Gneiss, pronounced as ‘nice’, is one type of metamorphic rock found all over in the world today. This metamorphic rock is usually identified by distinctive bands of lighter and darker materials as well as its coarse surface. The banding, or foliation as it is commonly called, is the result of a metamorphic change that occurred in the Earth’s crust. The method of formation for gneiss is similar to the formations of the other metamorphic rocks, only gneiss is one of the higher grades of metamorphic rock.
Metamorphism
The formation of metamorphic rocks occurs as a result of either igneous or sedimentary rock being subjected to pressure and heat over a long period and is known as metamorphism. Regional metamorphism, which is the formation method of gneiss, occurs when the material is deeply buried under the ground, at depths of several miles. At these depths the temperature is hotter and the pressure is very high. Under these conditions, the rock will undergo several changes. The grade of metamorphic rocks is determined by the level of temperature and pressure. High grade have higher temperature and pressure while low grade do not.
Formation of gneiss
In the case of gneiss, the temperature and pressure are quite high as gneiss is one of the highest grades of metamorphic rock. Temperatures may reach between 500 and 800 degrees Celsius (900 to 1500 degrees Fahrenheit) while pressures can reach upwards of over 100,000 lbs per square inch (psi), which is about 12,000 times greater than normal atmospheric pressure which people experience on the surface of the planet.
While metamorphism is occurring, the base material (either igneous or sedimentary) will begin to change. Minerals that melt at lower temperatures will approach their melting point but never actually melt. The other minerals that can stand the higher temperatures will remain less changed by the process. Under the conditions of regional metamorphism, minerals will begin to migrate and segregate themselves which will cause the formation of a new material that has a different structure from the base material. The migration and segregation of minerals forms the characteristic bands that gneiss is known for. In some cases there is a stark contrast between the bands but in others it is hard to differentiate.
Gneiss forming from sedimentary rock is called paragneiss and can form from shale, sandstone or basalt while gneiss forming from igneous rock is called orthogneiss and typically forms from granite, gabbro, or diorite. Most gneiss has very a similar composition to granite. The common mineral components are quartz, feldspar, micas, and silicates. Other minerals may include hornblende, biotite, and muscovite. Sometimes it is difficult to properly distinguish gneiss from schist which is just below gneiss in terms of metamorphic grade.
Uses of gneiss
As a result of the similarities to granite, most gneiss is used for building materials. This could include pave stones, countertops, flooring, architectural pieces, gravestones, facing, and a variety of other construction or building uses. Gneiss is temperature resistant, durable, and weather resistant.
References:
http://web.eng.fiu.edu/prieto/geo1/G1-Lecture06-Metamorphic-Rocks.pdf
