The topics covered in this chapter can be summarized as follows:

Section Summary
6.1 Metamorphism and Plate Tectonics Metamorphism is controlled by five main factors: the composition of the parent rock, the temperature to which the rock is heated, the amount and type of pressure, the volumes and compositions of aqueous fluids that are present, and the amount of time available for metamorphic reactions to take place. Almost all metamorphism can be explained by plate tectonic processes. Most regional metamorphism takes place in areas where mountain ranges have been created, which are most common at convergent boundaries. Contact metamorphism takes place around magma bodies in the upper part of the crust, which are also most common above convergent boundaries.
6.2 Classification of Metamorphic Rocks Metamorphic rocks are classified on the basis of texture and mineral composition. Foliation is a key feature of metamorphic rocks formed under directed pressure.  Foliated metamorphic rocks include slate, phyllite, schist, and gneiss. Metamorphic rocks that do not tend to be foliated, even if formed under directed pressure, include marble and quartzite. Geologists also classify metamorphic rocks based on some key minerals—such as chlorite, garnet, andalusite, and sillimanite—that form at specific temperatures and pressures.
 6.3 The Rock Cycle The three types of rocks are igneous: formed from magma; sedimentary: formed from fragments of other rocks or precipitation from solution; and metamorphic: formed when existing rocks are altered by heat, pressure, and/or chemical action. The rock cycle summarizes the processes that contribute to cycling of rock material among these three types. The rock cycle is driven by Earth’s internal heat, and by processes happening at the surface, which are driven by solar energy.
 Lab 6 Exercises Metamorphic rocks are classified according to their mineral composition, and texture (foliated or massive). Knowing the diagnostic properties of the common metamorphic minerals like chlorite, muscovite, biotite, and garnet will help you estimate the metamorphic grade of the rock (low, intermediate, or high grade). Just as with mineral samples, different samples of the same rock may not always look exactly the same, but they can always be identified by closely examining the mineral composition and texture. For example, schist can contain a wide variety of new metamorphic minerals with different colours and shapes, but all schists are characterized by schistose foliation.


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A Practical Guide to Introductory Geology by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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