It has a glorious banding which is apparent on microscopic scale and hand specimen. Its mineral content is mainly feldspar. They do not, however, carry large quantities of platy minerals such as micas or chlorites. The rock shines white after the rains and attracts attention of the visitors who excitedly climb the rock mass to the tower at the top to get a panoramic view of the gardens and the city. c) In Karighatta – Kunigal areas also, a similar age is indicated. Augen gneiss (from the German augen, meaning "eyes") is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite. The banding may be oriented nearly parallel to the Earth’s surface (horizontal dip) or may have a steep dip. Geology Institute India, 1, 47 (1906). b) In the Dharwar Craton studies done on the trondhjemites of Holenarsipur, major crust forming event are attributed through the Rb–Sr whole rock isochron ages. Its mineral content is mainly feldspar. New World Encyclopedia writers and editors rewrote and completed the Wikipedia article Gneiss 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. 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. The Archean gneisses and schists, which are the oldest rocks of the Indian Shield, constitute a considerable area of Peninsular India. It contains characteristic elliptic or lenticular shear-bound feldspar porphyroclasts within the layering of bands of quartz, biotite, and magnetite. The earliest rock was a dark coloured amphibolite which was converted into grey biotite gneiss during migmatization. Other bands contain platy or elongate minerals with evidence of preferred orientation. Gneiss resembles granite, except that the minerals are arranged into bands. If a rock shows minerals occurring in distinct bands, it is probably gneiss. The grey biotite gneiss during migmatization was first intruded by grey porphyritic granite and later by pink granites. Gneissic rocks usually have medium to coarse foliation (aligned by directed pressure) and are largely recrystallized. Gneiss is formed by high-grade regional metamorphic processes from formations that were formerly either igneous or sedimentary rocks. The study of gneiss has helped scientists understand the geological processes by which this metamorphic rock can be formed. The Acasta gneiss complex, Northwest Territories, Canada. This is especially true with mica-rich parting planes. One of the towers at the southern end of the city is the Lalbagh tower erected over the Lalbagh hill which is made up of Peninsular Gneiss, now identified as a National Geological Monument. Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying composition, while other bands contain granular minerals with an interlocking texture. Mem. Geology Survey India, 70 (1936 and 1940). This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Based on this estimate, it is the oldest known rock in the world. [3], The monument, located in the southern part of Bengaluru, 4 kilometres (2 mi) from the Legislature office complex of the Karnataka Government (Vidhana Soudha) within the Lalbagh gardens is easily approachable by road (Metro Map of the city in the Info box gives exact location). The supracrustal rocks of the Dharwar Group of the southern Indian Peninsula, as depicted in the geological map of South India, have the Peninsular Gneiss as the basement rock formation, also stated to be the remobilized basement. This term was first fashioned by W.F.Smeeth of the Mysore Geological Department in 1916 based on the first scientific study of this rock exposure. Gneiss is the principal rock over extensive metamorphic terrains. A common enclave is a dark grey to black rock called amphibolite containing the minerals plagioclase feldspar and hornblende. [2][5], The Lalbagh hill, which has been declared as a Geological Monument (plaque pictured) to represent the Peninsular Gneiss, has dark biolite gneiss of granitic to granodioritic composition containing streaks of biolite. Quartz, micas (such as muscovite and biotite), and hornblende are also often present. Peninsular Gneiss is a term coined to highlight the older gneissic complex of the metamorphics found all over the Indian Peninsula. Also, using a radiometric dating technique, researchers have identified the Acasta Gneiss (in Northwest Territories, Canada) as the oldest known crustal rock outcrop in the world. in accordance with New World Encyclopedia standards. it has been subjected to higher temperatures and pressures than schist. The Geological Survey of India identified one of the best exposures of this rock mass, protruding high above the ground as a hillock (pictured), dated 2.5 to 3.4 billion years, in the Bengaluru city in India at the famous Lalbagh gardens and declared it as a National Geological Monument to propagate the knowledge of the rock formation among the public, since Lalbagh is frequented by visitors from all over the world. Min. The term gneissose is used to describe rocks with properties similar to gneiss. Stern, R.A., and W. Bleeker. [8] Rec. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. Due to interactions with granite fluids, these enclaves have developed a border rich in biotite mica. Age of the world's oldest rocks refined using Canada's SHRIMP. Gneiss (pronounced "nice") is a metamorphic rock consisting mostly of quartz and feldspar and showing distinct layering or banding. Gneiss is a foliated metamorphic rock that is a common distribute type of rock high-grade regional metamorphic approaches from pre-current formations that have been initially both igneous or sedimentary rocks. In geological terms, it is an Archaean tonalite gneiss. 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, DC. Contact Department of Geological and Environmental Sciences Appalachian State University 033 Rankin Science West ASU Box 32067 Boone, NC 28608-2067 The gneisses and granites have been profusely invaded by still younger veins of coarse grained pegmatite which have cut up the gneisses into several irregular and lenticular shapes. Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. Gneiss (/ ˈ n aɪ s /) is a common and widely distributed type of metamorphic rock.Gneiss is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks.Orthogneiss is gneiss derived from igneous rock (such as granite).
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