The Concentration of the Heat-Producing Isotopes

Tens of thousands of heat flow measurements have been made both in the continents and the oceans. Because the oceanic crust participates in the plate tectonic cycle and the continental crust does not, we can consider these regions separately.

The mean heat flow for all continents is 65 ± 1.6 mW m−2. Regions of high heat flow in the continents are generally restricted to active volcanic areas. Examples are the lines of volcanoes associated with ocean trenches – the Andes, for example – and regions of tensional tectonics such as the western United States. The areas of high heat flow associated with volcanic lines are generally quite small and do not contribute significantly to the mean heat flow. Similarly, areas of tensional tectonics are quite small on a global basis. Broad regions of continental tectonics, such as the collision zone Temperatures Between Layers of Rock Types Depth (m) Temp. (◦C) Rock Type k (Wm–1 K–1)

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380 18.362 Sandstone 3.2 402 18.871 Shale 1.7 412 19.330 Sandstone 5.3 465 20.446 Salt 6.1 475 20.580 Sandstone 3.4  510 21.331 Shale 1.9 515 21.510 extending from the Alps through the Himalayas, have near-normal surface heat flows. Therefore, regions of active tectonics and mountain building make a relatively small contribution to the mean continental heat flow.

In stable continental areas, the surface heat flow has a strong correlation with the surface concentrations of the radioactive, heat-producing isotopes. This correlation, which is discussed in detail in Section 4–8, is illustrated in

Approximately one-half of the surface heat flow in the continents can be attributed to the heat  production from the radioactive isotopes of uranium, thorium, and potassium in the continental crust. Surface heat flow systematically decreases with the age of the surface rocks in stable continental areas. Similarly, the oncentration of the radioactive isotopes in the surface rocks also decreases with the age of the rocks. This decrease is attributed to the progressive effects of erosion that remove the near-surface rocks with the largest concentrations of the heat-producing isotopes. The onclusion is that the decrease in surface heat flow with age in stable continental areas can be primarily attributed to the decrease in the crustal concentrations of the heat-producing isotopes.

The mean measured heat flow for all the oceans is 101 ± 2.2 mW m−2.

The concentration of the heat-producing isotopes in the oceanic crust is about one order of magnitude less than it is in the continental crust. Also, the oceanic crust is about a factor of 5 thinner than the continental crust.

Therefore, the contribution of heat production by the radioactive isotopes in the oceanic crust to the surface heat flow is negligible (∼2%). The most striking feature of heat flow measurements in the oceans is the  systematic dependence of the surface heat flow on the age of the seafloor. This can be understood as a consequence of the gradual cooling of the oceanic lithosphere as it moves away from the mid-ocean ridge. This process is analyzed in detail in Section 4–16, where it is shown that conductive cooling of the initially hot oceanic mantle can explain quantitatively the observed heat flow–age relation. The dependence of the oceanic heat flow measurements on age is given in Figure 4–25. The total heat flow from the interior of the Earth Q can be obtained by multiplying the area of the continents by the mean continental heat flow and adding the product of the oceanic area and the mean oceanic heat flow. The continents, including the continental margins, have an area Ac = 2×108 km2. Multiplying this by the mean observed continental heat flow, 65 mW m−2, we get the total heat flow from the continents to be Qc = 1.30 × 1013 W. Similarly, taking the oceans, including the marginal basins, to have an area Ao = 3.1 × 108 km2 and a mean observed heat flow of 101 mW m−2, we find that the total heat flow from the oceans is Qo = 3.13× 1013 W. Adding the heat flow through the continents and the oceans, we find that the total surface heat flow is Q = 4.43×1013 W. Dividing by the Earth’s surface area A = 5.1 × 108 km2, we get 87 mW m−2 for the corresponding mean surface

heat flow.