OL DOINYO NYUKIE
Obsidian is an uncommon igneous rock that varies in
chemistry from felsic to intermediate to mafic to alkaline. Chemical
analysis can show whether an obsidian is rhyolitic, dacitic, etc. The
rock shown below is a trachyte obsidian from a small volcano in the East
African Rift Valley called Ol
Doinyo Nyukie (several variant spellings exist). Eastern Africa is
being ripped apart by the Afar Hotspot and quite a few volcanoes having unusual
lava chemistries are present along the rift zone. Ol Doinyo Nyukie is a
Quaternary-aged volcano that has principally erupted phonolites and trachytes -
both lithologies are intermediate extrusive igneous rocks noticeably high in
alkali content (sodium & potassium). This rock is almost 100% glass,
but there are some small, scattered phenocrysts of feldspar.
Locality:
Ol Doinyo Nyukie (01º 09’ South latitude, 36º 21’ East longitude), a small
volcanic center on the southern side of the Mt. Suswa caldera, southern Kenya,
eastern Africa.
Trachyte obsidian (7.9 cm across) from the Quaternary-aged Ol Doinyo
Nyukie Volcano, southern Kenya.
Some info. provided by Tony Peterson.