The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 136

6.4.1 Geothermal energy
The temperature of the Earth's interior increases with depth. This geothermal
gradient varies in different parts of the world from 15 to 75 o C/km and creates a
heat flow leading to a heat loss escaping at the crust. The amount of heat escaping
through the Earth's surface is a consequence of the superposition of four components
(Jaupart and Mareschal, 2004):
Q = Q C + Q L + Q B + Q T (6.1)
where Q B is the heat input at the base of the lithosphere due to mantle convection,
Q T is a long-term transient due to cooling after a major tectonic or magmatic
perturbation and Q L is the radiogenic heat production in the mantle part of the
lithosphere. Finally Q C is the radiogenic heat production of the crust which is due
to the decay of the radioactive elements 238 U, 235 U, 232 U and 40 K either in the crust
or the upper mantle. For geological provinces older than 100 million years, Q L ,
Q B and Q T are combined into a single parameter called mantle heat flow, denoted
Q M .
There are different ways to estimate the bulk crustal heat flow of the Earth.
Some estimates (O'Nions et al., 1979; Allègre et al., 1988; Galer et al., 1989) are
obtained by redistributing the heat producing elements in the bulk silicate Earth
between the continental crust and the various reservoirs in the mantle. They require
assumptions regarding the structure of the convecting mantle, the composition and
the homogeneity of the reservoirs. Other estimates are based on measurements ei-
ther from representative rock types and their proportions in crustal columns derived
from geophysical profiles (Haack, 1983; Condie, 1993; Wedepohl, 1995; Borodin,
1999) or on large-scale production data sets (Eade and Fahring, 1971; Shaw et al.,
1994; Gao et al., 1998).
Jaupart and Mareschal (2004) estimated the bulk crustal heat production di-
rectly from the heat flow data and local studies of the crustal structure. These
authors obtained the values of heat production for three age groups: Archean,
Proterozoic, and Phanerozoic (see Table 6.2). The average heat production was es-
timated to be between 0.79 and 0.95 Wm 3 and the crustal heat flow component
ranged from 32 to 38 mWm 2 , considering an average crustal thickness of 40 km.
According to these numbers, the continental crust contributes between 5.8 to 6.9
TW of the total energy budget of the Earth 1 . Active provinces and continental
margins represent 30% of the total volume of the crust; a 50% error in their heat
production would lead to a 15% error in the global budget. These percentages dif-
fer from the values given by Skinner (1986), in which the flow is estimated to be
63 mWm 2 or 32.3 TW across the entire planetary surface (not only the crust).
Extrapolating the values given by Jaupart and Mareschal (2004) to the Earth's
entire surface, would lead to an average geothermal energy contribution 2 of 17.9
TW.
1 Should the continental crust be 7.310 18 m 3 .
2 For a total surface of the Earth of 5.1210 14 m 2 .