Ol Doinyo Lengai, "Mountain of God" in the Maasai language, is an
active volcano located
in the Gregory Rift,
south of Lake Natron within
the Arusha Region of Tanzania. Part of the volcanic system of the East African Rift, it uniquely
produces natrocarbonatite lava. The
1960 eruption of Ol Doinyo Lengai led to geological investigations that finally
confirmed the view that carbonatite rock is derived from magma.
Geology
Ol Doinyo Lengai is unique among active volcanoes in
that it produces natrocarbonatite lava, a unique occurrence of volcanic carbonatite. A few older extinct carbonatite volcanoes are
located nearby, including Homa Mountain.
Lava
Whereas most lavas are rich in silicate minerals, the lava of
Ol Doinyo Lengai is a carbonatite. It is rich in the rare sodium and potassium carbonates, nyerereite and gregoryite. Due to this unusual composition, the lava erupts at
relatively low temperatures of approximately 510 °C (950 °F). 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, often less viscous than water. The sodium and potassium
carbonate minerals of the lavas erupted at Ol Doinyo Lengai are unstable at the
Earth's surface and susceptible to rapid weathering, quickly turning from black to grey in colour. The
resulting volcanic landscape is different from any other in the world.
Ecology
The carbonatite ash spread over the surrounding
grasslands leads to a uniquely succulent, enriched pasture. This makes the area
a vital stage on the annual wildebeest beast migration, where it becomes the nursery
for the birth of several thousand calves.
Eruptive activity
1883-1915
The record of eruptions on the mountain dates to 1883.
Flows were recorded between 1904 and 1910 and between 1913 and 1915.
1917
A major eruption in June 1917 deposited volcanic ash
up to 48 kilometres (30 mi) away.
1926
An eruption took place for several months in 1926.
1940
An eruption between July and December 1940 deposited
ash as far as Loliondo,
100 kilometres (62 mi) away.
1950s
Several minor eruptions of lava were observed in 1954,
1955, and 1958.
1960s
Minor eruptions of lava were observed in the early
1960s.
A major eruption occurred on 14 August 1966.
Geologists J. B. Dawson and G. C. Clark visited the crater a week later and
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 they 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.
2007
Volcanic activity in the mountain caused daily earth
tremors in Kenya and Tanzania
from 12 July 2007 until 18 July 2007 at 8.30pm in Nairobi. The strongest tremor measured 6.0 on the Richter scale.
Geologists suspected that the sudden increase of tremors was indicative of the
movement of magma through the Ol Doinyo Lengai. The volcano erupted on 4
September 2007, sending a plume of ash and steam at least 18 kilometres (11 mi) downwind and
covering the north and west flanks in fresh lava flows.
2008
The 2007 eruption continued intermittently into 2008.
At the end of February it was reported to be gathering strength, with a major
outburst taking place on 5 March. Periods of inactivity were followed by
eruptions on 8 and 17 April. Eruptive activity continued until late August
2008. A visit to the summit in September 2008 discovered that lava emission had
resumed from two vents in the floor of the new crater. Visits to the crater in
March/April 2009 showed that this activity appeared to have ceased.
2010
In October 2010, two separate lava flows and a small
lava lake were photographed in an overflight.
2013
The volcano resumed natrocarbonatite lava flow which
started to fill the large crater from the 2007-2008 eruption. As of July 2013,
there is a large active hornito on the western edge of the crater floor. During
June, residents near the volcano reported several earthquakes. The new crater
is inaccessible and climbers have only occasionally glimpsed into it.