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The Primeval Era The Virunga Mountains consist of a string of volcanoes straddling the western branch of a huge system of cliff-bounded valleys known as the Great African Rift. The rift valleys run down the center of the African continent along the crest of a broad topographic uplift. This part of Africa is being pulled apart in an east-west direction. The pairs of cliffs forming the valley walls mark where the ground has faulted and split apart, allowing the huge blocks of rock between them to slip downward to form the long linear valleys. The uplift of the crests of the cliffs and the land behind them is caused partly by the inflow of hot rock from the earth's upper mantle and partly by the heating and thermal expansion of cool rocks near the earth's surface. Volcanoes, such as the Virungas, often form along rift valleys where molten rock from the depths is able to reach the surface through the rift faults. The six volcanoes constituting the center and eastern sections of the Virunga chain are now extinct. The westernmost volcanoes of the chain, Nyamuragira and Nyiragongo, are very much alive, being two of the most continually active volcanoes in the world.
The rich montane flora included herbs and vines gorillas found nourishing. Consequently, when wandering mountain gorillas entered the Virungas from their ancestral ranges to the north and west, they were able to establish a viable colony. Other mountain gorilla colonies formed in the montane forests growing on the highlands of the east side of the Rift Valley, but these became the eastern outposts of gorilla expansion because the land farther east was lower in elevation and too dry to support the plants the gorillas needed for food. It is not known just when the mountain gorillas moved into the Virungas, but they must have come when montane-type rainforests extended over a broader area westward and to much lower elevations than at present. Montane forests require a cool and wet climate that last existed in Central Africa during the recent Ice Epoch, implying that the gorillas colonized the Virungas before the beginning of the present warm climate. The climate of Central Africa began to dry out and warm up about 9,000 years ago. As the climate warmed, the range of the montane forests and of the mountain gorillas living in them retreated to progressively higher elevations, finally isolating the mountain gorillas in small islands of habitability along the crests of the higher mountain ranges.
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The bases of the Virunga volcanoes are at an elevation
of 1,220-1,525 m (4,000-5,000 ft), and their peaks reach elevations of 3,660-4,270 m
(12,000-14,000 ft), high enough to squeeze vast amounts of water out of passing air
masses. The resulting heavy rainfall supports the growth of a dense montane rainforest on
the volcanoes' sides. Left: Mount Karisimbi, the highest of the Virunga volcanoes, is often capped by
snow. The intense heat of the tropical sun is offset by the cooling effects associated
with increased elevation. Photo:
Courtesy of the Dian Fossey Gorilla Fund