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 coast of 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 in South 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 northern hemisphere 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 direct connections 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 kilometers in extreme cases. The first serious advocates for large horizontal displacements were two visionaries, F. B. Taylor and Alfred Wegener (Figure 1.7). Continental drift was not widely discussed until the publication ofWegener's famous book (Wegener, 1915; see also Wegener, 1924), but Taylor deserves to share the credit for his independent and somewhat earlier account (Taylor, 1910). Wegener's book includes his highly original picture of the breakup

Figure 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 (all Earth). (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 that was 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 the driving 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

Figure 1.8. Harold Jeffreys (1891-1989), the most influential theorist in the early debate over continental drift and mantle convection.

Mantle Rotation Earth

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," arguing that this term better characterized its elastic strength. Paradoxically, Jeffreys was at the same 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 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 convection in 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 the study 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 found to 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 in a 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 top of 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 see that the energy to drive subsolidus convection in the mantle and its attendant geological consequences (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 decay of 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 the same time, Wegener came to realize that his own proposed mechanism was inadequate for continental drift. He apparently considered the possibility of mantle convection, and made passing 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.

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