The heat flow through oceanic and continental crust and the heat loss of the Earth

Abstract

Simple thermal models based on the creation and cooling of the lithosphere can account for the observed subsidence of the ocean floor and the measured decreased in heat flow with age. In well‐sedimented areas, where there is little loss of heat due to hydrothermal circulation, the surface heat flow decays uniformly from values in excess of 6 µcal/cm² s (250 mW/m²), for crust younger than 4 Ma (4 m.y. B.P.), to close to 1.1 µcal/cm² s (46 mW/m²) through crust between 120 and 140 Ma. After 200 Ma the heat flow is predicted to reach an equilibrium value of 0.9 µcal/cm² s (38 mW/m²). The surface heat flow on continents is controlled by many phenomena. On the time scale of geological periods the most important of these are the last orogenic event, the distribution of heat‐producing elements, and erosion. To better understand the effects of age, each continent is separated into four provinces on the basis of radiometric dates. Reflecting the preponderance of Precambrian crust, two of these provinces cover the Archean to the middle Proterozoic, and the third covers the late Proterozoic to the Mesozoic. The mean heat flow decreases from a value of 1.84 µcal/cm² s (77 mW/m²) for the youngest province to a constant value of 1.1 µcal/cm² s (46 mW/m²) after 800 Ma. The nonradiogenic component of the surface heat flow decays to a constant value of between 0.65 and 0.5 µcal/cm² s (25 and 21 mW/m²) within 200–400 Ma. Using theoretical models, we compute the heat loss through the oceans to be 727 × 10 10 cal/s (30.4 × 10 12 W). The comparison between the theoretical and measured values allows an estimate of 241 × 10 10 cal/s (10.1 × 10 12 W) for the heat lost owing to hydrothermal circulation. We show that the heat flow through the marginal basins follows the same relation as that for crust created at a midocean spreading center. These basins have a corresponding heat loss of 71 × 10 10 cal/s (3.0 × 10 12 W). The heat loss through the continents is calculated from the observations and is 208 × 10 10 cal/s (8.8 × 10 12 W). Our estimate of the value for the shelves is 67 × 10 10 cal/s (2.8 × 10 12 W). The total heat loss of the earth is 1002 × 10 10 cal/s (42.0 × 10 12 W), of which 70% is through the deep oceans and marginal basins and 30% through the continents and continental shelves. The creation of lithosphere accounts for just under 90% of the heat lost through the oceans and hence about 60% of the worldwide heat loss. Convective processes, which include plate creation and orogeny on continents, dissipate two thirds of the heat lost by the earth. Conduction through the lithosphere is responsible for 20%, and the rest is lost by the radioactive decay of the continental and oceanic crust. We place bounds of between 0.6 and 0.9 µcal/cm² s (25 and 38 mW/m²) for the mantle heat flow beneath an ocean at equilibrium and between 0.40 and 0.75 µcal/cm² s (17 and 31 mW/m²) for the heat flow beneath an old stable continent. The computed range of geotherms for an equilibrium ocean overlaps the range of stable continental geotherms below a depth of 100 km. The mantle heat flow beneath a continent decays with a thermal time constant similar to that of the oceanic lithosphere. The continental basins subside with the same time constant. These observations are evidence that there is no detectable difference between the thermal structure of an equilibrium ocean and that of an old continent. Thus the concept of the lithosphere as a combination of a mechanical and a thermal boundary layer can be applied to both oceans and continents. We evaluate the constraints placed on models based on this concept by seismological observations. In the absence of compelling evidence to the contrary we favor these models because they provide a single explanation for the thermal structure of the lithosphere beneath an equilibrium ocean and a stable continent.