File:Orinoco Granite (garnetiferous convoluted gneiss) 12 (33170946154).jpg
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Summary
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| Description |
Gneiss is a common high-grade, foliated metamorphic rock characterized by alternating bands of light- and dark-colored minerals. It forms by metamorphism of schists or felsic-to-intermediate intrusive igneous rocks (e.g., granite). The rock shown above is a commercial decorative stone of undetermined provenance ("Orinoco Granite") The light-colored bands are quartz-rich. The dark-colored areas are rich in mafic minerals. The dark red spots (click on the photo to zoom in and look around) are garnets. The quartz bands are highly convoluted from metamorphism and orogenic deformation. This material resembles an injection gneiss with convoluted igneous dikes. The type of folding seen here is referred to as "ptygmatic". Ptygmatic folds are highly contorted folds with high amplitude-to-wavelength ratios. They occur in deformed rocks where high-viscosity areas (such as dikes or leucosomes) get folded, while the surrounding, low-viscosity, country rocks do not. |
| Date | |
| Source | Orinoco Granite (garnetiferous convoluted gneiss) 12 |
| Author | James St. John |
Licensing
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/33170946154 (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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| Date/Time | Thumbnail | Dimensions | User | Comment | |
|---|---|---|---|---|---|
| current | 06:21, 2 December 2019 | | 7,200 × 5,152 (18.57 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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