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Kilimanjaro is volcanic in origin. It is part of, and came about as a result of the forces that created, the Great Rift Valley. This rift in the earth's crust created by two tectonic plates pulling apart in the same way that North America tore itself away from Europe to form the Atlantic Ocean begins in Syria, stretches across the Red Sea and down Africa's eastern flank towards Kenya, where it splits in two to form the Central African Rift Valley and the East Africa Rift Valley. About a million years ago as part of the continuing geological activity Kilimanjaro was born when the tension within the earth's crust, and the thinning associated with it, caused the eruption of three volcanoes.
These were Shira, Mawenzi and Kibo, which of course now form what we know as Mount Kilimanjaro. They were all about 5,000m high but about half a million years ago Shira collapsed. The other two remained active, continuing to erupt lava, which eventually filled much of the caldera left by Shira's collapse and created what we see today as the Shira Plateau. The other two peaks continued to grow and the distance between them as distinct volcanoes diminished as the lava flows bridged the gap to form what we today see as the Kibo Saddle. At some point Mawenzi became extinct and given the constituency of its rock has been eroded into the bizarre shape we now see. Kibo continued to erupt and grow until about 100,000 years ago when it all but died apart from sending one last message of the activity below the surface when a massive chunk of mountain, nearly 1,000m high, broke off and careered downwards to form what we see today as the Barranco Valley. Today Kibo has three concentric craters. The outer rim, about 2.5km in diameter, with Gillman's and Stella Points, also contains the highest point on the mountain at Uhuru Peak. The middle crater is the Reusch crater and exhibits a number of active fumaroles. At its centre is the 400m wide and 130m deep Ash Pit.
The outer crater rim has been breached by many lava flows and the most dramatic of these is the Western Breach, known for being one of the more complex routes on the mountain.
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