East African mountains

Physiography

The Aberdare Range, of which the highest peak is Mount Lesatima (Satima), reaching a height of 13,120 feet, and the Mau Escarpment rise steeply from the eastern portion of the Eastern (Great) Rift Valley. To the west, beyond the Uasin Gishu Plateau, Mount Elgon emerges gently from a level of about 6,200 feet; but the spectacular cliffs of its western face dominate the lower plains of eastern Uganda, which lie at about 3,600 feet. The rim of Elgon’s caldera is approximately 5 miles (8 km) in diameter and contains several peaks, of which Wagagai, at 14,178 feet, is the highest.

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The Nyeri-Nanyuki corridor separates the Aberdare Range from Mount Kenya. The second highest mountain in Africa, Mount Kenya has a girth of about 95 miles at 8,000 feet, from which it rises boldly to its restricted summit zone. The craggy twin peaks of Batian (17,057 feet) and Nelion (17,022 feet) are closely followed in height by Lenana (16,355 feet).

Set amid low plateaus, Kilimanjaro is the highest mountain in Africa, rising to 19,340 feet (5,895 metres) at Uhuru peak on the Kibo cone. The generally smooth outlines of the cratered dome of Kibo are in marked contrast to the jagged form of Mawensi, or Mawenzi (17,564 feet); the two summits are connected by a saddle that lies at about 14,500 feet. Mount Meru, about 40 miles southwest of Kilimanjaro, attains an altitude of 14,978 feet.

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Africa’s Physical Features

The Ruwenzori Range runs parallel to the Western Rift Valley, to which it drops steeply. The fall to the uplands of western Uganda, however, is more gradual. At its base the range is some 80 miles long, and its greatest width is about 30 miles. The summit zone contains six distinct mountain massifs, which are separated by well-defined passes and deep river valleys. Mounts Baker and Gessi lie entirely within Uganda, while Mounts Stanley, Speke, Emin, and Luigi di Savoia form part of the Uganda-Congo frontier. Of the 10 peaks with heights of more than 16,000 feet, all but one are on Mount Stanley, which includes the highest peak, Margherita, at 16,795 feet.

The Virunga Mountains and their associated lava flows extend across the Western Rift Valley. In the west, Nyamulagira, Nyiragongo, and Mikeno are in Congo; Karisimbi—at 14,787 feet the highest of the Virunga volcanoes—and Visoke are centrally placed on the Congo-Rwanda frontier; and farther east Sabinio (Sabinyo), Mgahinga (Gahinga), and Muhavura, also known as the Mufumbiro Mountains, are on the Rwanda-Uganda frontier. Not all the cones culminate in craters, but several have crater lakes.

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The relict glaciers that occur in the summit zones of Kilimanjaro, Mount Kenya, and the Ruwenzori have little erosive force. Their more powerful predecessors, however, extended down to altitudes between 12,000 and 10,000 feet and even lower and produced arêtes (sharp-edged ridges), cirques (glacial amphitheatres), rock tarns (rock basin lakes), U-shaped valleys, and moraines (boulders and other debris deposited by glacial action). Early glaciation also affected both Mount Elgon and the Aberdare Range. More than 30 small glaciers on the Ruwenzori together cover a surface of approximately 1.5 square miles (4 square km), most of which is on Mounts Stanley and Speke; the lowest valley glacier descends to about 14,000 feet. Of the glaciers remaining on Mount Kenya and covering less than 0.3 square mile, the largest are Lewis and Tyndall; the lowest tongue of ice reaches down to about 15,000 feet. On Kilimanjaro, Kibo crater is strewn with giant blocks of ice, and the outer rims are covered with ice reaching down to about 16,000 feet on the wet southwestern moorlands. The 20th century was marked in East Africa by a process of glacial retreat that has been rapid but neither constant nor continuous.

Geology

The peneplain of eastern Africa, dating from the Miocene Epoch (about 23 to 5.3 million years ago), has been subject to a general elevational movement. The shoulders of the rift valleys have risen intermittently to produce highlands on which lavas that have been ejected from fissures in the Earth’s surface have in some instances added considerable height. The most dramatic uplift is that of the Ruwenzori, the only East African mountains that are not volcanic. The ancient plateau surface of gneisses and schists was upfaulted on the west and upwarped on the east. Movements along the faults continue, and the Ruwenzori system is an important earthquake epicentre.

Kilimanjaro is a volcano of complex structure and alkaline lavas situated at an intersection of fault lines. Shira was the first volcano of the group to become inactive, followed in turn by Mawensi and Kibo. The latter retains its caldera—1.5 miles in diameter and 600 feet deep—within which there are found successive inner cones and craters as well as fumaroles (holes or vents that emit gases).

The long-extinct volcano of Mount Kenya has been much denuded, and the highest peaks consist of the crystalline nepheline-syenite (a granular rock of alkalic feldspar, nepheline, and other minerals), which plugged the former vent. Around this core are gently dipping lavas, agglomerates, and tuffs.

Mount Elgon is part of the Eastern Volcanics in Uganda, which consist of soda-rich lavas and associated fragmental tuffs and agglomerates. The Western Volcanics are represented by the Virunga Mountains, of which Nyamulagira and Nyiragongo have remained active into the 21st century. Major eruptions occurred in 1912, 1938, 1948, the 1970s, and 2002. On several occasions a lava stream reached the shores of Lake Kivu. The 2002 Nyiragongo eruption destroyed much of Goma, Congo.