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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 developmentof modern science, virtually every famous Natural Philosopher has conjectured on the state ofthe deep interior and its relationship to the Earth’s surface. And every one of these thinkers
has come to essentially the same general conclusion: despite the obvious solidity of the
Earth beneath our feet, the interior must have ﬂowed, in order to create the complex surface
geology 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 of mantle convection, we ﬁnd it is impossible
to identify one particular time or event, or one particular individual, as being decisive in either
its 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 pioneers
whose insights were crucial at certain times. These insights deserve special recognition
and, 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 Earth was solid. Descartes
imagined 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 subsurface
channels would explain his discovery of the secular variation of the geomagnetic ﬁeld. Both
Newton 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 channels
and 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 elasticity
and G. H. Darwin’s (1898) investigation of the tides indicated that the Earth was not only
solid to great depths, but also “more rigid than steel.”In the eighteenth and early nineteenth centuries, resolution of an old controversy led
geologists to accept the idea of a hot and mobile Earth interior. This controversy centered on
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Historical BackgroundFigure 1.1. An early depiction of the Earth’s interior, showing channels of ﬂuid connected to a molten central
core. This was the prevailing view prior to the nineteenth century.

the origin of rocks and pitted the so-called Neptunists, led by Abraham Werner, who thought
all rocks were derived by precipitation from a primeval ocean, against others who held that
igneous rocks crystallized from melts and were to be distinguished from sedimentary rocks
formed by surﬁcial processes. Their most inﬂuential member was an amateur scientist, James
Hutton (Figure 1.2), who advanced the concept of uniformitarianism, that the processes
evident 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 to
the 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 receptive
to 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, rather
than 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 evidence
and was not widely held.

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1.1 Introduction

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

The idea that ﬂow in the Earth’s interior is a form of thermal convection developed
slowly. 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 in
the Earth was ﬁrst suggested by W. Hopkins in 1839 and the ﬁrst interpretations of geologicalobservations using convection were made by Osmond Fisher (1881). Both of these presumed
a 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 cooled
from 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 sufﬁciently
large. 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 himself
recognized. It has since been shown that Bénard’s cells were driven as much by surface
tension gradients as by gradients in buoyancy. Still, he correctly identiﬁed the essentials ofthermal convection, and in doing so, opened a whole new ﬁeld of ﬂuid mechanics. Motivatedby the “interesting results obtained by Bénard’s careful and skillful experiments,” Lord
Rayleigh (1916; Figure 1.5) developed the linear stability theory for the onset of convection© Cambridge University Press

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Historical BackgroundFigure 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 1920
with Reabouchansky on the right.

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

in a horizontally inﬁnite ﬂuid layer between parallel surfaces heated uniformly from below
and 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.

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1.2 Continental Drift

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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 revealed
that the ﬁt between these continents extends beyond coastline geometry. Mountain belts inSouth America match mountain belts in Africa; similar rock types, rock ages, and fossil
species are found on the two sides of the Atlantic Ocean. Thus the southern hemisphere
geologists 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. Geologists
recognized that tropical climates had existed in polar regions at the same times that arctic climates had existed in equatorial regions. Also, the evolution and dispersion of plant
and 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 century
assumed that relative motions on the Earth’s surface, including motions of the continents
relative 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 widely
discussed until the publication of Wegener’s famous book (Wegener, 1915; see also Wegener,
1924), but Taylor deserves to share the credit for his independent and somewhat earlier
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Historical BackgroundFigure 1.6. The remarkable “ﬁt” between the continental margins of North and South America and Greenland,
Europe, and Africa (from Bullard et al., 1965). This ﬁt was one of the primary early arguments for continental
drift.

and subsequent drift of the continents, and his recognition of the supercontinent Pangaea (allEarth). (Later it was argued (du Toit, 1937) that there had formerly been a northern continent,
Laurasia, and a southern continent, Gondwanaland, separated by the Tethys ocean.) Wegener
assembled a formidable array of facts and conjecture to support his case, including much thatwas subsequently discredited. This partially explains the hostile reception his book initially
received. However, the most damaging criticisms came from prominent geophysicists such
as 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 differential
centrifugal forces, respectively) were physically inadequate. (Wegener was a meteorologist
and recognized that the Earth’s rotation dominated atmospheric ﬂows. He proposed that these© Cambridge University Press

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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 continental
drift.) At the same time, seismologists were exploring the Earth’s deep interior, and were
impressed by the high elastic rigidity of the mantle. In his inﬂuential book, The Earth(Jeffreys, 1929), Sir Harold Jeffreys (Figure 1.8) referred to the mantle as the “shell,” arguingthat this term better characterized its elastic strength. Paradoxically, Jeffreys was at the same

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Historical Backgroundtime making fundamental contributions to the theory of convection in ﬂuids. For example,
he showed (Jeffreys, 1930) that convection in a compressible ﬂuid involved the difference
between the actual temperature gradient and the adiabatic temperature gradient. This result
would 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 rigid mantle was implausible on mechanical grounds. The realization that a solid
could 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 forces meant that some other mechanism had to be foundto drive the motion of the continents with sufﬁcient power to account for the observed deformation of the continental crust, seismicity, and volcanism. In addition, such a mechanism
had to operate in the solid, crystalline mantle.
Question 1.1: What is the source of energy for the tectonics and volcanism of
the solid Earth?
Question 1.2: How is this energy converted into the tectonic and volcanic
phenomena we are familiar with?
The mechanism is thermal convection in the solid mantle, also referred to as subsolidus
mantle 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 of
the layer is less dense than the cold ﬂuid at the top of the layer; this results in gravitational
instability. 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 fundamental
process 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) derives
from 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 continental
drift, 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 in
his own ﬁeld of meteorology. But Wegener chose not to promote it as the cause of continental
drift, 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 thermal
convection was a viable mechanism for ﬂow in the solid mantle, capable of driving continental 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 concluded that the available estimates of mantle viscosity were several orders of magnitude less

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1.3 The Concept of Subsolidus Mantle Convection

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

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

than that required for the onset of convection. He also outlined a general relation between
the ascending and descending limbs of mantle convection cells and geological processes,
illustrated in Figure 1.10. Holmes argued that radioactive heat generation in the continents
acted as a thermal blanket inducing ascending thermal convection beneath the continents.
Holmes was one of the most prominent geologists of the time, and in his prestigious textbook
Principles of Physical Geology (Holmes, 1945), he articulated the major problems of mantle
convection 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 the Earth’s surface© Cambridge University Press

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Historical Backgroundis still rebounding from the load of ice during the last ice age. By treating the mantle as a
viscous ﬂuid, Haskell was able to explain the present uplift of Scandinavia if the mantle has
a viscosity of about 1020 Pa s. Remarkably, this value of mantle viscosity is still accepted
today. Although an immense number (water has a viscosity of 10−3 Pa s), it predicts vigorous
mantle convection on geologic time scales.
The viscous ﬂuid-like behavior of the solid mantle on long time scales required an explanation. How could horizontal displacements of thousands of kilometers be accommodated
in 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 mechanisms 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 quantitatively explained the viscosity determined from observations of postglacial rebound. At
temperatures 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 than
104 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 mantle convection. Figure 1.11 shows an ingenious apparatus built by Griggs to demonstrate
the effects of mantle convection on the continental crust. Griggs modeled the crust withsand–oil mixtures, the mantle with viscous ﬂuids, and substituted mechanically driven
rotating cylinders for the thermal buoyancy in natural convection. His apparatus produced crustal roots and near-surface thrusting at the convergence between the rotating
cylinders; when only one cylinder was rotated, an asymmetric root formed with similarities 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 present
understanding.
In retrospect, these papers were far ahead of their time, but unfortunately their impact
was 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 idea
came slowly. The vast majority of the Earth Sciences community remained unconvinced
about the signiﬁcance of mantle convection. We can identify several reasons why the Earth
Science community was reluctant to embrace the concept, but one stands out far above
the others: the best evidence for mantle convection comes from the seaﬂoor, and until the
middle of the twentieth century the seaﬂoor was virtually unknown. The situation began tochange in the 1950s, when two independent lines of evidence conﬁrmed continental drift
and 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.

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