arthur Holmes (1931, 1933; Figure 1.9) was the first to establish quantitatively that thermalconvection was a viable mechanism for flow in the solid mantle, capable of driving con-tinental drift. Holmes made order of magnitude estimates of the conditions necessary forconvection, the energetics of the flow, and the stresses generated by the motion. He con-cluded that the available estimates of mantle viscosity were several orders of magnitude less

1.3 The Concept of Subsolidus Mantle Convection 9

Figure 1.9. Arthur Holmes (1890–1965), the firstprominent advocate for subsolidus mantle convection.

Figure 1.10. Arthur Holmes’ (1931) depiction of mantle convection as the cause of continental drift, thirtyyears prior to the discovery of seafloor spreading.

than that required for the onset of convection. He also outlined a general relation betweenthe ascending and descending limbs of mantle convection cells and geological processes,illustrated in Figure 1.10. Holmes argued that radioactive heat generation in the continentsacted as a thermal blanket inducing ascending thermal convection beneath the continents.Holmes was one of themost prominent geologists of the time, and in his prestigious textbookPrinciples of Physical Geology (Holmes, 1945), he articulated the major problems of mantleconvection much as we view them today.

The creep viscosity of the solid mantle was first determined quantitatively by Haskell(1937). Recognition of elevated beach terraces in Scandinavia showed that theEarth’s surface

10 Historical Background

is still rebounding from the load of ice during the last ice age. By treating the mantle as aviscous fluid, Haskell was able to explain the present uplift of Scandinavia if the mantle hasa viscosity of about 1020 Pa s. Remarkably, this value of mantle viscosity is still acceptedtoday.Although an immense number (water has a viscosity of 10−3 Pa s), it predicts vigorousmantle convection on geologic time scales.

The viscous fluid-like behavior of the solid mantle on long time scales required an expla-nation. How could horizontal displacements of thousands of kilometers be accommodatedin solid mantle rock?

Question 1.3: Why does solid mantle rock behave like a fluid?

The answer was provided in the 1950s, when theoretical studies identified several mech-anisms for the very slow creep of crystalline materials thereby establishing a mechanicalbasis for the mantle’s fluid behavior. Gordon (1965) showed that solid-state creep quan-titatively explained the viscosity determined from observations of postglacial rebound. Attemperatures that are a substantial fraction of the melt temperature, thermally activatedcreep processes allow hot mantle rock to flow at low stress levels on time scales greater than104 years. In hindsight, the flow of the crystalline mantle should not have been a surprisefor geophysicists since the flow of crystalline ice in glaciers had long been recognized andaccepted.

In the 1930s a small group of independent-minded geophysicists including Pekeris(1935), Hales (1936), and Griggs (1939) attempted to build quantitative models of man-tle convection. Figure 1.11 shows an ingenious apparatus built by Griggs to demonstratethe effects of mantle convection on the continental crust. Griggs modeled the crust withsand–oil mixtures, the mantle with viscous fluids, and substituted mechanically drivenrotating cylinders for the thermal buoyancy in natural convection. His apparatus pro-duced crustal roots and near-surface thrusting at the convergence between the rotatingcylinders; when only one cylinder was rotated, an asymmetric root formed with similar-ities to a convergent plate margin, including a model deep sea trench. The early workof Pekeris and Hales were attempts at finite amplitude theories of mantle convection.They included explanations for dynamic surface topography, heat flow variations, andthe geoid based on mantle convection that are essentially correct according to our presentunderstanding.

In retrospect, these papers were far ahead of their time, but unfortunately their impactwas much less than it could have been. In spite of all the attention given to continental drift,the solid foundation of convection theory and experiments, and far-sighted contributions ofa few to create a framework for convection in the mantle, general acceptance of the ideacame slowly. The vast majority of the Earth Sciences community remained unconvincedabout the significance of mantle convection. We can identify several reasons why the EarthScience community was reluctant to embrace the concept, but one stands out far abovethe others: the best evidence for mantle convection comes from the seafloor, and until themiddle of the twentieth century the seafloor was virtually unknown. The situation began tochange in the 1950s, when two independent lines of evidence confirmed continental driftand established the relationship between the continents, the oceans, and mantle convection.These were paleomagnetic pole paths and the discovery of seafloor spreading. We willconsider each of these in turn.

1.4 Paleomagnetism 11

Figure 1.11. Early experiment on mantle convection by David Griggs (1939), showing styles of deformationof a brittle crustal layer overlying a viscous mantle. The cellular flow was driven mechanically by rotatingcylinders.

1.4 Paleomagnetism

Many rocks contain small amounts of magnetic and paramagnetic minerals that acquirea weak remnant magnetism at the time of crystallization of the rock. Thus igneous rockspreserve evidence of the direction of the Earth’s magnetic field at the time they were formed.In some cases sedimentary rocks also preserve remnant magnetism. Studies of the remnantmagnetism in rocks are known as paleomagnetism and were pioneered in the early 1950sby Blackett (1956) and his colleagues.

This paleomagnetic work demonstrated inconsistencies between the remnant magneticfield orientations found in old rocks and themagnetic field inwhich the rocks are found today.In some cases, corrections had to be made for the effects of the local tilting and rotation thatthe rocks had undergone since their formation. Even so, having taken into account these andother possible effects, discrepancies remained. Several explanations for these discrepancieswere given: (1) variations in the Earth’s magnetic field, (2) movement of the entire outershell of the Earth relative to the axis of rotation (i.e., polar wander), and (3) continental drift.

The systematic variations in the remnant magnetism strongly favored the third explana-tion. When the remnant magnetic vectors for a series of rocks with different ages from thesame locality were considered together, the orientation had a regular and progressive changewith age, with the most recent rocks showing the closest alignment with the present field.This is shown graphically by plotting a series of “virtual magnetic poles.” For each rock inthe time series, a “pole position” is derived from its magnetic inclination and declination.If sufficient points are plotted, they form a curved line terminating near the present pole forthe youngest rocks; when rocks in North America and Europe are compared, the openingof the North Atlantic is clearly illustrated as shown in Figure 1.12. In the late 1950s thesestudies were taken by their proponents as definitive evidence supporting continental drift

12 Historical Background

European Path

Cu

S-D

N. Amer.Path

S-D

P Cu

TruTrl

Tr

K

K

C

C

P

Figure 1.12. Polar wander paths based on observations from North America and Europe. Points on the pathare identified by age in millions of years (C = Cambrian 540–510; S-D = Silurian/Devonian 440–290;Cu = Upper Carboniferous 325–290; P = Permian 290–245; Tr, l, u = Triassic, lower, upper 225–190;K = Cretaceous 135–65). The shapes of the paths are approximately the same until the Triassic when thecontinents began to separate.

(Runcorn, 1956, 1962a). The opponents of continental drift argued that the results could bedue to variations in the structure of the Earth’s magnetic field.

There was another important result of the paleomagnetic studies. Although consistentand progressive changes in the magnetic inclination and declination were observed for eachcontinent, the polarity of the remnant magnetic field was highly variable and in some casesagreed with the polarity of the present field and in others was reversed (Cox et al., 1963,1964). The recognition that virtually all rocks with reversed polarity had formed withinspecific time intervals, regardless of latitude or continent, led to the conclusion that thereversed magnetic polarities were the result of aperiodic changes in the polarity of theEarth’s magnetic field. These reversals were to play a key role in quantifying the seafloorspreading hypothesis, discussed in the next section.

1.5 Seafloor Spreading

In the decades leading up to the 1950s, extensive exploration of the seafloor led to thediscovery of a worldwide range of mountains on the seafloor, the mid-ocean ridges. Ofparticular significance was the fact that the mid-Atlantic ridge bisects the entire AtlanticOcean. The crests of the ridges have considerable shallow seismic activity and volcan-ism. Ridges also display extensional features suggesting that the crust is moving awayon both sides. These and other considerations led to the hypothesis of seafloor spreading(Dietz, 1961;Hess, 1962, 1965), wherein the seafloormoves laterally away from each side ofa ridge and new seafloor is continuously being created at the ridge crest bymagma ascendingfrom the mantle.

1.6 Subduction and Area Conservation 13

It was immediately recognized that the new hypothesis of seafloor spreading is entirelyconsistent with the older idea of continental drift. New ocean ridges form where continentsbreak apart, and new ocean crust is formed symmetrically at this ocean ridge, creating a newocean basin. The classic example is the Atlantic Ocean, with the mid-Atlantic ridge beingthe site of ocean crust formation. In the process of seafloor spreading, the continents aretransported with the ocean floor as nearly rigid rafts.

This realization was a critical advance in defining the role of mantle convection. It shiftedthe emphasis from the continents to the seafloor. The drifting blocks of continental crustcame to be seen as nearly passive participants in mantle convection, whereas the dynamicsof the process were to be found in the evolution of the ocean basins.

Striking direct evidence for the seafloor spreading hypothesis came from the linear mag-netic field anomalies that parallel the ocean ridges (Mason, 1958).Vine andMatthews (1963)proposed that these linear anomalies represent a fossilized history of the Earth’s magneticfield. As the oceanic crust is formed at the crest of the ocean ridge, the injected basalticmagmas crystallize and cool through the Curie temperature, and the newly formed rock ismagnetized with the polarity of the Earth’s magnetic field.

The magnetic anomaly produced by the magnetized crust as it spreads away from theridge is a record of the Earth’s magnetic field at the time the crust formed (Figure 1.13).Since it had just been shown that the Earth’s magnetic field is subject to aperiodic reversalsand these reversals had been dated from studies of volcanics on land (Cox et al., 1964), therate of movement of the crust away from the ridge crest was determined from the spacingof the magnetic stripes (Vine and Wilson, 1965; Pitman and Heirtzler, 1966; Vine, 1966;Dickson et al., 1968). A comprehensive summary of the worldwide distribution of magneticstripes was given by Heirtzler et al. (1968).

A typical space–time correlation for the East Pacific Rise is given in Figure 1.13. Thedata correlate the distance from the ridge crest to the position where the magnetic anomalychanges sign with the time when the Earth’s magnetic field is known from independentevidence to have reversed. For this example, with 3.3Myr seafloor lying 160km from theridge axis, the spreading rate is 48mmyr−1, and so new seafloor is being created at a rateof 96mmyr−1.

The Vine–Matthews interpretation of the magnetic anomalies was confirmed by thedrilling program of the Glomar Challenger (Maxwell et al., 1970). After drilling through thelayer of sediments on the ocean floor, the sediment immediately overlying the basaltic crustwas dated. The ages of the basal sediments as a function of distance from the mid-Atlanticridge are compared with the magnetic anomaly results for the same region in Figure 1.14.The agreement is excellent, and the slope of the correlation gives a constant spreading rate of20mmyr−1. The conclusion is that the Atlantic Ocean is widening at a rate of 40mmyr−1.

1.6 Subduction and Area Conservation

The observed rates of seafloor spreading are large when compared with those of many othergeological processes. With the present distribution of ridges, they would be sufficient todouble the circumference of the Earth in several hundred million years (5% of its age). Ifthe seafloor spreading hypothesis is correct, the Earth must either be expanding very rapidlyor surface area must in some way be consumed as rapidly as it is generated by the spreadingprocess. Each of these alternatives had a champion: S. Warren Carey chose the former andHarry Hess chose the latter.

14 Historical Background

E

W

(a)

Reversed Profile

Observed Profile

150 0 150 km

500γ

Gilber

t

Gauss

Mat

uyam

a

Brunh

es

Mat

uyam

a

Gauss

Gilber

t

Mid-

Ocean

Ridg

e

3.3 Million Years3.3

Brunh

es

Zone of Cooling and Magnetization

(b)

Figure 1.13. (a) Magnetic profile across the East Pacific Rise at 51◦S; the lower profile shows the observedprofile reversed for comparison. (b) The spreading ocean floor. Hot material wells up along the oceanic ridge.It acquires a remanent magnetism as it cools and travels horizontally away from the ridge. At the top of thediagram are shown the known intervals of normal (gray) and reversed (white) polarity of the Earth’s magneticfield, with their names. The width of the ocean-floor stripes is proportional to the duration of the normal andreversed time intervals.

Carey (1958, 1976) had been concerned about the geometrical reconstruction of formercontinental masses. These studies led him to view the Earth as undergoing rapid expansion.This view did not receive wide support and it will not be considered further. There is nosatisfactory physical explanation of how such a large volume expansionmight have occurredand it is inconsistent both with estimates of the change in the Earth’s rate of rotation andwith paleomagnetic constraints on changes in the Earth’s radius.

1.6 Subduction and Area Conservation 15

Figure 1.14. Age of the oceanic crust versus dis-tance from the crest of the mid-Atlantic ridge, asinferred from the magnetic anomaly pattern andfrom the oldest sediments.

0

20

40

60

80

0 500 1000 1500

Distance from Ridge Crest, km

Age,Myr

Magnetic(Dickson et al., 1968)Sediments(Maxwell et al., 1970)

Hess (1962) favored an Earth of constant volume and believed that there were zones inthe oceans where the evidence suggested that surface area might be lost. He drew attentionto the deep ocean trenches of the Pacific and other oceans, which had been studied for manyyears.

It had been recognized in the thirties by the Dutch geophysicist Vening Meinesz (1937,1948) that very large gravity anomalieswere associatedwith trenches. The trencheswere alsoassociated with unusual seismic activity. Over most of the Earth, earthquakes are restrictedto the outer fifty or so kilometers; at ocean trenches, however, earthquakes lie within aninclined zone that intersects the surface along the line of the trench and dips downward intothe mantle at angles ranging from 30 to 80◦. Along these zones, seismic activity extends formany hundreds of kilometers, in some cases as deep as 700km.

Hess’s view, and hewas followedbyVine andMatthews (1963), was that trencheswere thesites of crustal convergence; new ocean floor is created at mid-ocean ridges, travels laterallyand is consumed at ocean trenches. The loss of surface area at trenches subsequently becameknown and understood as subduction. Hess also proposed a fundamental difference betweenoceanic and continental crust. The former was being continuously created at ocean ridgesand lost at ocean trenches. Because oceanic crust is relatively thin, the buoyant body forceswhich would resist subduction are relatively small. The crust of the continents, however, is5–6 times as thick and has a slightly greater density contrast with the underlying mantle.As a consequence it cannot be subducted. This view was certainly influenced by the failureto identify any part of the floors of the deep oceans older than 200 million years and therecognition that continents commonly contained rocks several billion years old.

The concepts of continental drift, seafloor spreading, and subduction were combinedinto the plate tectonics model that revolutionized geophysics in the mid-to-late 1960s. Theessentials of the plate tectonics model will be discussed in the next chapter. Although platetectonics is an enormously successful model, it is essentially kinematic in nature. As theaccounts in this chapter indicate, the search for a fully dynamic theory for tectonics hasproven to be a far more difficult task. It has involved many branches of Earth Science,and continues to this day. This book describes the achievements as well as the questionsremaining in this search.