MT. KENYA VOLCANO
Mt. Kenya is an eroded, extinct volcano in
south-central Kenya, just next to the East African Rift Valley system (see photo).
The volcano was active principally during the Pliocene and Pleistocene.
The summit area of Mt. Kenya consists of majestic-looking, high spires of rock
- these rocks are shallow intrusives that filled up the throat of the old
volcano. The eroded slopes of the volcano consist of a variety of extrusive
rocks, including trachytes, basalts, phonolites, and kenytes.
Kenyte
(10.2 cm from top to bottom; 4.1 cm at its widest) - this is the second-rarest
lava type on Earth. It is only known
from two localities: 1) the extinct Mt. Kenya Volcano in Kenya and 2) the
active Mt. Erebus Volcano
in Antarctica. Chemically and
texturally, kenytes are porphyritic phonolites (intermediate,
alkaline-rich volcanic rocks) dominated by large phenocrysts of anorthoclase
feldspar ((K,Na)AlSi3O8).
So why aren't kenytes just called porphyritic
anorthoclase phonolites? Well, kenytes are different, but a gradational
spectrum of lithologies exists between kenyte lavas and porphyritic
anorthoclase phonolite lavas. The intended distinction is in the
characteristics of the matrix. Kenyte
matrix is supposed to be glassy-textured or homogeneously
cryptocrystalline-textured or composed of devitrified glass. Kenyte
matrix also often shows obvious flow-banding (see above sample).
This kenyte is Pleistocene in age, collected from the
southern slopes of the Mt. Kenya Volcano.
Large whitish-grayish crystals = anorthoclase feldspar phenocrysts.
Dark matrix = flow-banded, ~cryptocrystalline material.
Porphyritic anorthoclase phonolite (6.6 cm across) - this second Mt. Kenya rock has the
same chemistry and texture as a kenyte (= 1st rock shown above), but
the dark-colored matrix of this porphyritic phonolite is more coarsely-crystalline
than in kenyte, and also lacks flow-banding.
This rock is Pleistocene in age, and was collected
from an outcrop on the southern slopes of Mt. Kenya Volcano.
Large pale-colored crystals = anorthoclase feldspar phenocrysts.
Dark, finely-crystalline groundmass = dominated by sanidine & nepheline.
Porphyritic nepheline syenite (8.3 cm across) - this 3rd rock is derived
from the summit spires of Mt. Kenya Volcano.
It is a porphyritic nepheline syenite, a shallow intrusive igneous rock
that filled up the throat of the old volcano.
This rock was collected from a float boulder on the
southern slopes of Mt. Kenya, but derived from the summit spires. The
rock contains large phenocrysts of sanidine feldspar, smaller phenocrysts of
nepheline, and a groundmass of aegirine pyroxene, alkalic amphibole,
aenigmatite, and melanite.
Mostly synthesized from info. provided by Tony
Peterson & info. in Baker (1967) and Price et al. (1985).
References cited:
Baker (1967) - Geology of the Mount Kenya area. Geological
Survey of Kenya Report 79. 78 pp. 4 pls. 1 map.
Price et al. (1985) - Geochemistry of phonolites and
trachytes from the summit region of Mt. Kenya. Contributions to
Mineralogy and Petrology 89: 394-409.