Objectives of this chapter

We shall first review the mechanisms of heat transport and some basic thermodynamic considerations that are important for understanding thermal processes in the Earth, and the relationships between physical properties and the thermal regime. We shall also review the fundamental equations of conservation of energy and momentum.

Heat transport mechanisms

There are three basic mechanisms of heat transport: radiation, convection and conduction. Radiation is the transport of energy by electromagnetic waves; it is the only possible mechanism of heat transport in a vacuum. Convection (or advection) refers to the transport of energy by matter in movement. When energy is transferred from one part to another in a continuous medium without mass transport, it is said to be conducted. All three mechanisms are important in different parts of the Earth. The light that carries the energy from the sun to the Earth's surface is electromagnetic radiation, in the visible part of the electromagnetic spectrum; it is radiated back in the infra-red. In the oceans and atmosphere, convection is the dominant mechanism of heat transport. Within the solid Earth, all three mechanisms play a role at different depths: in the outer shell of the Earth, conduction dominates, but locally, one must include hydrothermal convection generated in porous and permeable rocks and magma ascent. Hydrothermal circulation develops in fractures and pores, which get closed by the confining pressure deeper than 10 km.