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Geology and geography
Kilimanjaro is a large dormant stratovolcano composed of three distinct volcanic cones: Kibo, the highest; Mawenzi at 5149 m; and Shira, the lowest at 4005 m. Mawenzi and Shira are extinct, while Kibo is dormant and could erupt again.

Uhuru Peak is the highest summit on Kibo's crater rim. The Tanzania National Parks Authority, a Tanzanian government agency, and the United Nations Educational, Scientific and Cultural Organization lists the height of Uhuru Peak as 5895 m, based on a British survey in 1952. The height has since been measured as 5,892 m in 1999, 5902 m in 2008, and 5,899 m in 2014.

A map of the Kibo cone on Mount Kilimanjaro was published by the British government's Directorate of Overseas Surveys (DOS) in 1964 based on aerial photography conducted in 1962 as the "Subset of Kilimanjaro, East Africa (Tanganyika) Series Y742, Sheet 56/2, D.O.S. 422 1964, Edition 1, Scale 1:50,000". Tourist mapping was first published by the Ordnance Survey in England in 1989 based on the original DOS mapping at a scale of 1:100,000, with 100 ft contour intervals, as DOS 522. West Col Productions produced a map with tourist information in 1990, at a scale of 1:75,000, with 100 m contour intervals; it included inset maps of Kibo and Mawenzi on 1:20,000 and 1:30,000 scales respectively and with 50 m contour intervals. In recent years, numerous other maps have become available, of various qualities.

Volcanology
The volcanic interior of Kilimanjaro is poorly known because there has not been any significant erosion to expose the igneous strata that comprise the volcano's structure.

Eruptive activity at the Shira center commenced about 2.5 million years ago, with the last important phase occurring about 1.9 million years ago, just before the northern part of the edifice collapsed. Shira is topped by a broad plateau at 3800 m, which may be a filled caldera. The remnant caldera rim has been degraded deeply by erosion. Before the caldera formed and erosion began, Shira might have been between 4,900 and 5,200 m high. It is mostly composed of basaltic lavas, with some pyroclastics. The formation of the caldera was accompanied by lava emanating from ring fractures, but there was no large-scale explosive activity. Two cones formed subsequently, the phonolitic one at the northwest end of the ridge and the doleritic Platzkegel in the caldera center.

Both Mawenzi and Kibo began erupting about 1 million years ago. They are separated by the Saddle Plateau at 4400 m elevation.

The youngest dated rocks at Mawenzi are about 448,000 years old. Mawenzi forms a horseshoe-shaped ridge with pinnacles and ridges opening to the northeast, with a tower-like shape resulting from deep erosion and a mafic dike swarm. Several large cirques cut into the ring. The largest of these sits on top of the Great Barranco gorge. Also notable are the East and West Barrancos on the northeastern side of the mountain. Most of the eastern side of the mountain has been removed by erosion. Mawenzi has a subsidiary peak, Neumann Tower, 4425 m.  Kibo is the largest cone on the mountain and is more than 24 km wide at the Saddle Plateau altitude. The last activity here, dated to 150,000–200,000 years ago, created the current Kibo summit crater. Kibo still has gas-emitting fumaroles in its crater. Kibo is capped by an almost symmetrical cone with escarpments rising 180 to 200 m on the south side. These escarpments define a 2.5 km caldera caused by the collapse of the summit.

Within this caldera is the Inner Cone and within the crater of the Inner Cone is the Reusch Crater, which the Tanganyika government in 1954 named after Gustav Otto Richard Reusch, upon his climbing the mountain for the 25th time (out of 65 attempts during his lifetime). The Ash Pit, 350 m deep, lies within the Reusch Crater. About 100,000 years ago, part of Kibo's crater rim collapsed, creating the area known as the Western Breach and the Great Barranco.

An almost continuous layer of lava buries most older geological features, except exposed strata within the Great West Notch and the Kibo Barranco. The former exposes intrusions of syenite. Kibo has five main lava formations:


 * Phonotephrites and tephriphonolites of the Lava Tower group, on a dyke cropping out at 4600 m, dated to 482,000 years ago.
 * Tephriphonolite to phonolite lavas "characterized by rhomb mega-phenocrysts of sodic feldspars" of the Rhomb Porphyry group, dated to 460,000–360,000 years ago.
 * Aphyric phonolite lavas, "commonly underlain by basal obsidian horizons", of the Lent group, dated to 359,000–337,000 years ago
 * Porphyritic tephriphonolite to phonolite lavas of the Caldera Rim group, dated to 274,000–170,000 years ago
 * Phonolite lava flows with aegirine phenocrysts, of the Inner Crater group, which represents the last volcanic activity on Kibo

Kibo has more than 250 parasitic cones on its northwest and southeast flanks that were formed between 150,000 and 200,000 years ago and erupted picrobasalts, trachybasalts, ankaramites, and basanites. They reach as far as Lake Chala and Taveta in the southeast and the Lengurumani Plain in the northwest. Most of these cones are well preserved, except the Saddle Plateau cones which were heavily affected by glacial action. Despite their mostly small size, lava from the cones has obscured large portions of the mountain. The Saddle Plateau cones are mostly cinder cones with terminal effusion of lava, while the Upper Rombo Zone cones mostly generated lava flows. All Saddle Plateau cones predate the last glaciation.

According to reports gathered in the 19th century from the Maasai, Lake Chala on Kibo's eastern flank was the site of a village that was destroyed by an eruption.

Glaciers
Kibo's diminishing ice cap exists because Kilimanjaro is a little-dissected, massive mountain that rises above the snow line. The cap is divergent and at the edges splits into individual glaciers. The central portion of the ice cap is interrupted by the presence of the Kibo crater. The summit glaciers and ice fields do not display significant horizontal movements because their low thickness precludes major deformation.

Geological evidence shows five successive glacial episodes during the Quaternary period, namely First (500,000 BP), Second (greater than 360,000 years ago to 240,000 BP), Third (150,000 to 120,000 BP), Fourth (also known as "Main") (20,000 to 17,000 BP), and Little (16,000 to 14,000 BP). The Third may have been the most extensive, and the Little appears to be statistically indistinguishable from the Fourth.

A continuous ice cap covering approximately 400 km2 down to an elevation of 3200 m covered Kilimanjaro during the Last Glacial Maximum in the Pleistocene epoch (the Main glacial episode), extending across the summits of Kibo and Mawenzi. Because of the exceptionally prolonged dry conditions during the subsequent Younger Dryas stadial, the ice fields on Kilimanjaro may have become extinct around 11,500 years BP. Ice cores taken from Kilimanjaro's Northern Ice Field (NIF) indicates that the glaciers there have a basal age of about 11,700 years, although an analysis of ice taken in 2011 from exposed vertical cliffs in the NIF supports an age extending only to 800 years BP. Higher precipitation rates at the beginning of the Holocene epoch (11,500 years BP) allowed the ice cap to reform. The glaciers survived a widespread drought during a three century period beginning around 4,000 years BP.

In the late 1880s, the summit of Kibo was completely covered by an ice cap about 20 km2 in extent with outlet glaciers cascading down the western and southern slopes, and except for the inner cone, the entire caldera was buried. Glacier ice also flowed through the Western Breach. The slope glaciers retreated rapidly between 1912 and 1953, in response to a sudden shift in climate at the end of the 19th century that made them "drastically out of equilibrium", and more slowly thereafter. Their continuing demise indicates they are still out of equilibrium in response to a constant change in climate over the past century.

In contrast to the persistent slope glaciers, the glaciers on Kilimanjaro's crater plateau have appeared and disappeared repeatedly during the Holocene epoch, with each cycle lasting a few hundred years. It appears that decreasing specific humidity instead of temperature changes has caused the shrinkage of the slope glaciers since the late 19th century. No clear warming trend at the elevation of those glaciers occurred between 1948 and 2005. Although air temperatures at that elevation are always below freezing, solar radiation causes melting on vertical faces. Vertical ice margin walls are a unique characteristic of the summit glaciers and a major place of the shrinkage of the glaciers. They manifest stratifications, calving, and other ice features. "There is no pathway for the plateau glaciers other than to continuously retreat once their vertical margins are exposed to solar radiation." The Kilimanjaro glaciers have been used for deriving ice core records, including two from the southern icefield. Based on this data, this icefield formed between 1,250 and 1,450 years BP.

Almost 85 percent of the ice cover on Kilimanjaro disappeared between October 1912 and June 2011, with coverage decreasing from 11.40 km2 to 1.76 km2. Between 1912 and 1953, there was about a 1.1 percent average annual loss of ice coverage. The average annual loss for 1953 to 1989 was 1.4 percent, while the loss rate for 1989 to 2007 was 2.5 percent. Of the ice cover still present in 2000, almost 40 percent had disappeared by 2011. Ice climber Will Gadd noticed differences between his 2014 and 2020 climbs. The glaciers are thinning in addition to losing areal coverage, and do not have active accumulation zones; retreat occurs on all glacier surfaces. Loss of glacier mass is caused by both melting and sublimation. While the current shrinking and thinning of Kilimanjaro's ice fields appear to be unique within its almost twelve-millennium history, it is contemporaneous with widespread glacier retreat in mid-to-low latitudes across the globe. In 2013, it was estimated that, at the current rate of global warming, most of the ice on Kilimanjaro will disappear by 2040, and "it is highly unlikely that any ice body will remain after 2060".

The Furtwangler Glacier on Kilimanjaro is a remnant of the ice cap that once covered the mountain. This has retreated dramatically over the last century with over 80 percent glacial retreat. The glacier is named after Walter Furtwangler, who along with Ziegfried Koenig, was the fourth to ascend to the summit of Kilimanjaro in 1912.

A complete disappearance of the ice would be of only "negligible importance" to the water budget of the area around the mountain. The forests of Kilimanjaro, far below the ice fields, "are [the] essential water reservoirs for the local and regional populations".

Drainage
Kilimanjaro is drained by a network of rivers and streams, especially on the wetter and more heavily eroded southern side and especially above 1200 m. Below that altitude, increased evaporation and human water usage reduce the water flows. The Lumi and Pangani rivers drain Kilimanjaro on the eastern and southern sides, respectively.

IUGS geological heritage site
In respect of it being 'the highest stratovolcano of the East African Rift that maintains a glacier on its summit', the International Union of Geological Sciences (IUGS) included 'The Pleistocene Kilimanjaro volcano' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organization defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'