Implications Of Heat Flow

The average vertical thermal gradient at the Earth's surface is about 25 °C km-1. If this gradient remained constant with depth, the temperature at a depth of 100 km would be 2500°C. This temperature is in excess of the melting temperature of mantle rocks at this depth, and so a fluid layer is implied. Such a molten layer does not exist because S waves are known to propagate through this region (Section 2.1.3). Two possibilities exist in explanation of this phenomenon: first, that heat sources are concentrated above a depth of

100 km; and second, that a more efficient mechanism than conduction operates below this depth whereby heat is transferred at a much lower thermal gradient. These processes can be distinguished by considering the variation in heat flow over the Earth's surface in conjunction with the variation in content of radioactive minerals of different crustal types.

Heat flow generally decreases with the age of the crust (Sclater et al., 1980). Within the oceans heat flow decreases from the ocean ridges to the flanking basins and it has been shown (Section 6.4) that this cooling correlates with a progressive thickening of the oceanic lithosphere and an increase in water depth. Similarly, the heat flow of backarc basins (Section 9.10) decreases with age, with the presently active basins exhibiting the greatest heat flow. Within continental regions the heat flow generally decreases with increasing time since the last tectonic event. Consequently, Precambrian shields are characterized by the lowest heat flow and young mountain belts by the highest.

The representation of the global pattern of heat flow is difficult because the density of the observations is highly variable so that the location of contours can be greatly biased by only a small number of measurements. Chapman & Pollack (1975) overcame the problem of limited observations in some areas by predicting the heat flow in those areas on the basis of the correlation of heat flow with the age of the oceanic lithosphere and the age of the last tectonic event to affect continental crust. In Fig. 12.3 their results are presented by a spherical harmonic analysis of the heat flow measured or predicted in 5° X 5° grid areas of the globe. This procedure imparts a certain smoothing of the true pattern, so that variations with wavelengths of less than about 3300 km are not represented. Figure 12.3 illustrates the high heat flow associated with the ocean ridge system and the youngest marginal basins of the western Pacific. Low heat flow values are associated with old oceanic crust and with Precambrian shields.

Histograms of heat flow measurements from oceans and continents are presented in Fig. 12.4. The greater dispersion of the oceanic values reflects variability arising from localized extreme values at the crests of ocean ridges. By contrast, there are fewer extreme high or low values present in the continental values. The mean of oceanic heat flow measurements is 67 mWm-2. However, this only represents the heat loss by conduction, and ignores the heat reaching the surface by the discharge of hot fluids such as water and lava. It is now recognized that the hydrothermal contribution accounts for about a quarter of the global heat loss, and that the average oceanic heat flow is 101 mW m-2. The mean continental heat flow is 65 mW m-2, including the small contribution from lavas. The global average heat flow is 87 mW m-2 (Pollack et al., 1993).

The majority of the heat escaping at the Earth's surface originates from the decay of long-lived radioactive isotopes of uranium, thorium and potassium (Section 2.13) which have half-lives of the same order as the age of the Earth. These isotopes are relatively

Figure 12.3 Pattern of global heat flow represented by spherical harmonic analysis. Contour interval 40 mW m 2 (after Chapman & Pollack, 1975).

Figure 12.4 Comparison of the heat flow from continents and oceans (redrawn from Pollack et al., 1993, by permission of the American Geophysical Union. Copyright © 1993 American Geophysical Union).

100 150

Figure 12.4 Comparison of the heat flow from continents and oceans (redrawn from Pollack et al., 1993, by permission of the American Geophysical Union. Copyright © 1993 American Geophysical Union).

enriched in the upper continental crust, and it has been estimated that their decay contributes 18-38 mW m-2 to the observed heat flow (Pollack & Chapman, 1977). Consequently up to about 60% of the heat flow in continental regions may be generated within the upper 10-20 km of the crust. The oceanic crust, however, is virtually barren of radioactive isotopes, and only about 4 mW m-2 can be attributed to this source. Over 96% of the oceanic heat flow must originate from beneath the crust, and so different processes of heat supply must act beneath continents and oceans (Sclater & Francheteau, 1970).

Thus, a large proportion of the continental heat flow is from sources concentrated at a shallow depth, and only a small sub-crustal component is required. Conversely, the majority of oceanic heat flow must originate at sub-crustal levels. Because of the melting problems discussed above, this heat must be transported under the influence of a low thermal gradient. The mechanism of heat transfer by convection is the only feasible process conforming to these constraints. Therefore, although heat transfer by conduction takes place within the rigid lithosphere, heat transfer by convection must predominate in the sublithospheric mantle. Indeed, conduction cannot occur to any great depth as the rate of heat transfer by this mechanism is much slower than required. The feasibility and form of such convection is discussed in the following sections.

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