Gneiss

Pure shear

Augen-gneiss

Road Cutting - geograph.org.uk - 820828

Yttre Ursholmen Kontakt Kosterdiabas i Nebulitisk-migmatitisk sedimentgnejs

Gnaisse Sete Voltas - Bahia - Brasil

Gneiss is a high-grade metamorphic rock characterized by its banded appearance and granular texture. It forms through the high-degree alteration of original rocks, either igneous or sedimentary rocks, under intense heat and pressure conditions deep within the Earth's crust. The process, known as metamorphism, results in the recrystallization of the rock's minerals, leading to the formation of new mineral assemblages that are stable under the conditions of high temperature and pressure.

Formation

Gneiss forms from the metamorphism of pre-existing rocks in conditions of high temperature and pressure, typically associated with mountain building processes. These conditions allow for the recrystallization of minerals without melting the rock. The process leads to the segregation of minerals into alternating light and dark bands, giving gneiss its characteristic appearance. The light bands are typically composed of quartz and feldspar, while the dark bands contain minerals such as biotite, amphibole, and garnet.

Types

There are several types of gneiss, classified based on their composition and the conditions under which they were formed. Some of the common types include:

• Orthogneiss: Derived from igneous rock, such as granite.
• Paragneiss: Derived from sedimentary rock.
• Migmatite: Features characteristics of both gneiss and igneous rock, indicating partial melting.

Characteristics

Gneiss is distinguished by its foliated texture, with minerals arranged into bands or layers. This foliation is a result of the differential stress and high temperatures involved in its formation, which aligns the minerals into sheets. Despite its foliated structure, gneiss does not split as easily as other metamorphic rocks, such as schist, due to its coarser and more granular texture.

Uses

Due to its strength and durability, gneiss has been used as a construction material for centuries. It is commonly used in building construction, landscaping, and as a decorative stone in architecture. Its resistance to weathering makes it suitable for exterior applications, while its unique banded appearance is valued for interior design projects.

Distribution

Gneiss is found in regions that have undergone intense metamorphism, often associated with the cores of ancient mountain ranges or the roots of modern mountains. These include parts of the Canadian Shield, the Scottish Highlands, the Himalayas, and the Scandinavian Mountains.