Gneiss


Gneiss
Gneiss rock
Augen gneiss from Rio de Janeiro, Brazil
Granitic gneiss from Enfield, New York

Gneiss (play /ˈns/) 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.

Contents

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

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


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Look at other dictionaries:

  • gneiss — [ gnɛs ] n. m. • 1779; all. Gneis ♦ Géol., minér. Roche métamorphique à grain grossier, où alternent les plages claires (quartz, feldspath) et foncées (mica, amphibole). ● gneiss nom masculin (allemand Gneiss) Roche métamorphique foli …   Encyclopédie Universelle

  • gneiss — gneiss; gneiss·oid; or·tho·gneiss; para·gneiss; gneiss·ic; gneiss·ose; …   English syllables

  • Gneiss — (n[imac]s), n. [G.] (Geol.) A crystalline rock, consisting, like granite, of quartz, feldspar, and mica, but having these materials, especially the mica, arranged in planes, so that it breaks rather easily into coarse slabs or flags. Hornblende… …   The Collaborative International Dictionary of English

  • gneiss|ic — «NY sihk», adjective. 1. like gneiss. 2. of or having to do with gneiss: »gneissic rocks …   Useful english dictionary

  • gneiss — ġnèiss s.m.inv. TS petr. roccia metamorfica, molto comune, di colore grigio chiaro, costituita in prevalenza da quarzo, feldspati e miche, utilizzata come materiale da costruzione {{line}} {{/line}} VARIANTI: gnais, gneis. DATA: 1796. ETIMO: dal… …   Dizionario italiano

  • gneiss — 1757, from Ger. Gneiss type of metamorphic rock, probably from M.H.G. gneist spark (so called because the rock glitters), from O.H.G. gneisto spark (Cf. O.E. gnast spark, O.N. gneisti) …   Etymology dictionary

  • gneiss — [nīs] n. [Ger gneis < OHG gneisto, a spark, akin to ON gneisti, OE gnast: from the luster of certain of the components] a coarsegrained, metamorphic rock resembling granite, consisting of alternating layers of different minerals, such as… …   English World dictionary

  • gneiss — gneissic, adj. /nuys/, n. a metamorphic rock, generally made up of bands that differ in color and composition, some bands being rich in feldspar and quartz, others rich in hornblende or mica. [1750 60; < G] * * * Medium to coarse grained… …   Universalium

  • Gneiss — Orthogneiss de la zone de cisaillement de l Ailao Shan Fleuve Rouge en Chine. Source [1]. Le gneiss est une roche métamorphique contenant du quartz, du mica, des feldspaths plagioclases et parfois du feldspath alcalin, tous suffi …   Wikipédia en Français

  • gneiss — [[t]naɪs[/t]] n. pet a metamorphic rock, generally made up of bands that differ in color and composition, some bands being rich in feldspar and quartz, others rich in hornblende or mica • Etymology: 1750–60; < G Gneis, ult. der. of OHG gneisto …   From formal English to slang


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