Terrestrial Heat Flow

The study of thermal processes within the Earth is somewhat speculative because the interpretation of the distribution of heat sources and the mechanisms of heat transfer are based on measurements made at or near the surface. Such a study is important, however, as the process of heat escape from the Earth's interior is the direct or indirect cause of most tectonic and igneous activity.

The vast majority of the heat affecting the Earth's surface comes from the Sun, which accounts for some 99.98% of the Earth's surface energy budget. Most of this thermal energy, however, is reradiated into space, while the rest penetrates only a few hundred meters below the surface. Solar energy consequently has a negligible effect on thermal processes occurring in the interior of the Earth. The geothermal energy loss from heat sources within the Earth constitutes about 0.022% of its surface energy budget. Other sources of energy include the energy generated by the gradual deceleration of the

6371 km

Figure 2.39 Comparison of the compositional and rheological layering of the Earth.

6371 km

Figure 2.39 Comparison of the compositional and rheological layering of the Earth.

Earth's rotation and the energy released by earthquakes, but these make up only about 0.002% of the energy budget. It is thus apparent that geothermal energy is the major source of the energy which drives the Earth's internal processes.

It is believed that the geothermal energy is derived in part from the energy given off during the radioactive decay of long-lived isotopes, in particular K40, U235, U238, and Th232, and also from the heat released during the early stages of the formation of the Earth. These isotopes would account for the present geothermal loss if present in proportions similar to those of chondritic meteorites. Radioactive decay is exponential, so that during the early history of the Earth the concentration of radioactive isotopes would have been significantly higher than at present and the thermal energy available to power its internal processes would have been much greater (Section 12.2). Currently accepted models for the formation of the Earth require an early phase of melting and differentiation of its originally homogeneous structure. This melting is believed to have been powered in part by thermal energy provided by the decay of short-lived radioactive isotopes such as Al26,

Fe60, and Cl36. The differentiation of the Earth would also have contributed energy to the Earth arising from the loss in gravitational potential energy as the dense iron-nickel core segregated to a lower energy state at the center of the Earth.

The heat flow through a unit area of the Earth's surface, H, is given by:

H = K — Sz where ST/Sz is the thermal gradient perpendicular to the surface and K the thermal conductivity of the medium through which the heat is flowing. The units of H are mW m-2.

On land, heat flow measurements are normally made in boreholes. Mercury maximum thermometers or thermistor probes are used to determine the vertical temperature gradient. Thermal conductivity is measured on samples of the core using a technique similar to the Lee's disc method. Although appearing relatively simple, accurate heat flow measurements on land are difficult to accomplish. The drilling of a borehole necessitates the use of fluid lubricants that disturb the thermal regime of the borehole so that it has to be left for several months to allow the disturbance to dissipate. Porous strata have to be avoided as pore water acts as a heat sink and distorts the normal thermal gradients. Consequently, it is rarely possible to utilize boreholes sunk for the purposes of hydrocarbon or hydrogeologic exploration. In many areas readings may only be undertaken at depths below about 200 m so as to avoid the transient thermal effects of glaciations.

Heat flow measurements are considerably easier to accomplish at sea. The bottom temperatures in the oceans remain essentially constant and so no complications arise because of transient thermal perturbations. A temperature probe is dropped into the upper soft sediment layer of the seabed and, after a few minutes' stabilization, the temperature gradient is measured by a series of thermistor probes. A corer associated with the probe collects a sediment sample for thermal conductivity measurements; alternatively, the role of one of the thermistors can be changed to provide a source of heat. The change in the temperature of this probe with time depends on the rate at which heat is conducted away from it, and this enables a direct, in situ measurement of the thermal conductivity of the sediment to be made.

A large proportion of geothermal energy escapes from the surface by conduction through the solid Earth. In the region of the oceanic ridge system, however, the circulation of seawater plays a major role in transporting heat to the surface and about 25% of the geothermal energy flux at the Earth's surface is lost in this way.

The pattern of heat flow provinces on the Earth's surface broadly correlates with major physiographic and geologic subdivisions. On continents the magnitude of heat flow generally decreases from the time of the last major tectonic event (Sclater et al., 1980). Heat flow values are thus low over the Precambrian shields and much higher over regions affected by Cenozoic orogenesis. Within the oceans the heat flow decreases with the age of the lithosphere (Section 6.5), with high values over the oceanic ridge system and active marginal seas and low values over the deep ocean basins and inactive marginal seas.

The average heat flux in continental areas is 65 mW m-2, and in oceanic areas 101 mW m-2, of which about 30% is contributed by hydrothermal activity at the mid-oceanic ridge system (Pollack et al., 1993). As 60% of the Earth's surface is underlain by oceanic crust, about 70% of the geothermal energy is lost through oceanic crust, and 30% through continental crust.

FURTHER READING

Anderson, D.L. (2007) New Theory of the Earth, 2nd edn. Cambridge University Press, Cambridge, UK.

Bott, M.H.P. (1982) The Interior of the Earth, its Structure, Constitution and Evolution, 2nd edn. Edward Arnold, London.

Condie, K.C. (2005) Earth as an Evolving Planetary System. Elsevier, Amsterdam.

Fowler, C.M.R. (2005) The Solid Earth: an introduction to global geophysics, 2nd edn. Cambridge University Press, Cambridge.

Jacobs, J.A. (1991) The Deep Interior of the Earth. Chapman & Hall, London.

Nicolas, A. (1989) Structure of Ophiolites and Dynamics of Oceanic Lithosphere. Kluwer Academic Publishers, Dordrecht.

Park, R.G. (1988) Geological Structures and Moving Plates. Blackie, London and Glasgow.

Ranalli, G. (1995) Rheology of the Earth, 2nd edn. Chapman & Hall, London.

Stein, S. & Wysession, M. (2003) An Introduction to Seismology, Earthquakes, and Earth Structure. Blackwell Publishing, Oxford.

Twiss, R.J. & Moores, E.M. (2006) Structural Geology, 2nd edn. W.H. Freeman, New York.

Continental drift

Solar Power Sensation V2

Solar Power Sensation V2

This is a product all about solar power. Within this product you will get 24 videos, 5 guides, reviews and much more. This product is great for affiliate marketers who is trying to market products all about alternative energy.

Get My Free Ebook


Post a comment