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 is 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 1020Pas. 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 mechanical basis 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 activated creep 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 surprise for geophysicists since the flow of crystalline ice in glaciers had long been recognized and accepted.

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 with sand-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 work of Pekeris and Hales were attempts at finite amplitude theories of mantle convection. They included explanations for dynamic surface topography, heat flow variations, and the 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 of a 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 to change 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 will consider each of these in turn.

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

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