The main object of this book is to study the physical processes that have determined, and continue to determine the thermal structure and evolution of the Earth. It has always been recognized that heat provides the energy that drives all the processes internal to the Earth, and that temperature controls the state and the mechanical behavior of the Earth's interior, but few of the available books focus on the thermal structure and evolution of the Earth and its geological systems. We intend to look at geodynamics from this thermal perspective and to provide the tools and data to determine temperatures at depth in a range of natural settings. We also aim at providing basic physical understanding of key aspects of heat transport in the Earth and thorough discussion of processes and physical properties that determine temperature at depth. It is impossible to discuss geological processes and environmental problems without thermal aspects and analysis must rely on sound grasp of physics as well as on knowledge of the parameters and variables at play.

There are many excellent books on the Earth's interior and on the dynamics of the Earth. In order not to forget any of them, and not to offend their authors, we shall not try to list them. Let us simply state that, although it has become in many ways obsolete, Jeffreys' The Earth (1959) has set a standard that is difficult to meet. During the past few decades, our understanding of the Earth's internal dynamics has completely changed.