By: Th.G. SAHAMA
88 pages, 34 b/w photos and b/w illustrations, 23 tables
Language: English with bilingual summary in English and French
This paper deals with the main cone of the Nyiragongo volcano (Zaire [now the Democratic Republic of Congo]) as it was before the violent eruption in 1977. A summary is presented of the field data, of the petrology and mineralogy of the rocks and of the evolution of the crater.
Four horizontal platforms characterized the crater: I (160-185 m below the rim), II (180 m below I), III (ca. 45 m below II), IV (a small incomplete remnant lower down). All these platforms collapsed in the 1977 eruption. The bottom of the crater consisted of a molten lava lake with a fluctuating level. The eastern part of the lake contained a large crag detached from the I (?) platform and floating on the lake, inclined towards SE.
The inner walls of the crater show the following succession of rocks, starting from the level of the I platform: pyroclastic strata with interbedded lavas and sills of bergalite (melilite-rich nephelinite), lava beds of nepheline-leucitite and leucite-nephelinite, nepheline-aggregate-lavas (leucite-melilite-nephelinite with aggregates of highly potassian nepheline) on top of the mountain. The strata below the level of the I platform and the lava lake melt consist of rocks equivalent to the nepheline-aggregate-lava.
The endogeneous inclusions and the strata of the inner pit, between the I and II platforms, often display signs of thermal metamorphism. Its textural and mineralogical characteristics are described. The xenoliths from the underlying bedrock, especially numerous on the II platform, consist mainly of fused granite. Gas-deposited glass and silica minerals have been found in vesicles and open cracks of lava. The evolution of the cone was initiated by the eruption of the bergalitic, highly gas-charged and carbonated top part of the ascending magma and was explosive in character. Subsequently, the cone grew through a quiet overflow of the ancient lava lake in which the differentiation by flotation played an important role. The gravity-controlled rise of the leucite and nepheline phenocrysts or aggregates affected the bulk composition of the lavas.
Tentatively, the composition of the Nyiragongo magma is considered a result of an enrichment in (mainly) the alkalies through volatile transfer in an already undersaturated deep-seated magma column.
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