Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated (aligned by directed pressure) 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, and so forth. Depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, and so forth. 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.
Gneiss resembles granite, except that the minerals are arranged into bands. Sometimes it is difficult to tell the difference between gneiss and a schist because some gneiss appears to have more mica than it really does. This is especially true with mica-rich parting planes. If a rock shows minerals occurring in distinct bands, it is probably gneiss.
The word "gneiss" is from an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.
Augen gneiss
Augen gneiss 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.
Etymology: from the German augen, meaning "eyes".
Acasta Gneiss
The Acasta Gneiss is the oldest known crustal rock outcrop in the world. It is an Archaean tonalite gneiss in the Slave craton in Northwest Territories, Canada.
The Acasta Gneiss is named for the nearby Acasta River east of Great Slave Lake some 350 km north of Yellowknife. The rock exposed in the outcrop formed just over four billion (4 x 109) years ago; an age based on radiometric dating of zircon crystals (4.03 Ga orthogneisses (Bowring & Williams, 1999), which are the oldest rocks in the world so far).
In 2003 a team from the Smithsonian Institute collected a four-tonne boulder of Acasta Gneiss for display outside the National Museum of the American Indian in Washington, D.C.
The Acasta outcrop is found in a remote area of the Tlicho land settlement.
According to some timings of the geological periods, it was formed in the Basin Groups unofficial period of the Hadean eon, which came before the Archean: see Timetable of the Precambrian.
See also
ReferencesISBN links support NWE through referral fees
- Blatt, Harvey, and Robert J. Tracy. 1995. Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed. New York: W.H. Freeman. ISBN 0716724383.
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
- Bowring, S.A., and Williams, I.S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, v. 134, 3-16.
- Stern, R.A., Bleeker, W., 1998. Age of the world's oldest rocks refined using Canada's SHRIMP. the Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada, v. 25, p. 27-31
External links
- Ancient rock begins pilgrimage to museum
- ANCIENT CONTINENT OPENS WINDOW ON THE EARLY EARTH Science, December 17,1999
- Yuichiro Ueno research homepage
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