Introduction
Metamorphic petrology is the
study of rocks which have been changed (metamorphosed) by heat and
pressure. They are broadly categorised into regional and contact.
Metamorphism is an extension of the process which forms sedimentary rocks
from sediment - diagenesis. 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 | |
Put your pointer over the picture to see the name. Pictures from Georgia College.
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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.
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