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Ol Doinyo Lengai

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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,[2] an unusually low temperature and highly fluid type of magma. Eruptions in 2007–2008 affected the surrounding region.

Ol Doinyo Lengai
Oldoinyo Lengai
Highest point
Elevation2,962 m (9,718 ft)[1]
Prominence1,360 m (4,460 ft) Edit this on Wikidata
Isolation16.68 km (10.36 mi) Edit this on Wikidata
Coordinates2°45′50″S 35°54′50″E / 2.764°S 35.914°E / -2.764; 35.914[1]
Geography
Ol Doinyo Lengai is located in Tanzania
Ol Doinyo Lengai
Ol Doinyo Lengai
Parent rangeEast African Rift
Geology
Mountain typeStratovolcano
Last eruption2024 AD

Name

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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.[3] Other names are Basanjo, Donjo Ngai, Duenjo Ngai, Mongogogura, Mungogo wa Bogwe, and Oldonyo L'Engai.[4]

Geography and geomorphology

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Ol Doinyo Lengai lies in the Arusha region of Tanzania,[5] 16 kilometres (9.9 mi) south of Lake Natron[6] and 120 kilometres (75 mi) northwest of the city of Arusha.[7] The summit was first explored between 1904 and 1915.[8] 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.[9]

Ol Doinyo Lengai is a symmetric cone[1] that rises more than 1,800 metres (5,900 ft) above the surrounding rift valley.[10] It has two craters on either side of the mountain summit,[11] which is formed by a 110-metre (360-foot) high ridge.[12] The floor of the northern crater is covered with lava flows that resemble pahoehoe lavas. Small cones[a] 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.[5] The southern crater is inactive and sometimes filled with water.[14] White volcanic ash deposits cover the slopes of the volcano,[11] which have large fractures on the western flank.[9] There are parasitic vents on Ol Doinyo Lengai's flanks,[15] 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.[16]

There are deposits of past debris avalanches around the volcano, especially on its northern flank;[17] one such event has left a scar on the volcano's flanks.[18] Their occurrence may have been influenced by regional fault systems.[19]

Geology

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Ol Doinyo Lengai is part of the Gregory Rift,[1] 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 4,000 kilometres (2,500 mi),[20] where there is high heat flow through a thinner crust.[21] In the Gregory Rift, spreading began about 1.2 million years ago[20] and is ongoing at a rate of about 3 millimetres per year (0.12 in/year).[22] The Natron Fault, the western boundary of the Gregory Rift in the area, passes just southwest of the volcano.[23]

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.[20] Over time, volcanic activity shifted northeastward to the present-day Ol Doinyo Lengai.[24] Other volcanoes in the area are Gelai to the northeast[b] and Ketumbeine southeast of Ol Doinyo Lengai; further away are the Olduvai Gorge to the west and Kilimanjaro mountain east of the volcano.[10]

Composition

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Most of the volcanic cone is formed by melilite, nephelinite, and phonolite.[c][26] Ol Doinyo Lengai is the only volcano on Earth known to have erupted carbonatitic lavas[d] during historical times,[1] although these rocks make up only a small fraction of the volcano[15] and only occur in the northern crater;[e][27] they only recently appeared on the volcano.[13] 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.[21]

Chemical composition:

The carbonatite lavas are rapidly chemically modified by rainfall[30] or covered by deposits condensing from fumarolic gases,[31] yielding secondary minerals like calcite, gaylussite, nahcolite, pirssonite, shortite, thermonatrite, and trona,[32] including various chlorides, fluorides,[f] and sulfates.[5] These rocks form crusts on the lava flows and within lava tubes.[13] Weathering on the silicic rocks has yielded zeoliths.[34]

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.[35] 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.[21] The phonolites appear to have a separate origin from the other volcanic rocks.[36] There appear to be two magma reservoirs under the volcano,[37] and its plumbing system is complex, involving regional tectonic structures.[38]

Volcanic gases

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Volcanic gas sampled at Ol Doinyo Lengai consists mostly of water vapor and carbon dioxide and originates in the mantle.[39] The volcano is a major source of volcanic carbon dioxide, producing about 80 kilograms per second (11,000 lb/min) of CO
2.[26]

Eruption history

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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.[15][40] 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;[15][41][12] growth of the volcanic cone was complete about 15,000 years ago,[1] when the Naisiusiu Beds were emplaced in the Olduvai Gorge.[42] 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.[17] The oldest natrocarbonatite lavas date to 1,250 years before present.[39] 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.[43] A large eruption deposited the Namorod Ash in the gorge, about 1,250 years ago,[34] and another about 600 years ago formed the so-called "Footprint Tuff".[34] Ol Doinyo Lengai is the only presently active volcano of the Gregory Rift.[10]

Records of eruptions go back to the 1880s.[44][g] The volcano is continually active, but there are seldom observations of its activity.[46] It erupts tephra and lava flows[11] from within the northern crater.[10] During the middle 20th century, the crater was about 200 metres (660 ft) deep; subsequently, lava flows filled it, and by 1998, lava was overflowing its rims.[1] The lava flows issue from cones within the crater and form lava ponds and lakes.[7] Explosive eruptions are less common, having been reported in 1917, 1940, 1966,[h] 1983 and 1993.[39][47] Oversteepened slopes produce landslides,[11] and erosion has cut gullies into volcanic deposits.[48] Steam jets have also been observed.[45]

There is evidence of underground magma intrusions.[22] Satellite observations have shown deformation of the volcano during eruptions,[49] and ground-based observations have identified movement in neighboring fault systems such as the Natron Fault caused by magma originating at Ol Doinyo Lengai.[50]

Recent eruptive period: 1983 and subsequent

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Summit of Ol Doinyo Lengai in February, 2006

After a phase of quiescence,[26] renewed activity commenced in 1983 and continues[11] with several interruptions to this day.[51] During the 1983 eruption, ashfall occurred at tens of kilometers from the volcano.[26] 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.[52]

A large explosive eruption began on the 4 September 2007, producing a 3-kilometre (1.9 mi)-high eruption column[53] and a new crater 100 metres (330 ft) deep and 300 metres (980 ft) wide.[54] The explosive activity continued into 2008, when the volcano settled back into the effusion of lava flows;[53] a cinder cone formed in the northern crater during the eruption.[55] Aerosol clouds from the eruption[56] extended over east Africa.[57] The 2007 eruptions forced the evacuation of three villages[58] and disturbed air travel in the touristically important area;[59] livestock fatalities and injuries to people led to requests that the government of Tanzania enact access restrictions to the volcano[60] and to increased awareness of the threat formed by the volcano.[61] Wild animals such as flamingos were also impacted by the eruption.[59] The eruption was preceded in July by seismic activity, which was frequently mistaken for renewed eruptions,[62] and the intrusion of a dyke less than 20 kilometres (12 mi) from Ol Doinyo Lengai.[38]

General appearance of lava flows

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White surface of solidified lava flows at Ol Doinyo Lengai, August 2001

Lavas erupted by Ol Doinyo Lengai initially have brown or black colors, but within days[45] to hours become white like snow.[11] The lavas of Ol Doinyo Lengai have temperatures of 540–593 °C (1,004–1,099 °F);[5] they are so cold that during the day they look like mudflows[i] or oil and glow only during the night.[7] They are highly fluid (reaching flow speeds of 1–5 metres per second (3.3–16.4 ft/s),[5] making them the most liquid of all known lavas, and form short (few tens of meters) and thin (few centimeters thick) lava flows.[11] More viscous flows containing silicic rocks have also been observed, for example during the 1993 eruption.[64]

Hazards

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Potential threats from Ol Doinyo Lengai eruptions are scarcely established.[65] Threats from eruptions at Ol Doinyo Lengai include lahars, landslides, lava flows, pyroclastic flows, volcanic bombs, volcanic gas, and volcanic ash fall.[66][9] Beginning in 2016, the volcano is being monitored by a seismometer and GNSS stations.[66]

Climate and vegetation

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Vegetation in the area consists mostly of grassland, which reaches an elevation of 1,750 metres (5,740 ft) above sea level.[9] Volcanic ash from Ol Doinyo Lengai influences the surrounding landscape, favoring the growth of nutrient-rich plants.[67] Precipitation falls during two wet seasons in March–May and October–December.[9]

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  •  
    Ol Doinyo Lengai erupting in March 2008
  •  
    Satellite image (2009) of Ol Doinyo Lengai after an explosive eruption
  •  
    Ol Doinyo Lengai in February 2012
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    Aerial photo of Oldoinyo Lengai in January 2011
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    Image of 1966 eruption
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    Crater of Ol Doinyo Lengai in January 2011

See also

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Notes

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  1. ^ Known as hornitos.[13]
  2. ^ The Naibor Soito monogenetic volcanic field lies between Gelai and Ol Doinyo Lengai.[25]
  3. ^ Together they make up more than 90% of the cone.[13]
  4. ^ Carbonatites are magmas that consist of carbonate compounds.[10] At Ol Doinyo Lengai, they are made up of nyererite (Na
    2    Ca(CO
    3    )
    2    ) and gregoryite ((Na
    ,    K
    ,    Ca)
    2    CO
    3    ).[5]
  5. ^ Silicic lavas mostly issued from the southern crater.[13]
  6. ^ The volcanic rocks contain up to several percent chlorine and fluorine by weight.[33]
  7. ^ Eruptions have been recorded in 1880, 1894 (?), 1904, 1913-15, 1917, 1921, 1926, 1940-41, 1954-55, 1958, and 1960.[45]
  8. ^ 1966 saw explosive eruptions in August and October, which formed a deep crater.[11]
  9. ^ And have been confused for mud by non-volcanologists.[63]

References

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  1. ^ a b c d e f g GVP 2023, General Information.
  2. ^ Keller & Krafft 1990, p. 629.
  3. ^ Bernbaum 2022, p. 183.
  4. ^ GVP 2023, Synonyms & Subfeatures.
  5. ^ a b c d e f McFarlane, Lundberg & Belton 2004, p. 98.
  6. ^ Mangler et al. 2014, p. 43.
  7. ^ a b c Muthama, Mathu & Kamau 2012, p. 8.
  8. ^ Zaitsev, Keller & Billström 2009, p. 303.
  9. ^ a b c d e Rey et al. 2021, p. 72.
  10. ^ a b c d e Nyamweru 1988, p. 603.
  11. ^ a b c Sekisova et al. 2015, p. 1719.
  12. ^ a b c d e Gilbert & Williams-Jones 2008, p. 520.
  13. ^ Kervyn et al. 2010, p. 921.
  14. ^ a b c d e Mangler et al. 2014, p. 44.
  15. ^ Klaudius & Keller 2006, p. 174.
  16. ^ a b Delcamp et al. 2015, p. 7.
  17. ^ Delcamp et al. 2015, p. 8.
  18. ^ Delcamp et al. 2015, p. 17.
  19. ^ a b c Mollel & Swisher 2012, p. 274.
  20. ^ a b c Radebaugh, Barnes & Keith 2020, p. 1.
  21. ^ a b Jones et al. 2019, p. 2517.
  22. ^ Jones et al. 2019, p. 2522.
  23. ^ Mollel & Swisher 2012, p. 276.
  24. ^ Ho & Wauthier 2022.
  25. ^ a b c d Oppenheimer 1998, p. 55.
  26. ^ Klaudius & Keller 2006, p. 173.
  27. ^ Oppenheimer 1998, p. 60.
  28. ^ Morogan & Martin 1985, p. 1114.
  29. ^ Robertson et al. 2014.
  30. ^ Gilbert & Williams-Jones 2008, p. 524.
  31. ^ Zaitsev, Keller & Billström 2009, p. 302.
  32. ^ Mangler et al. 2014, p. 51.
  33. ^ a b c Hay 1989, p. 80.
  34. ^ Jones et al. 2019, p. 2518.
  35. ^ Mangler et al. 2014, p. 48.
  36. ^ Daud Masungulwa et al. 2021.
  37. ^ a b Biggs et al. 2021, p. 3.
  38. ^ a b c Fischer et al. 2006.
  39. ^ Mollel & Swisher 2012, p. 278.
  40. ^ Klaudius & Keller 2006, p. 176.
  41. ^ Hay 1989, p. 78.
  42. ^ Makongoro et al. 2022, p. 209.
  43. ^ Meshili & Kwon 2020, p. 401.
  44. ^ a b c Nyamweru 1988, p. 604.
  45. ^ Nyamweru 1990, p. 389.
  46. ^ Kervyn et al. 2010, p. 926.
  47. ^ Nyamweru 1990, p. 387.
  48. ^ GVP 2023, Deformation history.
  49. ^ Jones et al. 2019, p. 2525.
  50. ^ GVP 2023, Eruption history.
  51. ^ Kervyn et al. 2010, p. 915.
  52. ^ a b Kervyn et al. 2010, p. 914.
  53. ^ Laxton 2020, p. 438.
  54. ^ Kervyn et al. 2010, p. 924.
  55. ^ Muthama, Mathu & Kamau 2012, p. 9.
  56. ^ Muthama, Mathu & Kamau 2012, p. 15.
  57. ^ Vye-Brown et al. 2014, p. 4.
  58. ^ a b Vye-Brown et al. 2014, p. 25.
  59. ^ Vye-Brown et al. 2014, p. 2.
  60. ^ Biggs et al. 2021, p. 9.
  61. ^ Kervyn et al. 2010, p. 916.
  62. ^ Nyamweru 1988, p. 610.
  63. ^ Dawson et al. 1994, p. 799.
  64. ^ Rey et al. 2021, p. 79.
  65. ^ a b Dye et al. 2022, p. 30.
  66. ^ Morrison & Bolger 2014, p. 619.

Sources

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