This paper deals with the geological structure and the petrologie evolution of Karisimbi, the highest volcano in the Virunga region.
As this paper is intended to be understood by non-geologists, a brief review about the methods used by volcanologists, should make things clear for the reader.

The field-work data enabled us to describe the morphology, structure and the evolution of Karisimbi.
The results of the laboratory studies are summarized in the section petrography-petrochemistry, where some problems concerning nomenclature and interpretation of chemical data are discussed as well.

Petrographical and petrochemical information leads us to the origin and the evolution of magmas, which is the ultimate purpose of every petrologist. In the case of Karisimbi, it is suggested that its petrologie evolution took place by simultaneous fractional crystallization and contamination by crustal rocks.

Finally, the ages of some typical Karisimbi lavas have been determined by a radiometric dating method (K-Ar), bearing in mind that large errors on these ages are inevitable.

KEYWORDS : geology, volcanoes, petrology, Virunga, differentiation