File:Vesicular porphyritic trachyte (Magadi Trachyte (Plateau Trachyte), Pleistocene, 700 ka to 1.4 Ma; northern end of Lake Magadi, Kenya Rift segment of the East African Rift Valley).jpg
![File:Vesicular porphyritic trachyte (Magadi Trachyte (Plateau Trachyte), Pleistocene, 700 ka to 1.4 Ma; northern end of Lake Magadi, Kenya Rift segment of the East African Rift Valley).jpg](https://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Vesicular_porphyritic_trachyte_%28Magadi_Trachyte_%28Plateau_Trachyte%29%2C_Pleistocene%2C_700_ka_to_1.4_Ma%3B_northern_end_of_Lake_Magadi%2C_Kenya_Rift_segment_of_the_East_African_Rift_Valley%29.jpg/799px-thumbnail.jpg?20220619082247)
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[edit]DescriptionVesicular porphyritic trachyte (Magadi Trachyte (Plateau Trachyte), Pleistocene, 700 ka to 1.4 Ma; northern end of Lake Magadi, Kenya Rift segment of the East African Rift Valley).jpg |
English: Magadi Trachyte (Plateau Trachyte) (field of view ~4.3 centimeters across) - vesicular porphyritic trachyte sample from an outcrop at the northern end of Lake Magadi, in the Kenya Rift segment of the East African Rift Valley.
Quite a few volcanoes along the East African Rift Valley have lavas of unusual chemistries (e.g., natrocarbonatite, phonolite). The rift valley also has flood lavas in places. The rock seen here is from a flood lava that covers part of the rift valley floor in far-southwestern Kenya. This is representative of the Magadi Trachyte (or Plateau Trachyte), a flood trachyte deposit erupted during the Early and Middle Pleistocene (~700 ka to ~1.4 Ma). The rock itself is a vesicular porphyritic trachyte. Trachytes are alkaline, intermediate extrusive rocks (it’s one of the alkaline equivalents of the more common lava types dacite & andesite). The large whitish crystals are phenocrysts of sanidine feldspar ((K,Na)AlSi3O8 - potassium-sodium aluminosilicate). The sanidine phenocrysts look whitish in the photo, but they are actually colorless & nicely transparent to translucent. The fine-grained, darker matrix of trachytes is usually a mixture of finely-crystalline sanidine, pyroxene, a little sodic plagioclase, maybe a little quartz, etc. Trachyte lavas typically have high viscosities, and are not prone to flowing long distances as are basalt lavas. However, some trachyte lavas, in a few places, have indeed spread out once erupted - the Magadi Flood Trachyte is a great example. |
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Source | https://www.flickr.com/photos/47445767@N05/14850565899/ |
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@N05/14850565899. It was reviewed on 19 June 2022 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
19 June 2022
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current | 08:22, 19 June 2022 | ![]() | 3,006 × 1,362 (2.23 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/14850565899/ with UploadWizard |
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Orientation | Normal |
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Horizontal resolution | 72 dpi |
Vertical resolution | 72 dpi |
Software used | Adobe Photoshop CS2 Macintosh |
File change date and time | 19:49, 25 August 2014 |
Color space | Uncalibrated |
Image width | 3,006 px |
Image height | 1,362 px |
Date and time of digitizing | 10:59, 25 July 2007 |
Date metadata was last modified | 15:49, 25 August 2014 |
IIM version | 2 |