Metamorphic Rock Characteristics Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 1, 2011


When naming metamorphic rock, the rules are more flexible than that of igneous or sedimentary rock naming. Since metamorphic rock tends to change in composition and texture as temperatures and pressures change, the naming changes.

For example, shale is a fine-grained, clastic sedimentary rock containing quartz, clays, calcite, and some feldspar. With the start of low-grade metamorphism, muscovite and chlorite begins to form. Transformed shale is called slate . If the slate meets with further metamorphism, the mineral grains grow and intermediate-grade metamorphism happens with foliation and mica forms. Continued metamorphism causes the formation of even larger, coarsegrained rock with high schistosity and is known as schist . Then at high-grade metamorphism, the minerals group into separate bands with layers of mica-like minerals such as quartz and feldspar. This type of high-grade metamorphic rock is called gneiss from an old German word, gneisto , meaning to sparkle. Figure 8-5 shows the different types of metamorphic rock formed as temperature and pressure increase.

Metamorphic Rock

Fig. 8-5. Shale undergoes metamorphic change depending on pressure and temperature.

So naming then depends on what can be seen. Slate and phyllite describe textures, while gneiss is described by the large mineral grains (that are easily seen) being named first. So a specific gneiss might be named, quartz–plagioclase–biotite–garnet gneiss. In this way, another geologist would have a pretty good idea of all that the rock contained. Nongeologists would probably just call it garnet!

The Internet has several sites that provide photos of metamorphic rock types. There are even photos that illustrate the complete metamorphic rock series.

Practice problems of this concept can be found at: Metamorphic Rock Practice Test

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