Also known as the “Mountain of God” in the Maasai language, the Ol Doinyo Lengai is one of the unique phenomena that nature has put forth. Located over an active continental rift zone of East Africa, the volcano and its cold, black lava that flows as freely as water, has captured the attention of many geologists and archaeologists in the past. Despite being quite active with over ten eruptions in the last hundred years, the volcano is quite harmless. So harmless, in fact, that a man has even fallen into the lava and survived.
Located on the Gregory Rift, Tanzania, and on part of the East African Rift, Ol Doinyo Lengai is an active volcano of unique significance. Due to its unusual composition, the lava erupts at a relatively low temperature, 510 0C (950 0F), much lower than normal lavas at 1200 0C (2192 0F).
The present-day, cone shape of the mountain was geologically formed about 15,000 years ago. Historical eruptions have come in all varieties: huge, moderate and small. Furthermore, the lava seeps out of craters at the bottom of the summit. Because of all the eruptions through the many years, the depth and morphology of the northern crater have changed drastically from steep crater walls about 200 meters deep in the mid-20th century to shallow platforms filling the crater. By 1998, the crater was full and overflowing. In July 2000, several carbonatite eruptions took place and the cone rapidly grew at that time in both shape and size.
Ol Doinyo Lengai’s lava is made of natrocarbonatite, rare sodium and potassium carbonate minerals, unlike the usual sodium oxide and potassium oxide silicate lavas. The carbonate minerals are extremely susceptible to weathering and react upon contact with atmospheric moisture turning white in few hours.
Most volcanoes produce lava that is rich in silicates, but this volcano produces lava rich in rare sodium and potassium carbonates: nyerereite and gregoryite. The gray ash it produces also makes the surrounding ground extremely rich in nutrients, resulting in pastures that became the birthplace for thousands of calves and a spot to migrate to for many animals. The gray ash is also hydrated in the air, turns white, and becomes brittle and powdery. Finally, it is washed off by the rains.
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Due to the low temperatures, the lava appears black under daylight and its iridescence can only be seen at night. The lava is also much less viscous than normal lava which travels at 10 kilometers per hour, and this lava flow can easily outpace a running human.
The low temperature makes the lava look black in sunlight which is different from the red glow seen in most volcanic lavas. Moreover, due to the atypical composition of lava, it is much more fluid than other lavas. The viscosity of this lava is often similar to water because it lacks the ability to form molecular chains. This is also why the melting point is far lower than silicate-rich lava. Sometimes, the lava freezes or solidifies when it erupts into the air and shatters on the ground like glass.
The volcano has two structural units, Lengai I and Lengai II, which were primarily formed from pyroclastic deposits. While the volcano was around 370,000 years old, Lengai II was formed only about 10,000 years ago from a collapse of the north flank of Lengai I.
Sixty percent of the volcano, from the south flank to most of its base which was part of the original structure, is now Lengai I. More recently, Lengai II was formed as a result of a scar left behind during the collapse of its north flank. The pyroclastic deposits from which the two structures are formed suggest that there was an explosive activity during their formation. The natrocarbonatite forms at Lengai I can be explained by the formation of combeite and wollastonite. This formation is done by separation of natrocarbonatite.
In a rare incident, during an expedition in 2007, a local Maasai porter fell into one of the lava flows in the crater. He was able to climb out of it sustaining severe burns becoming the only person to have fallen into lava and survived.
The Maasai man had sustained severe burns on one of his arms and both of his legs and was hospitalized. Minor intercrater eruptions in this volcanic mountain also can take various forms such as lava flows, lava fountaining, or strombolian activities which create rock fragments known as “tephra.” These activities, which result from effusion at the crater floor, are mostly observed around cones.
