File:Gneiss (Joshimath Formation, Proterozoic; outcrop at Joshimath, Uttarakhand State, Indian Himalayas) 3 (26708323062).jpg
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Gneiss from the Precambrian of India. (field of view ~7.35 cm across) Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy. Gneiss is a high-grade, foliated metamorphic rock. It differs from lower grade foliated metamorphics, such as slate, phyllite, and schist, by having alternating bands of light and dark minerals. Metamorphism has been so intense that minerals have segregated into discrete zones. Most gneisses form by metamorphism of schists or granites. The gneiss shown above is from the Himalayan Mountains of India and has well-defined foliation with crenulations. The dark bands are rich in biotite mica. The light-colored bands are dominated by quartz and feldspar. The small red crystals are garnet. Stratigraphy: Joshimath Formation, lower Vaikrita Group, Higher Himalayan Crystalline Belt, Proterozoic Locality: outcrop in the town of Joshimath, Uttarakhand State, Himalayan Mountains, northern India Some info. from: Pandey et al. (1981) - Rb-Sr age data for the gneissic rocks from Dhakuri, Joshimath and Guptakashi areas of Central Crystalline Zone, Kumaon Himalaya, U.P. pp. 56/1-56/6 in: National Symposium on Mass Spectrometry - Progress in Research, Applications, and Instrumentation, Bhabha Atomic Research Centre, December 21-23, 1981. |
| Date | |
| Source | Gneiss (Joshimath Formation, Proterozoic; outcrop at Joshimath, Uttarakhand State, Indian Himalayas) 3 |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/26708323062 (archive). It was reviewed on 2 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 06:18, 2 December 2019 | | 3,268 × 2,491 (4.48 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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