Lengai is a volcano located in the north of Tanzania and is
part of the volcanic system of the Great Rift Valley in
Eastern Africa. It is located in the eastern Rift Valley,
south of both Lake Natron and Kenya. It is unique among
active volcanoes in that it produces natrocarbonatite lava,
a unique occurrence of volcanic carbonatite, which means its
lava is only 510 degrees Celsius (or 950 degrees
Fahrenheit). A few older extinct carbonatite volcanoes are
located nearby, including Homa Mountain.
Whereas most lavas are rich in silicate minerals, the lava
of Ol Doinyo Lengai is rich in the rare sodium and potassium
carbonates, nyerereite and gregoryite. Due to this unusual
composition, the lava is erupted at relatively low
temperatures (approximately 500-600 degrees Celsius). This
temperature is so low that the molten lava appears black in
sunlight, rather than having the red glow common to most
lavas. It is also much more fluid than silicate lavas. The
sodium and potassium carbonate minerals of the lavas formed
by Ol Doinyo Lengai are unstable at the Earth's surface and
susceptible to rapid weathering, quickly turning from black
to grey in color. The resulting volcanic landscape is
different from any other in the world. The chemical makeup
of the lava has been compared to dish soap.
"Ol Doinyo Lengai" means "Mountain of God" in the language
of the native Maasai people.
The flank of the volcano.
The record of eruptions on the mountain dates to 1883, and
flows were also recorded between 1904 and 1910 and again
between 1913 and 1915. A major eruption took place in June
1917, which resulted in volcanic ash being deposited about
48 kilometres away.
A similar eruption took place for several months in 1926 and
between July and December 1940, resulting in the ash being
deposited as far as Loliondo, which is 100 kilometres away.
Several minor eruptions of lava were observed in 1954, 1955,
1958 the early 1960s.
In modern times, Ol Doinyo Lengai erupted on August 14,
1966. Two geologists — J. B. Dawson and G. C. Clark — who
visited the crater a week later, reported seeing “a thick
column of black ash” that rose for approximately three
thousand feet above the volcano and drifted away northwards
towards Lake Natron. When the two climbed the cone-shaped
vent, they reported seeing a continuous discharge of gas and
whitish-grey ash and dust from the centre of the pit.
Volcanic activity in the mountain has caused daily earth
tremors in Kenya and Tanzania beginning on July 12, 2007.
The latest to hit parts of Nairobi city was recorded on July
18, 2007 at 8.30pm (Kenyan Time). The strongest tremor has
measured 6.0 on the Richter scale. Geologists suspect that
the sudden increase of tremors is indicative of the movement
of magma through the Ol Doinyo Lengai. The volcano finally
erupted on September 4, 2007, sending a plume of ash and
steam at least 18 kilometers downwind and covering the north
and west flanks in fresh lava flows. The eruption has
continued intermittently into 2008, as of the end of
February it was reported that the eruption appeared to be
gathering strength, with a major outburst taking place on
March 5. During April periods of inactivity have been
followed by eruptions on April 8 and 17. Eruptive activity
continued until late August 2008. A visit to the summit in
September discovered that lava emission had resumed from two
vents in the floor of the new crater. .