Mount Kilimanjaro (Its Geology)- Africa Natural Tours ( africanaturaltours.com )

Mount Kilimanjaro (Its Geology): Africa Natural Tours

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Geology of Mount Kilimanjaro

The interior of the volcanic edifice is poorly known, given the lack of large scale erosion that could have exposed the interiors of the volcano. Eruptive activity at the Shira centre 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 3,800 metres (12,500 ft), 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 m (16,000 ft) and 5,200 m (17,000 ft) high. It is mostly composed of basic 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 centre. Both Mawenzi and Kibo began erupting about 1 million years ago. They are separated by the "Saddle Plateau" at 4,400 metres (14,400 ft) 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 which has a tower like shape resulting from deep erosion and a mafic dyke swarm. Several large cirques cut into the ring, the largest of these sits on top of the Great Barranco gorge. Also notable are the Ost 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 named Neumann Tower (4,425 metres (14,518 ft)).

Kibo is the largest cone and is more than 15 miles (24 km) wide at the "Saddle Plateau" altitude. The last activity here has been dated to between 150,000 and 200,000 years ago and created the current Kibo summit crater. Kibo still has gas-emitting fumaroles in the crater. Kibo is capped by an almost symmetrical cone with escarpments rising 180 metres (590 ft) to 200 metres (660 ft) on the south side. These escarpments define a 2.5-kilometre-wide (1.6 mi) caldera^[15] 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 25^th time (out of 65 attempts during his lifetime). The Ash Pit, 350 metres (1,150 ft) 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 lavas buries older geological features, with the exception of exposed strata within the Great West Notch and the Kibo Barranco. Kibo has five main lava formations: Phonotephrites and tephriphonolites of the "Lava Tower group", on a dyke cropping out at 4,600 metres (15,100 ft), 482,000 years ago, Tephriphonolite to phonolite lavas "characterized by rhomb mega-phenocrysts of sodic feldspars" of the "Rhomb Porphyry group", 460,000–360,000 years ago, aphyric phonolite lavas, "commonly underlain by basal obsidian horizons", of the "Lent group", 359,000–337,000 years ago, porphyritic tephriphonolite to phonolite lavas of the "Caldera rim group", 274,000–170,000 years ago and 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, with the exception of the Saddle Plateau cones that 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.

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