NO MORE FRICTIONLESS PISTONS- TEACHING BASIC THERMODYNAMICS TO UNDERGRADUATES

Most texts on thermodynamics use examples from engineering or physics, such as frictionless pistons and heat engines, so that geology students may not see the relevance of thermodynamics to the earth sciences. Here I develop a simple example in structural geology using frictional melting on faults to illustrate basic thermodynamic principles. The occurrence of pseudotachylytes in the geologic record underscores the importance of frictional heating as a geologic process. Frictional melting is ideally suited to illustrate several concepts such as: mechanical work and thermal energy, and conversion from one to the other, different types of thermodynamic systems (open, closed, adiabatic), and the principle of conservation of energy (First law). By defining the system such that the fault zone is within the system walls, frictional energy can be readily accounted for. Frictional melting (and also seismic slip) can be regarded as an adiabatic system, because the heat generated cannot escape on the time scale of the slip event. The temperature at which the heat is generated naturally introduces the concepts of reversibility, thermodynamic efficiency, and entropy as a measure of irreversibility. Entropy can also be viewed as a measure of the quality of thermal energy (or ability to do work) depending on the temperature of the system. That the entropy of an isolated system can only increase (Second law) also follows from considering frictional melting as an adiabatic system. The fundamental concepts above can be developed assuming only high school algebra and chemistry and are prerequisite for more advanced thermodynamic applications in the earth sciences.