Positioned on the Pre-Cambrium bedrock of the Trans Nzoia Plateau, Mt. Elgon is similar to other such volcanoes in East Africa in that it is associated with the formation of the Rift Valley. Similar to other Rift Valley Volcanoes, one thing sets Mt. Elgon apart…its age! Although there is no verifiable evidence of its earliest volcanic activity, Mt. Elgon is estimated to be at least 24 million years old, making it the oldest extinct volcano in East Africa. This presents a striking comparison to Mt. Kilimanjaro, the highest mountain in Africa at 5895m, which is just over one million years old. Judging by the diameter of its base, it is a common belief among geological experts that Mt. Elgon was once the highest mountain in Africa, however erosion has played a significant role in reducing the height to its present elevation of 4321m. Mt. Elgon is now the eighth highest massif on the African continent!
During the Miocene Epoch (12-20 million years ago) Mt. Elgon’s volcanic activity was probably violent. In fact, the bulk of Mt. Elgon was formed from the build-up of debris (older rock) ejected from the enlarged main vent. In contrast, lava flows began at a relatively late stage of its development. Elgon disgorged mainly fluid basic lavas, creating a low convex profile (4° slope) known as a shield volcano. Given that the overlying lava is harder and more resistant to erosion than the underlying debris, Elgon has weathered in an uneven fashion, resulting in a number of caves and the characteristic step-like cliffs and bluffs well known to Elgon’s lower slopes.
The last major eruption occurred in the early Pleistocene Epoch (12 million years ago). Only minor eruptions occurred since, the most recent being about 2 million years ago. When the main vent became blocked, horizontal fissures were created from which lava spilled out. Wanale Ridge, the 20km long escarpment of “peninsula” that extends west from the main mountain towards Mbale town , is the result of this process.
The caldera (crater) is one of Mt. Elgon’s most spectacular geological features. Following intense eruptions, the magma chamber drained and the hardened, over-lying volcanic cone collapsed, forming the caldera. Approximately 8km in diameter, Mt. Elgon’s caldera is one of the largest intact calderas in the world. Weathered hills and peaks several hundred meters above the caldera floor form a wide ring around the caldera. The highest peaks in Uganda are: Wagagai (4321m), Mubiyi (4211m), and Jackson’s Summit (4161m), while Sudek (4302m), and Koitobos (4222m) Peaks are located in Kenya.
During the Pleistocene Epoch (1.5 million years ago), glaciers filled the caldera and extended down (on the outer slopes) to about 3500m. The glaciers left behind evidence of their presence. Small lakes (mires) were carved out of the ground’s surface and the accumulation of glacial debris (moraines) can be observed. As global temperatures increased at the end of the Pleistocene Epoch, the glaciers began to melt. As a result, the powerful run-off water eroded and cut through the caldera rim in two places. The main break in the rim is now known as the Suam Gorge.