Oldoinyo Lengai Volcano

Oldoinyo Lengai Volcano (2962m) is the only volcano to erupt sodium carbonatite lavas in historical times.  These lavas have a significantly lower melting point (around 500'C) than “normal” silicate lavas (around 1200'C) and their weak incandescence can only be observed at night.  Documented activity is characterized by short explosive eruptions of predominantly silicate ashes which leave a funnel-shaped crater, followed after an eruptive pause by a phase of intracrater activity involving intermittent effusion of carbonatite (soda) lavas.  The current effusive phase started in 1983 and has completely filled the crater formed by the 1967 explosive eruption.

View of Oldoinyo Lengai volcano from East. The summit contains an inactive S crater and active N crater, the position of which is indicated by the light-coloured carbonatite deposits on the flanks, July 2004.	View along main road of nearby Masai village towards North side of Oldoinyo Lengai. White carbonatite deposits are visible near summit, July 2004.

Oldoinyo Lengai volcano consists of various types of peralkaline (Na₂O- and K₂O-rich) silicate lavas. Two main structural units are recognized. The remains of the initial structure, "Lengai I" form the south flank and much of the base of the volcano and account for 60% of its volume. "Lengai II" is more recent and formed in the scar left behind by a major flank collapse of the N flank of Lengai I about 10000 years ago. Flank collapses feature in the history of many volcanoes and are dealt with in more detail in the sections on Stromboli and Augustine volcanoes. Both "Lengai I and Lengai II" are formed primarily from pyroclastic deposits, suggesting mainly explosive activity during the cone-building phases. Natroarbonatites, which form less than 5% of the structure (mainly in and around the N crater), appear to be a recent feature of Lengai activity. Lengai I is made of phonolite (14-17% alkaline). Lengai II is predominantly nephelinite (15-21% alkali). These lavas contain between 53 and 43% silicate, with a gradually decreasing trend during evolution of the structure (Klaudius and Keller, 2006 (Lithos 91:173-190)). Although these lavas show a gradual increase in alkalinity and a gradual decrease in silica, they are far removed from natrocarbonatite lavas which have over 40% alkali content and usually less than 0.5% silica.

View from summit ridge of Oldoinyo Lengai, August 2000.	View from summit ridge of Oldoinyo Lengai, July 2004.

Most lavas contain 40-80% silicate, whereas carbonatites usually contain significantly under 10%. Carbonatites have over 50% volume of carbonate materials. The most commonly found carbonatite deposits are rich in calcite (CaCO₃). Deposits of natrocarbonatites are extremely rare, although this may reflect the fact that the sodium and potassium carbonate minerals nyerereite and gregoryite making up much of their composition are rapidly weathered. At Lengai, as the erupted anhydrous natrocarbonatite cools on the surface, hydration rapidly occurs, changing the colour of the material from dark grey to an off-white colour usually within a matter of days. The process can be observed particularly well when raindrops fall onto fresh natrocarbonatite deposits each leaving lighter marks on the surface. Hydration and subsequent alteration by chemical reactions and leaching out of certain constituents eventually results in a weak material which can be crushed to powder between ones fingers. The rapid weathering has the effect that sites of fresh lava emission can be easily recognized as darker areas on the crater floor.

Hornitos on crater floor, August 2000. Dark grey area is recently erupted lapilli field.	Hornitos on crater floor, July 2004. Dark lava flow is only hours old.

Less than 400 carbonatite deposits are known and only few of these represent eruption of carbonatite lavas at the earths surface. The formation of carbonatites is thought to result from differentiation of mixed magmas as they cool on approaching the earths surface. As magma temperatures fall, silicate minerals crystallize, increasing relative levels of carbonates in the melt. The carbonate-rich magma eventually seperates from the silicate magma and can be erupted as carbonatite lava if it reaches the surface. The process of natrocarbonatite formation at Lengai can possibly explained by natrocarbonatite seperating from the Lengai II-forming combeite and wollastonite bearing nephelinite after combeite crystallization (Dawson 1998 (J. Petrology 39:2077-2094)). The processes occurring in the magma chamber are thought to also account for the predominance of carbon dioxide in the gases emitted by the volcano.

A pocket of natrocarbonatite coexists with nephelenitic melts in Lengai's magma chamber(s). This can be deduced from eruption of mixed material during larger eruptions. For example, the tephra from the minor 1993 eruption was predominantly silicate with inclusion of small globules of natrocarbonatite. All larger explosive eruptions have involved mixed silicate-natrocarbonatite tephras with varying relative compositions. Explosive eruptions of Lengai have been documented in 1917, 1940-41 and 1966-67. Tephra deposits surrounding Lengai suggest that significantly larger eruptions have occurred every several hundred years (last about 450 years ago) before historical records began.

Minor intracrater eruptions of natrocarbonatites can take several forms, reminiscent of activity at other volcanoes. Lava flows can be observed, as can lava fountaining or strombolian activities or flank failures of cones. The observation that one cone erupted an A'a flow shortly after which another erupted a Pahoehoe flow (personal observation 2004) is interesting , since the difference could not be accounted for by the topology of the area of emplacement or by an obviously different flow rate and could result from differences in temperature or composition of lava in individual cones within the crater. Such variance in types of products produced by intracrater activity is repeatedly observed.

Nighttime eruption of hornito in center of crater, July 2004.	Nighttime eruption of hornito in center of crater, July 2004.
Lava flow from vent in flank of hornito, July 2004	Lava cascading down flank of hornito, July 2004.
Channeled carbonatite Pahoehoe flow, Crater floor, July 2004.	Strombolian activity of vent in flank of hornito, July 2004.

Although Lengai is in a little-developed region of Tanzania, many tour operators now offer trips to the area. The most common route up Lengai is from the North and involves a near 2000 meter steep ascent. Local guides and porters can be hired at Ngaro Sero village nearby which also provides rudimentary camping possibilities. Camping near the summit is possible in the relatively safe inactive south crater or even in the actice crater. This is however dangerous since lava flows have unexpectedly reached camps in the past. Unfortunately, the local Massai are demanding more and more unreasonable amounts of money for climbing and particularly camping in "their" volcano. I have even been prevented from taking photos of Lengai by aggressive money-demanding Massai near a village south of Lengai.

Unfounded reports of major eruptions have circulated in the press repeatedly in recent years. However, on 31. August 2007 and the following days vast amounts of carbonatite lava were erupted from the summit and flowed down to the base of the volcano. By the 6th of September, a massive cinder cone had formed in the middle of the crater, destroying most other features. Activity since the initial effusive phase of the eruption has involved nearly daily ash eruptions of varying duration and magnitude. Several km high columns have been observed with visible incandescence and occasional lightning bolts. Indeed, in March 2008 some extremely violent eruptions with towering eruption columns have been observed. These would probably have been fatal for anyone in the vicinity of the crater.

As an interesting little extra, the photos below show the inside (!) of an inactive hornito at Lengai in July 2004. A small hole that had formed its flank allowed the rare opportunity to climb inside and view the interior of the structure.

Top of hornito viewed from inside, July 2004.	Stalagtites inside hornito, July 2004.

Further Photos

Photovolcanica Full Index