The World's oddest volcano

Oldoinyo Lengai is the world's strangest volcano, everything about it is an enigma. It is not an exaggeration to suggest that this relatively small volcano in East Africa challenges the whole of Igneous Petrology and our models of volcanism.


Situated in the African Rift Valley in Northern Tanzania it sits directly west of its large cousin, Kilimanjaro. Initially discovered in the late 1800's it was only categorized in 1960, when JB Dawson took a look at its chemistry. The eruptions this volcano produces are dominated by sodium carbonate: a key component of soap. You could wash your hands with it.


Lengai's first recorded eruption was in 1940, followed by minor eruptions in 1954, 1955 and 1958. In 1966 JB Dawson and GC Clark observed a significant Vulcanian activity that developed into a Plinian eruption. This involved cone formation and an ash fountain from the mouth, followed by a decrease in activity. Similar large eruptions occurred in 1988 and 1993.

The vast majority of this world's volcanic eruptions have a basaltic or acidic composition. In these eruptions the primary element is silicon in the form of silica, and these between them represent the bulk chemistry of The Earth's Crust. These can be explained by either the (partial) melting of the Earth's mantle or the reprocessing/re-melting of the Crust. Carbonate rocks are found almost exclusively in the oceans (limestone is calcium carbonate) and form in biogenic processes or by precipitation. These are among the worlds most thoroughly studied rocks and are well understood in the context of ocean chemistry.

Carbonatites - carbonaceous igneous rocks, are an enigma. They had begun to gain credence before 1960, with extremely rare intrusive examples having been found around the world since 1920. These units are all inactive, with Lengai the only active example known. The drawback here is that every other example is either calcium carbonate (volcanic limestone) or calcium-magnesium carbonate (volcanic dolomite). Since the day it was discovered geologists have been trying to use Oldoinyo Lengai as a key to understanding the rest. But this solution has proved elusive, and it is becoming accepted that the rest have little in common with Lengai.

Lengai challenges the only models we have managed to produce for the rest. When people think they have cracked the formation of Canadian carbonatites, Lengai sits there like a square peg in a round hole. Geologists have been butting there heads at these questions for years: Where is this sodium carbonate forming within the Earth and why? There is no phase in the Earths Mantle that would produce carbonates, it simply should not be forming. How does it fit into its tectonic setting? The situation in the African Rift suggests that Lengai is produced by Mantle Plume related volcanism, that it is linked to dynamic rifting but even within these models it makes little sense to have carbonates.

Even taken as an individual volcano, ignoring its origin, Lengai is a misfit. Here are a few of its bizarre individual features:

This volcano is extremely water soluble, after a strong rainfall a significant proportion of it will have dissolved. Due to its solubility, Lengai has no preservation potential, if it stopped erupting tomorrow and we waited 500 years, there would be nothing there to see. This makes it extremely difficult to judge how common they are.

This volcano has an eruptive temperature of 500°C (as opposed to the 1200°+ temperatures of a basalt volcano like Kilauea) meaning you can walk right next to it without heat problems.

This lava is extremely low viscosity; It runs like water across the landscape.

She sits there, in the middle of Africa, challenging us to explain her.... And we fall short. Why does this magma exist? How does it form? What the hell is this thing doing on Planet Earth? If you are ever in Tanzania, she is worth a visit and a moments thought, to put your foot on a scientific enigma is a rare experience, and she is baffling.

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