Geothermy

a division of geophysics that studies the thermal state and history of the interior of the earth. Solar heat penetrates only into the topmost layers of the earth’s crust. Diurnal soil temperature variations extend to a depth of 1.2-1.5 m; annual variations, to 10-20 m. The heat associated with solar radiation does not penetrate further, although a regular increase in temperature with increasing depth has been established, indicating the existence of sources of heat inside the earth. Heat flows continuously from the depths to the surface of the earth and is scattered into surrounding space. The density of the heat flow is given by the product of the geothermal gradient and the coefficient of thermal conductivity. A considerable part of the heat flow is radiogenic heat—that is, heat evolved in the breakdown of radioactive elements present in the earth.

The temperature of the earth’s interior within the boundaries of dry land is determined directly in shafts and boreholes by means of electric thermometers. Instruments for recording the thermal gradient are used for measurements on the ocean floor. Laboratory measurements are made to determine the thermal conductivity of rocks, and measurements show that the change of temperature with depth at various places varies from 0.006 to 0.15 deg/m. The density of heat flow is more constant and is closely connected with the tectonic structure. Very rarely does it extend beyond the limits of 0.025 to 0.1 watts per sq m (W/m²), or 0.6-2.4 microcalories per (cm²-sec) [μcal/(cm² sec)]; individual values attain 0.3 W/m² [8 μcal/(cm^2.-sec)]. Precambrian crystalline shields are characterized by low values [up to 0.04 W/m², or 0.9 μ,cal/(cm^2.. sec)]; platforms, by medium values [0.05-0.06 W/m², or 1.1-1.5 μcal/(cm^2. sec)]; and technically active regions (midocean ridges, rifts, and regions of modern orogenesis), by high values [0.07-0.1 W/m², or 1.7-2.6 μcal/(cm²-sec)]. On the average, oceans, continents, and the earth give the same values; about 0.05 W/m², or 1.2 μcal/(cm²sec); however, that figure is not very reliable, since most of the earth’s surface has not yet been examined.

The earth’s temperature may be measured directly to a depth of only a few kilometers. Below that the temperature is estimated indirectly from the temperature of volcanic lavas and from certain geophysical data. At depths of over 400 km, only probable temperature limits can be obtained. Here it is considered that the temperature of the Gutenberg layer is near the melting point, and that below it the melting point rises (because of a rise in pressure) faster than the actual temperature, and that at the boundaries of the earth’s core the material of the interior remains solid, although the core (apart from the subcore) is molten. The probable temperature limits for various depths are given in Table 1.

Thus, the geothermal gradient decreases greatly with depth. The energy of the total heat flow coming from the earth is about 2.5 x 10¹³ W, which is about 30 times greater than that of all the electric power stations in the world but 4,000 times less than the amount of heat the earth receives from the sun. Consequently the heat coming from the earth’s interior does not affect the climate.

An explanation of the earth’s thermal history requires data about the original content of radioactive material of the various shells of the earth, about their shifts from one geosphere to another, about the energies and rates of decomposition, about the earth’s age, about the amount of heat received by the planet during its formation, and about the amount of heat evolved and absorbed in the various mechanical, physical, and chemical processes in the earth’s interior. The coefficients of thermal conductivity, the specific heat of the material of the interior, and the temperature and pressure at various depths and on the earth’s surface should also be taken into account.

Estimated values make it possible to sketch such a picture of the earth’s thermal history. Immediately after the planet was formed from a cluster of meteoric bodies, its interior temperature was probably 700°-2000° C. Calculations for the earth assuming a silicate core show that, apart from the nucleus and possibly the Gutenberg layer, it was never molten. The deep interior of the earth is warming up slowly (by a few degrees in 10₇ years), whereas its upper layers (several hundred kilometers) are cooling still more slowly.

Geothermic research is of great theoretical significance for various types of earth studies. Its role is particularly great in constructing and evaluating tectonic hypotheses. For example, geothermic data contradict the thermal contraction hypothesis and other hypotheses that postulate that the earth’s heat loss is much greater than the observed values. Geothermic measurements are also used practically; they assist in prospecting for oil and minerals and in the preparation for using the earth’s heat for industrial and domestic purposes.