Ol Doinyo Lengai, located in northern Tanzania, is one of the world’s most scientifically intriguing volcanoes. The local Maasai people refer to this active peak as “The Mountain of God.” While structurally similar to many others, the volcano is fundamentally unique due to the chemical make-up of the material it erupts. Its distinct nature is defined by an unusual lava composition that sets it apart from every other active volcano on Earth.
Physical Classification and Geological Setting
Ol Doinyo Lengai has the classic, steep-sided profile of a composite volcano, or stratovolcano, rising approximately 1,800 meters above the surrounding plains. This conical shape is built up by alternating layers of lava flows and ash deposits over many eruptions. The summit features a steep cone with an active crater where effusive and explosive activity is focused.
The volcano’s existence is directly linked to its location within the East African Rift Valley (EARV), a major zone of continental extension. This immense geological feature is slowly pulling the African tectonic plate apart, causing the crust to thin and fracture. This rifting process creates pathways that allow magma to ascend from deep within the Earth’s mantle.
Ol Doinyo Lengai is situated in the eastern branch of the rift, known as the Gregory Rift. The thinning crust here generates the highly alkaline and carbon-dioxide-rich magmas that feed the volcano. The geological stresses of the rift valley provide the necessary conditions for its unusual chemistry to develop.
The Defining Chemical Composition
The feature that distinguishes Ol Doinyo Lengai is the lava it erupts, which is classified as natrocarbonatite. This composition is extremely rare, making Ol Doinyo Lengai the only currently active volcano on Earth to produce it. In contrast to the vast majority of lavas, which are rich in silicate minerals, natrocarbonatite contains essentially no silica.
Instead, the lava is dominated by sodium and potassium carbonate minerals. The two primary components are nyerereite (sodium, calcium, and carbonate) and gregoryite (mainly sodium carbonate). These minerals are water-soluble and unstable when exposed to the atmosphere, which dictates the lava’s unusual surface behavior.
The magma is highly enriched in alkali elements and carbon dioxide. This unique composition allows the molten material to maintain a liquid state at temperatures significantly lower than typical silicate lavas. The natrocarbonatite composition provides scientists with a rare window into deep mantle processes.
Unusual Eruption Characteristics
The high concentration of sodium and potassium carbonates results in lava that behaves unlike any other volcanic material. Natrocarbonatite is the coolest lava on Earth, erupting at an exceptionally low temperature of approximately 500 to 600 degrees Celsius. This is roughly half the temperature of common basaltic lava, which typically erupts above 1,000 degrees Celsius.
Because of the cool eruption temperature, the lava does not exhibit the bright red-orange glow seen in most eruptions during the day. Freshly erupted natrocarbonatite is dark gray or black, glowing only faintly red once the sun sets. This low temperature is also responsible for the lava’s extremely low viscosity, making it exceptionally fluid.
Natrocarbonatite flows almost like water or thin oil, often moving faster than typical silicate lavas. Immediately after extrusion, the dark-colored lava begins a rapid transformation due to its mineral instability. The nyerereite and gregoryite minerals react quickly with atmospheric moisture, causing the lava to weather within hours into a white or light gray powder.