Ol Doinyo 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.[3]
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. [4].