of the magmas that form the continents and as a filter that selectively passes incompatible trace element-rich fluids rising from below.
Convergence of tectonic plates causes contraction in the crust, resulting in mountain ranges with many folds and faults. The formation of mountain ranges transports rock masses through changes of pressure and temperature. The rock masses respond by changing texture, structure, composition, and mineralogy as they approach equilibrium with the new conditions. During the complex changes, dissociation reactions release volatile components and solutions migrate through the rock masses. Study of these metamorphic changes enhances information about the thermal structure of the continental lithosphere and of mountain ranges, the time scales for mountain building, and the mechanisms and scales of fluid flow through the crust.
A long-term approach of metamorphic petrologists to understanding mountain-building processes has been to calibrate naturally occurring mineral assemblages in terms of the depth, a pressure equivalent, and the temperature at which they last equilibrated. This approach is complemented by the forward approach, in which the thermal response of the rocks to tectonism is determined by computer modeling of the transient temperature distribution in a rock mass having specified properties, as it is depressed into warmer regions or uplifted toward the surface at specified rates. Combining these two approaches promises greater understanding of processes. Experimental determination of mineral reactions as a function of pressure, temperature, and different volatile components—such as H2O, CO2, and O2—provides a depth-temperature framework, or grid, of reaction boundaries for the location of many common mineral assemblages. Reactions in various rock types were initially modeled in terms of phase diagrams, using observations of natural mineral assemblages. A second generation of grid models was derived through the combination of petrological observations and experimental results on selected mineral reactions. Sufficient thermodynamic data are now becoming available for the calculation of a third generation, allowing the whole family of grids to be calculated once the thermodynamic parameters have been chosen. Predictions of mineral reactions and compositions, based entirely on thermodynamic data, agree well with petrological observations, but additional experimental work is required before further refinements can be made.
The application of temperature and pressure estimates—called geothermometry and geobarometry—to certain rocks in the European Alps yielded the surprising result that these continental rocks had been buried to a depth of at least 100 km, at a temperature of 800°C or more. It had been assumed that light, relatively buoyant continental rocks had not been buried very deeply. The explanation of how these rocks returned to the surface from such depths without completely recrystallizing is another challenge for structural geologists. In recent years investigations in mountain ranges on other continents have suggested that similar occurrences are not uncommon.
The principles of geothermometry and geobarometry, when applied to zoned minerals or to incompletely reacted mineral assemblages, help to define the paths of depth, or pressure, and temperature (P-T paths) followed by the individual rocks. These paths represent part of the tectonic history of the whole rock mass, or mountain range, and provide important insight into geological processes. Interpretation involves unraveling details of crustal thickening, folding, uplift, and local heating by igneous intrusions. Chemical data on the zonation of minerals can provide a wealth of information on the thermal processes that took place during mountain building as well as on details such as the growth history of minerals. Future work coupling inverse theory, experimental diffusion, and crystal growth data with new high-technology measurements of chemical zonation in minerals will provide an exciting revelation of the kinetic history chemically preserved in rocks that have undergone a wide variety of tectonic excursions in the Earth. This information will open a new window into the internal workings of the crust and upper mantle and will help to test our current views on the workings of plate tectonics. Improvements in analytical instruments have recently produced a highly significant advance. Ion-microprobe measurements of ages within a single zoned garnet in a rock provide, in addition to the P-T path, information about time (t). The newly developed laser probe dating technique produces results accurate enough to discriminate between several episodes of mountain building by measurements made on a single crystal.
Rates of mountain building can be estimated as well. For example, when measurements of radiometric ages are matched with the geobarometric and geothermometric evidence, P-T-t paths are derived that yield rates of geological processes associated