Geology Reference
In-Depth Information
L. Stephanie
L.Rudolf
L.Kioga
L.Albert
Elgon
Baringo
Kenya
Naivasha
L. Magadi
Nat
ron
Kalimnajaro
Ruwenzor
L.Edward
Birunga Volcanoes
L.Kivu
Lake
L.Eyasi
L.Tanganyika
Mombasa
Pemba
Zanzbar
Mafia
Rungwe
L.Mweru
L.Bangweulu
L.Nyasa
Ozambique
Shire
Highlands
Fig. 4.8 Map of the Great Rift Valley of Africa from Zambesi to Ethiopian border (after Holmes
1965)
The two active volcanoes namely, Nyiragongo and Nyamuragira (Fig.
4.10
),
which are separated by less than 15 km, are characterized by the production of
contrasting lava
flows. The Nyamuragira is a typical Birunga volcano erupting
K-basanites and more evolved derivatives such as K-hawaiites. These lavas are
feldspar, olivine and clinopyroxene-phyric with leucite phenocrysts or leucite in the
groundmass. The Nyiragongo volcano however, is known for erupting leucite
nephelinites and melilitites comprising nepheline, leucite and melilite with olivine
and clinopyroxene.
Sahama (1973) studied samples of lavas of Shaheru-Nyiragongo-Baruta, and
found that they are mainly latites, leucite phonolites and leucite tephrites. Except for
tephrites other rocks of Nyiragongo are nepheline-normative. In tephrites, leucite
predominates over nepheline. The content of clinopyroxene is far more than that of
olivine in the melanocratic rocks. There is continuous increase in the (Na
2
O+K
2
O)
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