Page 30 - Geothermal Energy Renewable Energy and The Environment

Sources of Geothermal Heat: Earth as a Heat Engine 13

Continental crust
Lower mantle
Upper mantle
Oceanic crust
Core
Descending
limb

Ascending
limb
FIGUre 2.1 (See color insert following page 17.0..) Interior of the Earth, shown in a cut-away that shows
the outer edge of the liquid core (reflecting orange sphere), the lower mantle (yellow), upper mantle (pink and
purple), and the crust. Ascending limbs of convection cells are shown as the orange-tinted plumes extending
from the lower mantle through the upper mantle to the base of the crust. Descending limbs of convection
cells are shown as the darker purple features extending into the mantle from the base of the crust. (From U.S.
Geological Survey, http://geomag.usgs.gov/about.php.)

the details and timing of the process remain a matter of substantial scientific uncertainty and
interest, the end result was that the Earth segregated into layers with distinct chemical compositions
(Figure 2.1).
The mantle, which surrounds the core, has a thickness of about 2890 kilometers. It is composed
of minerals that are relatively low in silica and that have relatively high density. The density of the
minerals is a reflection of the fact that the high pressures in the interior of the Earth favor mineral
structures that have relatively small volumes per formula unit (i.e., per mole). Such structures do
not readily accommodate large atoms, such as potassium (K), rubidium (Rb), thorium (Th), or
uranium (U), as well as a host of other elements. As a result, the mantle tends to be composed of
silicate minerals, oxides, and other high-density minerals with high contents of atoms with relatively
small atomic radii such as magnesium (Mg), titanium (Ti), calcium (Ca), some aluminum (Al), and
low abundances of larger atoms. The exclusion of atoms with relatively large atomic radii has the
consequence of depleting the mantle in elements that have relatively high proportions of radioactive
isotopes (e.g., K, Rb, Th, and U).
The crust, which floats on the mantle, is of two types. Oceanic crust underlies the global oceans
and has a thickness that varies between 6 and 10 km. Continental crust, of which all the major
landmasses of the Earth are composed, varies in thickness from 30 to 60 km. Oceanic crust is
formed at regions where the upwelling portions of convection cells in the mantle reach the surface
(see the later section in this chapter on Plate Tectonics). Magma that forms during the upwelling
process is extruded at ocean ridges and solidifies as oceanic crust. Because this crust is formed
directly from the mantle, which is low in radioactive elements, the oceanic crust also has a low
abundance of radioactive elements.
The continental crust, on the other hand, is largely composed of the material that was incompatible
with the high-density minerals in the mantle. As a result, the continental crust is richer in lower
density minerals that are made up of relatively large atoms. Included in this suite of minerals are
those that can readily accommodate K, Rb, Th, and U. Consequently, the continental crust holds the
largest global reservoir of radioactive elements (Shih, 1971; Table 2.2). Sixty percent of the heat that
exists in continents is derived from the radioactive decay of these four elements.
The contribution of radioactive decay of these elements to the surface heat flow is described by
the following relationship (Birch, Roy and Decker, 1968; Lachenbruch, 1968).

q = q + D × A, (2.1)
0