Metamorphic petrology is the study of rocks which have been changed (metamorphosed) by heat and pressure.
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They are broadly categorized into regional and contact. Metamorphism is an extension of the process which forms sedimentary rocks from sediment: lithification. However, all types of rocks; igneous, sedimentary and metamorphic, can all be metamorphosed. During metamorphism no melting takes place. All the chemical reactions which take place occur in the solid-state.
Factors Controlling Characteristics
The characteristics of a metamorphic rock depend on the following factors:
1. Composition of parent rock
2. Temperature and Pressure of metamorphism
The composition of the parent rock does not usually change during metamorphism (if it does it is then called metasomatism). The changes are the due to the minerals changing. A basalt which has around 50% of silica will produce a metabasalt with 50% silica.
Temperature and pressure affect the rock in terms of the mineral assemblage which is stable at the pressure and temperature obtained. The minerals stable at the pressure and temperatures that metamorphic rocks reach are simulated in a lab. This allows geologists to look at a mineral assemblage and give a (good) estimate of the pressure and temperature that the sample was exposed to. This gives tectonic information which is useful in other branches of geology.
Fluid changes the chemical composition of the rock being metamorphosed and hence is called metasomatism. The addition of fluid can radically change the rock.
Time has an important role as a rock which is heated to an extreme temperature for a short (years) period of time will not be altered too much. A rock heated for a longer period of time (millions of years) will show changes.