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 shown below 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 k.y. to ~1.4 m.y.).


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.


Magadi Trachyte (Plateau Trachyte) (field of view ~4.3 cm 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.



Some info. provided by Tony Peterson.



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