Mount Kilimanjaro was born of the catastrophic movements in the earth's crust that created the Great Rift Valley that runs from the Red Sea through Tanzania to Southern Africa. The rift valley is an example of a constructive margin, where new crust is exposed as two continental plates pull away from each other. Around 25 million years ago East Africa was a huge flat plain that buckled and ruptured after the African and Eurasian continental plates rebounded off each other causing huge rifting and weak spots in the thinning crust that led to the formation of many volcanoes in the region. Where the original valley was deepest, the volcanic activity was greatest, eventually forming the huge volcanoes of Ngorongoro on the rift itself and a string of volcanoes to the east including Kenya, Meru and Kilimanjaro.
The rift valley is still active today and Kilimanjaro is the result of comparatively recent volcanic activity. Starting around 750,000 years ago, the mountain originally consisted of three large vents, Shira, Kibo and Mawenzi, which came together as they grew in altitude. Eventually, the Shira cone collapsed and became extinct, followed by Mawenzi. The Kibo cone, however, remained active and about 360,000 years ago endured a massive eruption that released a flow of black lava to completely cover the old Shira caldera to the west and created the saddle between Kibo and Mawenzi to the west. Kibo eventually levelled out at its present altitude and became dormant. Since then it has been periodically been covered with ice sheets and glaciers. Around 100,000 years ago a huge landslide on the southwest face created the Barranco Wall, a major feature of some climb routes and Kibo's last eruption formed the caldera, containing the Inner Crater and Ash Pit.
The summit of Kilimanjaro was previously completely covered by an ice cap more than 100m deep with glaciers ranging well down the mountain to below 4000m. At present only a small fraction of that glacial cover remains, the most visible and impressive sections being those around the spectacular northern and south-eastern ice fields.
These next few paragraphs are an Extract from The Ice Cap Retreat by Jonathan Amos in San Francisco. The beautiful icefields on the top of Mount Kilimanjaro in East Africa could completely melt away in the next 20 years if the earth continues to warm at the rate many scientists now claim. The calculation comes from Professor Lonnie Thompson of Ohio State University, who has made an aerial survey of the famous Tanzanian peak. He said comparisons with the previous mapping showed 33% of Mt Kilimanjaro's ice had disappeared in the last two decades, 82% had gone since 1912. Studies on other tropical peaks revealed a similar picture, he told the annual meeting of the American Association for the Advancement of Science.
He warned this melting could have serious repercussions for drinking water supply, crop irrigation, hydroelectric production and tourism. He said, "retreating glaciers is just one of many symptoms of the dramatic changes in climate that we are likely to experience within our lifetime." However, evidence of a retreat of this ice sheet was first observed by Hans Meyer, the first Westerner to make the summit, who reported in 1898 that the ice limit had withdrawn by over 100m since his first ascent 8 years earlier. This rapid change is therefore perhaps not entirely due to recent global warming, but rather a result of a longer-term cycle of climatic events. Studies by Sheffield University during the 1950s reported that Kilimanjaro has had a long history of glacial advance and retreat coinciding with a sequence of eight glaciations. The present ice cap is probably the result of the worldwide drop in temperature experienced between 1400AD and 1700AD and suggests that there have been several long periods when Kilimanjaro was devoid of ice. The current retreat is the result of a general increase in the temperature of the earth over many hundreds of years.