Geology Reference
In-Depth Information
(leucite + clinopyroxene + K-feldspar + glass). Analyses of leucitites from New
South Wales, Australia are given in Table
4.5
.
4.3 Potassium-Rich Silica-Deficient Rocks from Africa
4.3.1 Birunga Volcanic Field
The rift system in Africa is extended from the Red Sea in the north to Malawi
(Wooley and Jones 1987) in the south for a distance of 2,800 km, and can be
divided broadly into three major rifts: the Ethiopian Rift, the Kenyan or Gregory
Rift and the Western Rift. Holmes (1965) mentioned that these rifts are located on
plateaux. The Ethiopian Rift cuts across the Ethiopian Highlands and the Kenyan
and Western Rifts across the East African Plateau. According to Rogers et al.
(1998) the dimensions of these circular plateaux are similar to those of uplifted
areas of the oceans associated with oceanic island (Courtney and White 1986).
Alkali volcanism in this region is associated with crustal warping and there is
possibly a plate tectonic control (Bailey 1964). The width of the rift valleys at Lake
Albert, Lake Tanganyika, Rukwa, Lake Rudolf, Lake Natron, Ruaha and Lake
Nyasa, varies between 30 and 55 km. The rift valley is constituted of the following
segments (Fig.
4.8
):
(1) The Nyasa section and its branches towards the southern end of the central
plateau (Rukwa, and Ruaha):
(2) The Western Rift from lake Tanganyika through Lake Kivu, Lake Edward and
Lake Albert and Ruwenzori massif;
(3) The Gregory Rift, east of Lake Victoria and
(4) Lake Rudolf and the Ethiopian section.
The topography broadly correlates with a negative Bouger anomaly (Bullard
1936). This suggests that plateaux are dynamically supported by convective activity
in the underlying asthenosphere (Ebinger and Sleep 1998; Rogers et al. 1998).
There is also a general consensus that the rift system is supported probably by more
than one plume (Macdonald 1994).
The Birunga province is situated on the international boundaries of Zaire,
Rwanda and Uganda in the western branch of the East African Rift System
(Fig.
4.9
). The Birunga volcanic
field has 8 volcanoes (Fig.
4.10
), namely, Mikeno,
Magahinga, Muhavura, Visoke, Sabinyo, Karisimbi, Nyamuragira and Nyiragongo.
Six of the eight volcanoes are now extinct; only Nyiragongo and Nyamuragira are
active. There are several smaller volcanic cones also. According to Rogers et al.
(1998) the most easterly volcano Muhavura has a very youthful morphology and
appears to be intermediate in age between the currently active volcanoes and more
deeply eroded Sabinyo.
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