Language
Country/Area
No result found
The amazing formation of Gneiss: How do high temperatures and high pressures shape the texture of the Earth?
Gneiss (granite schist) is a fairly common and widely distributed metamorphic rock known for its unique formation and color variation.
This metamorphic rock is formed during a metamorphic process of high temperature and high pressure and is usually the product of a formation composed of igneous or sedimentary rocks undergoing metamorphism. The conditions for its formation include a pressure range of 2 to 15 kbar, or even higher, and a temperature exceeding 300°C (572°F). Gneiss almost always shows band-like structural features, which are formed by alternating dark and light bands and do not feature distinct cracks.
Gneiss is a common rock type in the ancient layers of the continental crust. Some of the oldest rocks on Earth, such as Acasta Gneiss, rank among such rocks.
Description of Gneiss
In traditional English and North American usage, Gneiss is defined as a coarse-grained metamorphic rock showing a compositionally banded structure but with indistinct fission development. In short, this is a structure of mineral particles visible to the naked eye with distinct compositional layers but only a slight tendency to split along the layers. In Europe, the term is more widely applied to any coarse, mica-deficient high-grade metamorphic rock.
Gneiss is characterized by the band-like structure it exhibits, which is formed by the arrangement of minerals at high temperatures.
As defined by the British Geological Survey (BGS) and the International Federation of Geological Sciences (IUGS), Gneiss is considered a broad text category for metamorphic rocks with medium to coarse grains showing poorly developed plates -like structure and has a compositional layer more than 5 mm (0.20 in) thick. The nomenclature of Gneiss often carries descriptions of color and mineral composition, such as "garnet-mica Gneiss" or "grey-pink Gneiss".
Gneissic stripes
The minerals in Gneiss are arranged in band-like structures, a phenomenon known as Gneissic streaks. Darker bands contain more minerals such as magnesium and iron, while lighter bands contain more minerals such as sodium, aluminum and potassium. This band-like structure is formed when the compressive strength of the rock is greater than in other directions, and high temperatures reorganize these minerals.
During the formation of Gneiss, the rock is subjected to extreme shear forces that stretch and flatten the original material.
Different types of Gneiss
Augen gneissis a type of Gneiss metamorphosed from granite, characterized by the inclusion of rounded or lenticular grains in a matrix of smaller particles. Augen gneiss exhibits more significant metamorphic features relative to conventional Gneiss.
Migmatiteis Gneiss containing more than two different rock types, some of which manifest as ordinary Gneiss and the other resembling intrusive rocks, such as granite. This rock is generally thought to be the product of partial melting, and its components include the famous "Leucosome" and "Melanosome".
Distribution of Gneiss
Gneiss usually occurs where regional deterioration is more active, where the degree of deterioration can be as high as 600°C (1,112°F), while pressures can reach 2 to 24 kbar. Many rock types can metamorphose to Gneiss, so geologists institutionally add descriptions to each of them.
For example, Gneiss in the ancient Kraton region is often accompanied by greenstone belts, which have an extremely long geological history.
The existence of these Gneiss is not only a display of geological structure, but also a witness to the earth's change process.
Famous Gneiss instance
Acasta Gneiss is an ancient crustal debris located in the northwestern region of Canada that has undergone metamorphism for about 3.58 to 4.031 billion years. Another famous example is the Lewisian gneiss in Scotland, which is composed primarily of metamorphic volcanic and sedimentary rocks, while the Morton Gneiss in the United States is considered the oldest complete continental crustal mass in the country.
Purposes of Gneiss
As a building material, Gneiss is widely used in construction projects, and its aesthetic appearance makes it a popular choice in some cities. In addition, Gneiss is often used as a coarse aggregate for construction.
Gneiss not only plays an important role in geology, but its amazing formation process and distribution have also triggered people to think deeply about the evolution of the earth. Faced with these phenomena combined with nature, we can’t help but ask: Do other metamorphic rocks on the earth also hide similar stories?
