Gneiss is a very widespread rock type, especially in the lower - TopicsExpress



          

Gneiss is a very widespread rock type, especially in the lower parts of the continental crust but it is also a common rock on the surface in some places (Scandinavia, Canada, and other shield areas where crystalline rocks are not covered by a layer of sedimentary rocks). Most of the mineral grains of gneissose rocks are visible to the naked eye. Banding in gneiss is a result of mineral segregation into separate, typically light- and dark-colored layers. Light-colored layer is usually composed of feldspars and quartz. Most important dark minerals are hornblende and biotite. Individual bands are usually 1-10 mm in thickness. Layers larger than that imply that partial melting or the introduction of new material have probably taken place. Such rocks are called migmatites. It is often difficult to distinguish gneiss from migmatite because there is a gradational transition from one to another. Hence, terms like ‘migmatized gneiss’ are commonly used. It is not well understood how the segregation takes place but it must be the result of extreme pressure and shear stress deep in the crust.
Posted on: Wed, 24 Sep 2014 23:19:52 +0000

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