Kilimanjaro geology2017-12-05T07:00:35+00:00
  • Detail of HH Johnston's map of Kilimanjaro

 

Kilimanjaro geology

Rising 4800m above the East African plains, 270km from the shores of the Indian Ocean and measuring up to 40km across,Kilimanjaro is a bizarre geological oddity, the tallest freestanding mountain in the world and one formed, shaped, eroded and scarred by the twin forces of fire and ice. It is actually a volcano, or rather three volcanoes, with the two main peaks, Kibo and Mawenzi, the summits of two of those volcanoes. The story of its creation goes like this:

The Rift Valley

About three-quarters of a million years ago (making Kilimanjaro a veritable youngster in geological terms) molten lava burst through the fractured surface of the Great Rift Valley, a giant fault in the earth’s crust that runs through East Africa (actually, Kilimanjaro lies 50 miles from the East African Rift Valley along a splinter running off it, but that need not concern us here). The huge pressures behind this eruption pushed part of the Earth’s crust skywards, creating the Shira volcano, the oldest of the volcanoes forming the Kilimanjaro massif. Shira eventually ceased erupting around 500,000 years ago, collapsing as it did so to form a huge caldera (the deep cauldron-like cavity on the summit of a volcano) many times the size of its original crater.

The formation of Kibo and Mawenzi

The twin peaks of Kilimanjaro from a hotel terrace in Moshi

The twin peaks of Kilimanjaro from a hotel terrace in Moshi

Soon after Shira’s extinction, Mawenzi started to form following a further eruption within the Shira caldera. Though much eroded, Mawenzi has at least kept some of its volcanic shape to this day. Then, 460,000 years ago, an enormous eruption just west of Mawenzi caused the formation of Kibo. Continual subterranean pressure forced Kibo to erupt several times more, forcing the summit ever higher until reaching a maximum height of about 5900m. A further huge eruption from Kibo 100,000 years later led to the formation of Kilimanjaro’s characteristic shiny black stone – which in reality is just solidified black lava, or obsidian. This spilled over from Kibo’s crater into the Shira caldera and around to the base of the Mawenzi peak, forming the so-called Saddle. Later eruptions created a series of distinctive mini-cones, or parasitic craters, that run in a chain south-east and north-west across the mountain, as well as the smaller Reusch Crater inside the main Kibo summit. The last volcanic activity of note, just over 200 years ago, left a symmetrical inverted cone of ash in the Reusch Crater, known as the Ash Pit, that can still be seen today.

Kilimanjaro’s glaciers

Today, Uhuru Peak, the highest part of Kibo’s crater rim and the goal of most trekkers, stands at around 5895m. The fact that the summit is around five metres shorter today than it was 450,000 years ago can be ascribed in part to some improved technology, which has enabled scientists to measure the mountain more accurately; and in part to the simple progress of time and the insidious glacial erosion down the millennia. These glaciers, advancing and retreating across the summit, created a series of concentric rings like terraces near the top of this volcanic massif on the western side.

The Furtwangler Glacier on the Kibo summit

The Furtwangler Glacier on the Kibo summit

The Kibo peak has also subsided slightly over time, and about one hundred thousand years ago a landslide took away part of the external crater, creating Kibo Barranco or the Barranco Valley (home to one of the mountain’s more popular campsites).

The glaciers were also behind the formation of the valleys and canyons, eroding and smoothing the earth into gentle undulations all around the mountain, though less so on the northern side where the glaciers on the whole failed to reach, leaving the valleys sharper and more defined.

You can find out more about the mountain’s glaciers by visiting the Kilimanjaro glaciers page.

Is Kilimanjaro now an extinct volcano?

While eruptions are unheard of in recent times, Kibo is classified as being dormant rather than extinct, as anybody who visits the inner Reusch Crater can testify. A strong sulphur smell still rises from the crater, the earth is hot to touch, preventing ice from forming, while occasionally fumaroles escape from the Ash Pit that lies at its heart.

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