Ol Doinyo Lengai is an active
volcano
A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most oft ...
in northern
Tanzania
Tanzania, officially the United Republic of Tanzania, is a country in East Africa within the African Great Lakes region. It is bordered by Uganda to the northwest; Kenya to the northeast; the Indian Ocean to the east; Mozambique and Malawi to t ...
. 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, south of
Lake Natron and 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
Tourism is travel for pleasure, and the Commerce, commercial activity of providing and supporting such travel. World Tourism Organization, UN Tourism defines tourism more generally, in terms which go "beyond the common perception of tourism as ...
is increasingly important.
Ol Doinyo Lengai is a symmetric cone that rises more than above the surrounding rift valley. It has two craters on either side of the mountain summit, which is formed by a high ridge. The floor of the northern crater is covered with
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
s that resemble
pahoehoe lavas. Small cones with sizes ranging from to over 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
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...
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 , 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 . 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
The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
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
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...
,
galena,
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
,
monticellite,
sellaite, and
sphalerite
Sphalerite is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in Sedimentary exhalative deposits, sedimentary exhalative, Carbonate-hoste ...
form accessory components.
* The silicic lavas contain
combeite,
ijolites,
melanite,
nepheline,
phlogopite, and
pyroxene, as well as
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...
,
garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...
,
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
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
,
gaylussite,
nahcolite,
pirssonite,
shortite,
thermonatrite, and
trona, including various
chloride
The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
s,
fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...
s, 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
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
s 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
Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (Vesicular texture, vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from ...
sampled at Ol Doinyo Lengai consists mostly of water vapor and
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
and originates in the
mantle. The volcano is a major source of volcanic carbon dioxide, producing about 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
Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because ...
. An eruption 3,000-2,500 years
before present
Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because ...
produced a tephra fallout west of Ol Doinyo Lengai, that is presently being eroded by wind and forming
dune
A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat ...
s 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
Tephra is fragmental material produced by a Volcano, volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, ...
and
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
s from within the northern crater. During the middle 20th century, the crater was about 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
landslide
Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...
s, and erosion has cut
gullies into volcanic deposits. Steam jets have also been observed.
There is evidence of underground
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
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 -high
eruption column
An eruption column or eruption plume is a cloud of super-heated Volcanic ash, ash and tephra suspended in volcanic gas, gases emitted during an explosive eruption, explosive volcanic eruption. The volcanic materials form a vertical column or Plu ...
and a new crater deep and wide. The explosive activity continued into 2008, when the volcano settled back into the effusion of lava flows; a
cinder cone
A cinder cone or scoria cone is a steep, volcanic cone, conical landform of loose pyroclastic rock, pyroclastic fragments, such as volcanic ash, clinkers, or scoria that has been built around a volcanic vent. The pyroclastic fragments are forme ...
formed in the northern crater during the eruption.
Aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...
clouds from the eruption extended over east Africa. The 2007 eruptions forced the evacuation of three villages and disturbed
air travel
Air travel is a form of travel in vehicles such as airplanes, jet aircraft, helicopters, hot air balloons, blimps, Glider (aircraft), gliders, Hang gliding, hang gliders, parachuting, parachutes, or anything else that can sustain flight. in the touristically important area; livestock fatalities and injuries to people led to requests that the
government of Tanzania
The politics of Tanzania takes place in a framework of a unitary state, unitary presidential system, presidential Democracy, democratic republic, whereby the President of Tanzania is both head of state and head of government, and of a multi-party ...
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 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 ; 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 , making them the most liquid of all 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
lahar
A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock, pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a valley, river valley.
Lahars are o ...
s,
landslide
Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...
s,
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
s,
pyroclastic flows,
volcanic bombs,
volcanic gas
Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (Vesicular texture, vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from ...
, and
volcanic ash
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...
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 above sea level.
Volcanic ash
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...
from Ol Doinyo Lengai influences the surrounding landscape, favoring the growth of nutrient-rich plants. Precipitation falls during two
wet season
The wet season (sometimes called the rainy season or monsoon season) is the time of year when most of a region's average annual rainfall occurs. Generally, the season lasts at least one month. The term ''green season'' is also sometimes used a ...
s in March–May and October–December.
Gallery
Image:Aerial view of Ol Doinyo Lengai erupting in 2008.jpg, Ol Doinyo Lengai erupting in March 2008
Image:Ol Doinyo Lengai After Explosive Eruptions.jpg, Satellite image (2009) of Ol Doinyo Lengai after an explosive eruption
File:Ol Doinyo Lengai volcano in Tanzania 20120217.jpg, Ol Doinyo Lengai in February 2012
Image:OldoinyoLengaiAir.jpg, Aerial photo of Oldoinyo Lengai in January 2011
File:Oldoinyolengai.jpg, Image of 1966 eruption
File:Crater of Ol Doinyo Lengai (Jan 2011).jpg, Crater of Ol Doinyo Lengai in January 2011
See also
*
List of volcanoes in Tanzania
Notes
References
Sources
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External links
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Ol Doinyo Lengai, The Mountain of God *
Ol Doinyo Lengai at nationalgeographic.comOl Doinyo Lengai at Stromboli OnlineOl Doinyo Lengai at Volcano WorldSt Lawrence University ''Oldoinyo Lengai''''Fred Belton's Ol Doinyo Lengai site''''Ol Doinyo Lengai Photos 2001''Video of molten carbonatite lava flow eruptions at Ol Doinyo Lengai volcano
{{Authority control
Carbonatite occurrences
Volcanoes of the Great Rift Valley
Stratovolcanoes of Tanzania
Mountains of Tanzania
Active volcanoes
Geography of Arusha Region
Pleistocene stratovolcanoes
Holocene stratovolcanoes