Ol Doinyo Lengai

Ol Doinyo Lengai is an active volcano 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 by a hunter with an arrow. 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 km south of Lake Natron and 120 km northwest of the city of Arusha. The summit was first explored between 1904 and 1915. , 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 1800 m above the surrounding rift valley. It has two craters on either side of the mountain summit, which is formed by a 110 m high ridge. The floor of the northern crater is covered with lava flows that resemble pahoehoe lavas. Small cones with sizes ranging from 2 m to over 10 m 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 Ol Doinyo Lengai's 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. The East African Rift is a continental rift extending from eastern to southern Africa over a length of 4000 km, where there is high heat flow through a thinner crust. In the Gregory Rift, spreading began about 1.2 million years ago and is ongoing at a rate of about 3 mm/year. The Natron Fault, the western boundary of the Gregory Rift in the area, 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 mountain east of the volcano.

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.

Chemical composition:
 * 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 vapor and carbon dioxide and originates in the mantle. The volcano is a major source of volcanic carbon dioxide, producing about 80 kg/s of.

Eruption history
Radiometric dates obtained by geologists for the start of volcanic eruptions at Ol Doinyo Lengai range from more than 500,000 to 22,000 years ago. It formed in two stages, Lengai I consisting of phonolite that forms about 60% of the volume of Ol Doinyo Lengai and crops out in its southern part, and Lengai II formed by nephelinitic rocks; growth of the volcanic cone was complete about 15,000 years ago, when the Naisiusiu Beds were emplaced in the Olduvai Gorge. The volcano collapsed several times, including once between 850,000 and 135,000 years ago and another time between 50,000 and 10,000 years ago. The oldest natrocarbonatite lavas date to 1,250 years before present. An eruption 3,000-2,500 years before present produced a tephra fallout west of Ol Doinyo Lengai, that is presently being eroded by wind and forming dunes including the Shifting Sands of the Olduvai Gorge. A large eruption deposited the Namorod Ash in the gorge, about 1,250 years ago, and another about 600 years ago formed the so-called "Footprint Tuff". Ol Doinyo Lengai is the only presently active volcano of the Gregory Rift.

Records of eruptions go back to the 1880s. The volcano is continually active, but there are seldom observations of its activity. It erupts tephra and lava flows from within the northern crater. During the middle 20th century, the crater was about 200 m deep; subsequently, lava flows filled it, and by 1998, lava was overflowing its rims. The lava flows issue from cones within the crater and form lava ponds and lakes. Explosive eruptions are less common, having been reported in 1917, 1940, 1966, 1983 and 1993. Oversteepened slopes produce landslides, and erosion has cut gullies into volcanic deposits. Steam jets have also been observed.

There is evidence of underground magma intrusions. Satellite observations have shown deformation of the volcano during eruptions, and ground-based observations have identified movement in neighboring fault systems such as the Natron Fault caused by magma originating at Ol Doinyo Lengai.

Recent eruptive period: 1983 and subsequent
After a phase of quiescence, renewed activity commenced in 1983 and continues with several interruptions to this day. During the 1983 eruption, ashfall occurred at tens of kilometers from the volcano. The emission of a lava flow onto the western flank of Ol Doinyo Lengai in 2006 was accompanied by the formation of a pit crater on the summit.

A large explosive eruption began on the 4 September 2007, producing a 3 km-high eruption column and a new crater 100 m deep and 300 m wide. The explosive activity continued into 2008, when the volcano settled back into the effusion of lava flows; a cinder cone formed in the northern crater during the eruption. Aerosol clouds from the eruption extended over east Africa. The 2007 eruptions forced the evacuation of three villages and disturbed air travel in the touristically important area; livestock fatalities and injuries to people led to requests that the government of Tanzania enact access restrictions to the volcano and to increased awareness of the threat formed by the volcano. Wild animals such as flamingos were also impacted by the eruption. The eruption was preceded in July by seismic activity, which was frequently mistaken for renewed eruptions, and the intrusion of a dyke less than 20 km from Ol Doinyo Lengai.

General appearance of lava flows
Lavas erupted by Ol Doinyo Lengai initially have brown or black colors, but within days to hours become white like snow. The lavas of Ol Doinyo Lengai have temperatures of 540 - 593 C; they are so cold that during the day they look like mudflows or oil and glow only during the night. They are highly fluid (reaching flow speeds of 1 - 5 m/s, making them the most liquid known lavas) and form short (few tens of meters) and thin (few centimeters thick) lava flows. More viscous flows containing silicic rocks have also been observed, for example during the 1993 eruption.

Hazards
Potential threats from Ol Doinyo Lengai eruptions are scarcely established. Threats from eruptions at Ol Doinyo Lengai include lahars, landslides, lava flows, pyroclastic flows, volcanic bombs, volcanic gas, and volcanic ash fall. Beginning in 2016, the volcano is being monitored by a seismometer and GNSS stations.

Climate and vegetation
Vegetation in the area consists mostly of grassland, which reaches an elevation of 1750 m above sea level. Volcanic ash from Ol Doinyo Lengai influences the surrounding landscape, favoring the growth of nutrient-rich plants. Precipitation falls during two wet seasons in March–May and October–December.