Thematic Articles

Metamorphic rocks, formed at elevated temperature and pressure from pre-existing rocks inside mountain belts, provide a seemingly unpromising target for the application of equilibrium thermodynamics. This is because metamorphic rocks develop their mineral assemblages along a pressure–temperature (P–T) path, with pressure and temperature continuously changing along the path. However, in a successful model for the formation of such rocks, involving the essential role of fluid or melt, the mineral assemblages observed at the Earth’s surface can be considered to reflect a state of frozen-in equilibrium as the rocks are exhumed towards the Earth’s surface. Equilibrium thermodynamics applied to such mineral assemblages allow P–T information to be extracted. Currently the best way to do this is via calculated phase diagrams, the most powerful being P–T pseudosections. These diagrams portray the variation of mineral assemblages with P–T for a specified rock composition. Pseudosections allow the P–T conditions of the frozen-in equilibrium to be estimated, and can also give information on the P–T path followed. Such paths are an essential input in constraining the processes involved in mountain-building and the evolution of continental crust.

The chemical composition of groundwater results from the reaction of mineral dissolution and precipitation. We can use the thermodynamic approach to predict water composition under conditions where water and newly formed minerals are in equilibrium. Although some minerals exist in a state of equilibrium with water, other minerals are always unstable. In the latter case, we can evaluate the extent of the overall irreversible mass transfer between minerals and water to quantify the mineral surface area participating in the water–rock interaction. This parameter is fundamental to basic and applied research in areas such as the geological sequestration of CO2 and the safe geological storage of waste.

The burning of fossil fuels has increased the concentration of carbon dioxide (CO2) in the atmosphere from 280 ppmv (volume parts per million) to 385 ppmv over the last 200 years. This increase is larger than has occurred over the past 800,000 years. Equilibration of increasing amounts of CO2 with surface waters will decrease the pH of the oceans (called ocean acidifi cation) from a current value of 8.1 to values as low as 7.4 over the next 200 years. Decreasing the pH affects the production of solid CaCO3 by microorganisms in surface waters and its subsequent dissolution. CO2 dissolution in the ocean can also affect acid–base equilibria, metal complex formation, solid–liquid equilibria, and the adsorption of ions to charged surfaces. Thermodynamic principles can be used to understand these processes in natural waters.

Thermodynamic principles play a key role in almost all processes occurring in the Earth’s atmosphere. They are formidably expressed in the thermal stratification of the atmosphere, in the appearance of various regional and large-scale wind systems, as well as in the formation of clouds and precipitation. It is important to note that the application of simplified thermodynamics is usually sufficient to describe large-scale atmospheric processes. However, for an in-depth understanding of the microphysical structure of clouds, a detailed investigation of the complex thermodynamic cloud processes is needed.

All geological changes result from the transfer of matter and energy, the study of which is the goal of thermodynamics. Investigating natural processes thus necessarily involves thermodynamic considerations. This has long been practiced implicitly, as shown by the smart reflections made by “natural philosophers” from antiquity to the 18th century about topics ranging from atmospheric phenomena to the early history of the Earth. Since the early 19th century, investigations explicitly take advantage of a rigorous framework that deals with chemical and thermal aspects of the Earth’s activity. Far from being abstruse, these principles can in fact be summarized in a simple and concise way.