Page 65 - Global Tectonics

52 CHAPTER 2

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

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Earth’s rotation and the energy released by earthquakes, Fe , and Cl . The differentiation of the Earth would
but these make up only about 0.002% of the energy also have contributed energy to the Earth arising from
budget. It is thus apparent that geothermal energy is the the loss in gravitational potential energy as the dense
major source of the energy which drives the Earth’s iron-nickel core segregated to a lower energy state at
internal processes. the center of the Earth.

It is believed that the geothermal energy is derived The heat flow through a unit area of the Earth’s
in part from the energy given off during the radioactive surface, H, is given by:
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235
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decay of long-lived isotopes, in particular K , U , U ,
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and Th , and also from the heat released during the δ
early stages of the formation of the Earth. These iso- H = K T
topes would account for the present geothermal loss if z δ
present in proportions similar to those of chondritic
meteorites. Radioactive decay is exponential, so that where δT/δz is the thermal gradient perpendicular to
during the early history of the Earth the concentration the surface and K the thermal conductivity of the
of radioactive isotopes would have been signifi cantly medium through which the heat is flowing. The units

−2
higher than at present and the thermal energy available of H are mW m .

to power its internal processes would have been much On land, heat flow measurements are normally
greater (Section 12.2). Currently accepted models for made in boreholes. Mercury maximum thermometers
the formation of the Earth require an early phase of or thermistor probes are used to determine the vertical
melting and differentiation of its originally homoge- temperature gradient. Thermal conductivity is mea-
neous structure. This melting is believed to have been sured on samples of the core using a technique similar
powered in part by thermal energy provided by the to the Lee’s disc method. Although appearing relatively
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decay of short-lived radioactive isotopes such as Al , simple, accurate heat flow measurements on land are