Ol Doinyo Lengai is an active stratovolcano in northern Tanzania. It consists of a volcanic cone with two craters, the northern of which has erupted during historical time. Uniquely for volcanoes on Earth, it has erupted natrocarbonatite, an unusually low temperature and highly fluid type of magma. Eruptions in 2007–2008 affected the surrounding region.
Name The Maasai and Sonjo people refer to the volcano as "The Mountain of God", associated with a myth of the abode of the god Engai, who withdrew there after being hit with an arrow by a hunter. Other names are Basanjo, Donjo Ngai, Duenjo Ngai, Mongogogura, Mungogo wa Bogwe, and Oldonyo L'Engai.
Geography and geomorphology Ol Doinyo Lengai lies in the Arusha Region of Tanzania, 16 kilometres (9.9 mi) south of Lake Natron and 120 kilometres (75 mi) northwest of Arusha. The summit was first explored between 1904 and 1915. As of 2012, about 300,000 people live in the region, and livestock farming is the most important economic activity, although tourism is increasingly important. Ol Doinyo Lengai is a symmetric cone that rises more than 1,800 metres (5,900 ft) above the surrounding rift valley. It has two craters on either side of the summit, which is formed by a 110-metre (360-foot) high ridge. The floor of the northern crater is covered with lava flows that resemble pahoehoe lavas. Small cones with sizes ranging from 2 metres (6 ft 7 in) to over 10 metres (33 ft) occur in the crater and produce lava flows from their summits and, when they collapse, from their flanks. The southern crater is inactive and sometimes filled with water. White volcanic ash deposits cover the slopes of the volcano, which have large fractures on the western flank. There are parasitic vents on the flanks, such as Kirurum Crater on the western, the Nasira cones on the northern, Dorobo crater on the northeastern, and Oltatwa Crater on the eastern flank. There are deposits of past debris avalanches around the volcano, especially on its northern flank; one such event has left a scar on the volcano's flanks. Their occurrence may have been influenced by regional fault systems.
Geology Ol Doinyo Lengai is part of the Gregory Rift, which is part of the active East African Rift—a continental rift extending from eastern to southern Africa over a length of 4,000 kilometres (2,500 mi), where there is high heat flow through a thinner crust. The Gregory Rift began spreading began about 1.2 million years ago and is ongoing at a rate of about 3 millimetres per year (0.12 in/year). The Natron Fault, the western boundary of the Gregory Rift, passes just southwest of the volcano. The volcano is part of the Ngorongoro volcanic highland, a system of volcanoes that were active from the Miocene to present and which includes the Ngorongoro and other volcanoes. Over time, volcanic activity shifted northeastward to the present-day Ol Doinyo Lengai. Other volcanoes in the area are Gelai to the northeast and Ketumbeine southeast of Ol Doinyo Lengai; further away are the Olduvai Gorge to the west and Kilimanjaro to the east.
Composition Most of the volcanic cone is formed by melilite, nephelinite, and phonolite. Ol Doinyo Lengai is the only volcano on Earth known to have erupted carbonatitic lavas during historical times, although these rocks make up only a small fraction of the volcano and only occur in the northern crater; they only recently appeared on the volcano. The properties of Ol Doinyo Lengai's magmas have been used as an analogue for the conditions on carbon planets; these are planets which are rich in carbon.
The carbonatites contain a groundmass of fluorite and sylvite, while apatite, galena, magnetite, monticellite, sellaite, and sphalerite form accessory components. The silicic lavas contain combeite, ijolites, melanite, nepheline, phlogopite, and pyroxene, as well as apatite, garnet, sphene, and wollastonite. Xenoliths from the basement have been found and consist of gneiss and other metamorphic rocks, as well as ijolites, pyroxenites, and urtites. The carbonatite lavas are rapidly chemically modified by rainfall or covered by deposits condensing from fumarolic gases, yielding secondary minerals like calcite, gaylussite, nahcolite, pirssonite, shortite, thermonatrite, and trona, including various chlorides, fluorides, and sulfates. These rocks form crusts on the lava flows and within lava tubes. Weathering on the silicic rocks has yielded zeoliths. The chemical composition of the erupted rocks is not steady, with an increase of silicic magma emplacement noted after 2007–2008, after an episode of increased spreading in the Gregory Rift. The carbonatitic magmas appear to form through the separation of carbon-rich phases; the original magma is variously interpreted to be either nephelinitic or silicic. The phonolites appear to have a separate origin from the other volcanic rocks. There appear to be two magma reservoirs under the volcano, and its plumbing system is complex, involving regional tectonic structures.
Volcanic gases Volcanic gas sampled at Ol Doinyo Lengai consists mostly of water vapour and carbon dioxide and originates in the mantle. The volcano is a major source of volcanic carbon dioxide, producing about 80 kilograms per second (11,000 lb/min) of CO2.
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