GeologyRocksIntroduction to Metamorphic Petrology
About the author
Jon
Imperial College London
Introduction
Metamorphic petrology is the study of rocks which have been changed (metamorphosed) by heat and pressure. 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:
- Composition of parent rock
- Temperature and Pressure of metamorphism
- Fluid
- Time
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.
Classification
The classification of metamorphic rocks is split into contact and regionally metamorphosed rocks. After this it is divided according to the "amount" of metamorphism that has taken place and/or on the mineral content.
| Contact Metamorphism (based on mineral content) | |||
|---|---|---|---|
| Parent Rock | Metamorphic rock | Dominant Minerals | Characteristics |
| Limestone | Marble | Calcite | Interlocking grains. Fizzes in weak acid |
| Quartz Sandstone | Quartzite | Quartz | Sugary texture |
| Shale | Hornfels (Spotted Rock) | Micas | Dark colour |
| Regional Metamorphism (name based on degree of metamorphism) | |||
| Texture | Rock Name | Characteristics | |
| Slatey | Slate | Splits easily into sheets | |
| Between slate and schistose | Phylitte | Silky lustre, splits into wavy sheets | |
| Schistose | Schist | Pearly looking. Silky to touch | |
| Gneissic | Gneiss | Wavy, white and dark layers | |
|
Slate. Photo by Tim Ivanic |
Schist. Photo by Tim Ivanic |
|
Gneiss. Photo by Tim Ivanic |
Marble. Photo courtesy of Dr. Richard Busch, West Chester University |
Causes of Metamorphism
Contact
Caused by heating from an external source. Contact metamorphism occurs next to an igneous body. The degree of metamorphism decreases away from the body. This occurs at fairly shallow depths (<10km), as temperature not pressure is the dominating factor.
Regional
Regional metamorphism is caused by high pressure and temperatures usually during mountain building (oregenesis). The extremes of regionally metamorphic rocks are a high pressure, low temperature rock (called a blueschist) and a high pressure and very high temperature rock (called a granulite). If the rock is heated to the point of melting, but doesn't actually melt, it is called a migmatite.
Conclusion
Metamorphism is the transformation of pre-existing rocks by heat and pressure. Time is also an important factor. Metamorphism is a continuation of the porcess that turns sediment into rock (lithification). There are two main causes of metamorphism; contact, whih is caused by close proximity of a hot, igneous body; and regional, which is larger scale and is caused by tectonic forces. There are four main factors in deciding the type of rock produced by metamorphism: composition of parent rock, temperature and pressure of metamorphism, the role of fluids and time. Classification is based on the type of metamorphism, the amount of change and the composition of the parent rock.
References
C. Plummer & D. McGeary, 1997. Physical Geology.