Gneiss

Gneiss rock

Augen gneiss from Rio de Janeiro, Brazil

Granitic gneiss from Enfield, New York

Gneiss ( /ˈnaɪs/) is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.

Etymology

The etymology of the word "gneiss" is disputed. Some sources say it comes from the Middle High German verb gneist (to spark; so called because the rock glitters). It has occurred in English since at least 1757.^[1]

Other sources claim the root to be an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.

Composition

Gneissic rocks are usually medium- to coarse-foliated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, etc.

Gneiss displays compositional banding where the minerals are arranged into bands of more mafic minerals and more felsic minerals. This is developed under high temperature and pressure conditions.

Study of Gneiss Rock, Glenfinlas, the Trossachs, Scotland. A pen and ink study by John Ruskin, 1853, is now in the Ashmolean Museum, Oxford.

Types

Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissose is used to describe rocks with properties similar to gneiss.

Lewisian

Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. These are amongst the oldest rocks in Europe and some of the oldest in the world, having been formed in the Precambrian "super-eon", up to 3 billion years ago. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree.^[2] These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma.^[3] The gneiss's delicate pink colours are exposed throughout the islands and it is sometimes referred to by geologists as "The Old Boy".^[4]

Augen gneiss

Augen gneiss, from the German Augen [ˈaʊɡən], meaning "eye", is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.

Boulder of gneissic breccia on Osmussaar, Estonia, apparently thrown there by the Neugrund impact

See also

• List of rock types

References

• Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., pp. 359–365, Freeman, ISBN 0-7167-2438-3
• Gillen, Con (2003) Geology and landscapes of Scotland. Harpenden. Terra Publishing. ISBN 1903544092
• McKirdy, Alan Gordon, John & Crofts, Roger (2007) Land of Mountain and Flood: The Geology and Landforms of Scotland. Edinburgh. Birlinn. ISBN 9781841583570
• Murray, W.H. (1966) The Hebrides. London. Heinemann.

Footnotes

1. ^ Online Etymology Dictionary
2. ^ Gillen (2003) page 44.
3. ^ McKirdy et al. (2007) page 95.
4. ^ Murray (1966) p. 2

External links

Media related to Gneiss at Wikimedia Commons

•  Chisholm, Hugh, ed (1911). "Gneiss". Encyclopædia Britannica (11th ed.). Cambridge University Press. 
•  "Gneiss". New International Encyclopedia. 1906.