Mt. Kenya is a volcano, about 3 million years old, whose base diameter is about 120km. The original cone was possibly over 6000m. high but erosion, mainly glacial, has worn away the original upper part of this and left in its place a jagged glacial topography of knife-edge ridges, pyramidal peaks, U-shaped valleys and rock basins containing glacial lakes known as tarns. Many streams in steep-sided valleys form a radial drainage pattern covering the lower slopes.
The main peaks, Batian and Nelion, are part of the 'plug'. They are composed of nepheline-syenite which is a coarse-grained, intrusive igneous rock. The rocks of the lower slopes are of various different lavas and agglomerates. Eruptions from satellite vents produced such features as Ithanguni, Mugi Hill and the Giants Billiard Table.
During the period when global climates were significantly cooler than they are today and large ice sheets covered much of northern Europe and Canada, the glaciers on Mt. Kenya extended down to about 3000m. The erosive work of these glaciers was responsible for cutting the deep U-shaped valleys around the slopes of the cone. Within the valleys are found terminal moraines; these curved ridges of boulders and gravel represent the load of eroded material carried within and under the glacier. Moraines provide evidence as to the former extent of the glaciers, of the different episodes of glaciation, and of the various stages in the retreat of the glaciers. At present the glaciers are retreating at an accelerating rate; the ice is also becoming thinner. It has been estimated that if the present trend continues, in 25 years there may well be no permanent ice left on the mountain. Since records were first made in 1893, seven of the eighteen glaciers then recorded have disappeared.