How did volcanic lava help form Lake Kivu?

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East African mountains and lakes — East African mountains Mountains and lakes of East Africa.

Volcanic activity played a critical role in the formation and hydrological evolution of Lake Kivu. Geologically, the lake was once part of a larger body of water that occupied a structural trough in the Earth.1

The eruption of the Virunga Mountains resulted in significant volcanic outpourings along the lake's northern shore. This lava created a natural dam that blocked the lake's original northward drainage toward Lake Edward and the Nile system. Consequently, this blockage forced the lake's drainage to reverse, redirecting its outflow to the south through the Ruzizi River into Lake Tanganyika.12