Gneiss where can it be found

It is often difficult to distinguish it from migmatite because there is a gradational transition from one to another. 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. The protolith of gneiss may be an igneous rock , in this case it is called an orthogneiss. It forms probably because of shear in vicous granitic magma.

Paragneiss is a variety with a sedimentary protolith. Even in the latter case, gneissic banding has nothing to do with original layering of sedimentary rocks. These original features are completely obliterated by the metamorphic processes involved in the formation of this rock type.

Paragneiss in most cases is thought to be the end product of metamorphism of a pelitic clay-rich sedimentary rock shale , argillite, claystone, etc. Still deeper burial or more intense heating may result in migmatization and finally complete melting of gneiss. Despite being clearly oriented, this rock is not considered to be foliated because it is not fissile along the layering.

So, when hammered, gneiss behaves like a uniform homogenous rock. In this sense it is similar to igneous rocks like granite and gabbro and not similar to related metamorphic rocks like schist and phyllite which are foliated. It is important to note that gneiss is a rock type that is defined by its oriented texture, rather by its mineralogy or chemical composition.

Hence, qualifying terms are often added to the rock name: amphibolite gneiss or hornblende gneiss, for example. Gneiss is a product of regional metamorphism. This is a type of metamorphism which is associated with mountain building. Gneisses form deep below the forming mountain ranges and are exhumed many millions of years later when the mountains get carried away by the erosion.

The cores of the continental landmasses are typically composed of such grayish gneisses. This very old from the Archaean sample is from Karelia, Russia. Width of sample 16 cm. An augen gneiss from Estonia glacial erratic from the Finnish Bedrock. Width of sample 30 cm. The bands and lenses of foliation are clearly visible in this rock.

Image by the United States Geological Survey. Gneiss usually forms by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure. This alteration increased the size of the mineral grains and segregated them into bands, a transformation which made the rock and its minerals more stable in their metamorphic environment.

Gneiss can form in several different ways. The most common path begins with shale , which is a sedimentary rock. Regional metamorphism can transform shale into slate , then phyllite , then schist , and finally into gneiss. During this transformation, clay particles in shale transform into micas and increase in size. Finally, the platy micas begin to recrystallize into granular minerals.

The appearance of granular minerals is what marks the transition into gneiss. Intense heat and pressure can also metamorphose granite into a banded rock known as "granite gneiss. Folded Gneiss: A photograph of polished gneiss from the stock of a countertop vendor.

The view shown in the photo is about 12 inches across. Numerous tight folds are visible in this rock. Click to enlarge. Although gneiss is not defined by its composition, most specimens have bands of feldspar and quartz grains in an interlocking texture. These bands are usually light in color and alternate with bands of darker-colored minerals with platy or elongate habits. The dark minerals sometimes exhibit an orientation determined by the pressures of metamorphism.

Some specimens of gneiss contain distinctive minerals characteristic of the metamorphic environment. These minerals might include biotite , cordierite , sillimanite, kyanite , staurolite , andalusite , and garnet. Gneiss is sometimes named for these minerals, examples of which include "garnet gneiss" and "biotite gneiss. Garnet Gneiss: A photograph of polished garnet gneiss from the stock of a countertop vendor. Gneiss usually does not split along planes of weakness like most other metamorphic rocks.

Gneiss is typically associated with major mountain building episodes. During these episodes, sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such episodes.

Gneisses from western Greenland comprise the oldest crustal rocks known more than 3.