With the new global tectonics approach in the Earth Sciences, the quan titative aspects of the dynamics of rock-forming processes came into focus: geologists are no longer satisfied knowing the pressure-tempera ture conditions of the formation of a metamorphic rock or of the emplace ment of a magmatic body, but instead would like to learn the time history of these rocks as well, i. e., derive the temperature-pressure-time path and relate it to a tectonic process. To achieve this goal, a knowledge of both pressure-temperature-dependent equilibria and the time scales at which these equilibria...