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Thermodynamics deals with the equilibrium state of systems; applications of thermodynamic reasoning in geological sciences can therefore give more or less detailed pictures of what the Earth or parts of it should look like if the stable equilibrium state were attained. Thermodynamics is capable of pointing out the directions of the evolution of the Earth because it is known that all isolated natural systems that are at all capable of change undergo irreversible evolutions toward certain final goals beyond which no macroscopic changes are possible. Every geologic process must of necessity represent a step in the direction toward the state of stable equilibrium of the Earth as a whole provided that the Earth can be considered an isolated system. Thermodynamic reasoning applied in geology is therefore important in order to secure that we do not postulate processes which are impossible in the sense that they would lead the Earth away from the state of ultimate stability and maximum entropy.

It is found advantageous to distinguish among various basically different types of processes and their corresponding states of equilibria when dealing with petrogenesis and the evolution of the Earth’s crust.

(1) The mechanical transport process involves motion of matter in bulk (i.e., the moving units are too large to execute thermal agitation or Brownian motion). This kind of process is caused by mechanical instabilities in the Earth (the driving agency is mechanical potential differences), and it leads to stable mechanical equilibrium characterized by uniform and minimum values of mechanical potentials throughout the Earth, whereby the tendency for motion of matter in bulk vanishes.

(2) The chemical process is propelled by chemical potential differences and aims toward stable chemical equilibrium as characterized by uniform chemical potentials of minimum values throughout. The term chemical process is used in its broadest meaning. It involves chemical reactions, melting, vaporization, dissolution, and diffusion in gaseous, liquid, and solid states. In other words chemical processes comprise changes brought about by individual motion and rearrangement of atoms, ions, and molecules. Chemical thermodynamics provides a powerful tool for handling such things as crystallization of magmas, recrystallization of rocks, and replacement.

(3) The thermal transport process is characterized by flow of heat down temperature gradients which become smooth at thermal equilibrium.

(4) The electric transport process, which seems of limited petrogenetic significance, comprises unidirected currents of ions or electrons down electrical potential gradients. Electric equilibrium prevails when the electrical potential is uniform throughout.

(5) The nuclear process includes all types of nuclear reactions in the Earth. These processes must lead ultimately to a state of equilibrium among the various nuclei and their products.

If a certain kind of process runs as an isolated event in a system it generally starts relatively violently and then gradually quiets down as the driving force becomes exhausted and equilibrium is approached. This appears not to be true of many geologic processes, however, the violence of which is often cyclic. The reason is that the various kinds of geologic processes are intimately interconnected, hence progress in one process commonly creates potential energies which start or maintain other types of processes.

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