Mount Kenya is a stratovolcano that was active in the Plio-Pleistocene. The original crater was probably over 6,000 metres (19,700 ft) high; higher than Kilimanjaro. Since it became extinct there have been two major periods of glaciation, which are shown by two main rings of moraines below the glaciers. The lowest moraine is found at around 3,300 metres (10,800 ft). Today the glaciers reach no lower than 4,650 metres (15,260 ft). After studying the moraines, Gregory put forward the theory that at one time the whole summit of the mountain was covered with an ice cap, and it was this that eroded the peaks to how they are today.
The lower slopes of the mountain have never been glaciated. They are now mainly cultivated and forested. They are distinguished by steep-sided V-shaped valleys with many tributaries. Higher up the mountain, in the area that is now moorland, the valleys become U-shaped and shallower with flatter bottoms. These were created by glaciation.
When Mount Kenya was active there was some satellite activity. The north-eastern side of the mountain has many old volcanic plugs and craters. The largest of these, Ithanguni, even had its own ice cap when the main peaks were covered in ice. This can be seen by the smoothed summit of the peak. Circular hills with steep sides are also frequent in this area, which are probably the remains of small plugged vents. However, as the remaining mountain is roughly symmetrical, most of the activity must have occurred at the central plug.
The rocks that form Mount Kenya are mainly basalts, rhomb porphyrites, phonolites, kenytes and trachytes. Kenyte was first reported by Gregory in 1900 following his study of the geology of Mount Kenya.
The geology of the Mount Kenya area was first proposed to the Western Community by Joseph Thomson in 1883. He saw the mountain from the nearby Laikipia Plateau and wrote that it was an extinct volcano with the plug exposed. However, as he had only seen the mountain from a distance his description was not widely believed in Europe, particularly after 1887 when Teleki and von Höhnel ascended the mountain and described what they considered to be the crater. In 1893 Gregory's expedition reached the Lewis Glacier at 5,000 metres (16,400 ft). He confirmed that the volcano was extinct and that there were glaciers present. The first thorough survey by Europeans was not undertaken until 1966.