gnejsic

Gnejsic is a term used in geology to describe rocks that have undergone high-grade metamorphism. These rocks are typically characterized by a distinct foliation, which is the layering or banding of minerals. This foliation arises from the intense pressure and temperature conditions during their formation, causing minerals to recrystallize and align themselves in parallel planes. Gnejsic rocks often exhibit alternating bands of light and dark minerals, such as quartz and feldspar (light) and biotite or amphibole (dark).

The formation of gnejsic rocks usually occurs deep within the Earth's crust, often associated with mountain-building processes or tectonic plate collisions. The parent rocks, known as protoliths, can be igneous, sedimentary, or even older metamorphic rocks. Under extreme heat and pressure, the original mineral composition and texture of the protolith are transformed. Common examples of gnejsic rocks include granite gneiss, which is derived from granite, and paragneiss, which originates from sedimentary rocks like shale. The specific mineral assemblage and the intensity of foliation in a gnejsic rock provide clues about the metamorphic conditions it experienced.