8 - Geothermal processes

Summary

In this chapter we elaborate on the coupling between groundwater flow and heat transport that was addressed on a more theoretical level in Chapter 4. We begin by discussing the Earth's heat engine and conductive heat losses, and then address a variety of heat transfer problems to which fluid flow is critically important, including magma cooling and associated hydrothermal circulation; transport near the critical point of water; multiphase (steam–liquid water) flow; and the occurrence of hot springs and geysers. We will also briefly consider geothermal resources and volcanogenic ore deposits.

Active hydrothermal systems are often very dynamic from a geochemical point of view, because of the elevated temperatures and common occurrence of highly reactive (saline or acidic) fluids. However, in this chapter our emphasis is on the physical coupling between groundwater flow and heat transport, rather than the geochemical aspects. In that sense the heat transport theory outlined in Chapter 4 provides more relevant background than the solute transport theory of Chapter 3. Elsewhere we have addressed advective heat transfer by groundwater in a generalized geologic context (Section 5.4) and as it relates to ore genesis in sedimentary basins (Chapter 6) and hydrocarbon maturation (Section 7.1). Subsea hydrothermal systems – which have a globally significant influence on the Earth's thermal budget – are considered in detail in Chapter 13.

Crustal heat flow

The mean conductive heat flow measured very near the Earth's surface is approximately 70 mW/m² (e.g., Chapman and Pollack, 1975). Correcting for the effects of hydrothermal circulation in the oceanic crust (Section 13.4.1) brings the mean global heat flux to 87 mW/m² (Pollack and others, 1993). Integrated over the surface of the globe, this amounts to a heat loss of more than 4 × 10¹³ W. The sources of this heat are not completely resolved, but the radioactive decay of isotopes of uranium, thorium, and potassium is certainly the most significant. Cooling of an originally hot Earth and the gravitational energy released by its density segregation may or may not be important sources, depending on the presumed mechanism of planetary accretion and the rate and timing of core formation (Verhoogen, 1980).