How did lava from the Virunga volcanoes contribute to the formation of Lake Kivu?

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Cluster of small houses in a village nestled among green hills beside the wide Congo River, with more hills visible in the background, Democratic Republic of Congo. — Central Africa A small village in the hills along the banks of the Congo River, Democratic Republic of Congo, Central Africa.

Volcanic activity from the Virunga Mountains played a critical role in shaping the geography of Lake Kivu.

Historically, Lake Kivu was part of a larger body of water that filled a structural trough in the Earth, with its drainage flowing northward toward Lake Edward and eventually into the Nile system. Volcanic outpourings along the lake's northern shore created a massive lava dam that blocked this northern outflow. This geological obstruction forced a reversal of the lake's drainage, causing it to flow southward through the Ruzizi River into Lake Tanganyika.12