The peneplain of eastern Africa, dating from the Miocene Epoch (between 23.7 and 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 been active during the 20th century. Major eruptions occurred in 1938 and 1948 and during the 1970s, and on several occasions a lava stream has reached the shores of Lake Kivu.
"East African mountains"
Encyclopædia Britannica Online.