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 by an eruptive pause and then a phase of intracrater activity involving intermittent effusion of carbonatite (soda) lavas.  The penultimate effusive phase documented herein started in 1983 and completely filled the crater formed by the 1967 explosive eruption. In 2007-2008, another powerful eruptive phase occurred. Low level activity is currently starting to fill the large crater that was formed. The present page contains images and video from visits in 2000 and 2004.

Oldoinyo Lengai Volcano Oldoinyo Lengai, Village

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.

Lava Flow Ol doinyo Lengai Nighttime carbonatite lava flow on Oldoinyo Lengai volcano, Tanzania

Natrocarbonatite lava flowing from hornito T51 in July 2000

Nighttime lava flow from hornito T58B in 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. It encompasses both of the present craters. 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. Natrocarbonatites, 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 the 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 0.3% silica.

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 (Na2Ca(CO3)2) and gregoryite (Na2,K2,Ca(CO3)) 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 (see image in fumarolic deposits section). Hydration and subsequent alteration by chemical reactions and leaching out of certain constituents eventually results in a brittle easily powdered material. The rapid weathering has the effect that sites of fresh lava emission can be easily recognized as darker areas on the crater floor.

Hornitos Oldoinyo Lengai 2004 Comparison of hydrated and fresh natrocarbonatite lava, Oldoinyo Lengai

Hornitos on crater floor seen from E rim, July 2004. Dark lava flow is only hours old.

Front of flows on right image, older flow (hydrated so white), new flow (dark grey)

Crater, Summit View, Oldoinyo Lengai 2000 Hornitos Oldoinyo Lengai

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

Elevated central portion of crater viewed from E, July 2004

Less than 400 carbonatite deposits are known worldwide 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. The process of natrocarbonatite formation at Lengai can be partly explained by natrocarbonatite separating from the Lengai II-forming combeite and wollastonite bearing nephelinite after combeite crystallization (Dawson 1998 (J. Petrology 39:2077-2094)). The differentiation may indeed involve a combination of (i) fractional crystallization, wherein as silicate minerals crystallize during magma cooling sink, increasing relative levels of carbonates are left in the melt, and (ii) liquid immiscibility, wherein the strongly peralkaline Fe-rich nephelinite melt and natrocarbonatite melt separate when still in their liquid phases. Irrespective of the exact process, the carbonate-rich magma eventually separates from the silicate magma and can be erupted as carbonatite lava if it reaches the surface. Whilst too complex to explain in full detail here, a number of papers address carbonatite formation (e.g. Fischer et al. 2009. Nature 459, p. 77-80; Mitchell 2009. Contrib. Mineral. Petrol. 158, p.589-598; Keller et al. 2010. Bull. Volcanol. 72, p. 893-912). High levels of sodium in the Lengai lavas appear to be critical to the formation of stable carbonatites and these may be contributed by the regional geological setting in the Rift Valley.

As a result of the differentiation process, a pocket of natrocarbonatite coexists with nephelenitic melts in Lengai's magma chamber(s). Since the natrocarbonatite is less dense it tends to float on the top of the nephelenitic melts and thus is erupted during mild effusive eruptions. The mixed chamber content is evident from eruption of mixed material during explosive eruptions when the system is more disturbed. 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. Large explosive eruptions of Lengai have been documented in 1917, 1940-41, 1966-67 and 2007-2008. During these, the pocket of carbonatite are largely expelled, allowing access of primarily nephelenitic melts to the surface. 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. The activity is generally observed around cones (also referred to as Hornitos (although use of this terminology for cones at Lengai is technically questionable)) which result from sustained or repeated effusion at particular points on the crater floor. The cones generally sit on top of small lava reservoirs connected to the main conduit.

Nighttime Eruption Oldoinyo Lengai Moonlit Eruption Oldoinyo Lengai

Nighttime eruption of hornito T58B in center of crater, July 2004

Nighttime eruption of hornito T58B in center of crater, July 2004

Nighttime natrocarbonatite lava flow from cone T58B on Oldoinyo Lengai volcano, Tanzania Nighttime natrocarbonatite lava flow from hornito T58B on Oldoinyo Lengai volcano, Tanzania

Nighttime eruption of hornito T58B in center of crater, July 2004

Nighttime eruption of vent on SW side of hornito T58B, July 2004

Carbonatite Lava Flow Oldoinyo Lengai Lava flow field from T58B, Oldoinyo Lengai, 2004

Lava flow from vent in flank of hornito T58B, July 2004

Dark-coloured recent flow field from T58B, July 2004

Old hornito top sticking from fresher deposits, Oldoinyo Lengai Intracrater lava flow, Oldoinyo Lengai, people for scale

T58B shows mild spattering, older almost buried cone in foreground

Lava flow from T58B, people provide scale

Strombolian Eruption of natrocarbonatite lava, Oldoinyo Lengai Eruption Hornito Ol doinyo Lengai Strombolian Eruption, Oldoinyo Lengai

Strombolian activity of vent in flank of hornito T58B, July 2004

Batrocarbonatite lava gushing from hornito T58B on Oldoinyo Lengai volcano, Tanzania Batrocarbonatite lava gushing from hornito T58B on Oldoinyo Lengai volcano, Tanzania

Lava violently jetting from vent on T58B (1/4)

Lava violently jetting from vent on T58B (1/4)

Batrocarbonatite lava gushing from hornito T58B on Oldoinyo Lengai volcano, Tanzania Batrocarbonatite lava gushing from hornito T58B on Oldoinyo Lengai volcano, Tanzania

Lava violently jetting from vent on T58B (1/4)

Lava violently jetting from vent on T58B (1/4)

Flow field from overnight eruption of cone on crater terrace of Oldoinyo Lengai Carbonatite Lava Flow Oldoinyo Lengai

Flow field from overnight eruption of T49G, July 2004

Lava cascading down flank of hornito T49G, July 2004

Pahoehoe Flow of carbonatite lava, Oldoinyo Lengai Volcano Lava emerging from underground channel on flank of Oldoinyo Lengai

Lava cascading down flank of hornito T49G, July 2004

Channeled carbonatite Pahoehoe flow, Crater floor, July 2004

Lava from T49G emerges from underground channel on flank of volcano

July 2000 Images:

Summit View Oldoinyo Lengai Volcano 2000 Hornitos Oldoinyo Lengai 2000

View N from summit, July 2000. Dark area is huge flow emplaced on July 22

(day before expedition reached crater)

Lapilli / Lava deposits from July 22 eruption (presumably of T37E)

Weathering lapilli field, Oldoinyo Lengai Fumarolic deposits on fresh lapilli field, Oldoinyo Lengai Fumarolic gas deposits, Lapilli, Oldoinyo Lengai

View from E of vent - Rough lava flow field on left, white lapilli on right.

The lapilli in this area had weathered overnight.

Darker lapilli from same eruption with fumarolic deposits

Detail of middle image. Note, these lapilli did not weather, possibly due to a different composition

Hornito exploding, Oldoinyo Lengai volcano

T49B on morning of July 23

T37N1 exploding (center of image) on July 23, 13:04 local. Foreground is T51 with fresh flow field, T49B on left

Strongly hydrated lava flow, Oldoinyo Lengai Hornitos Oldoinyo Lengai

Flow field from T37N1 eruption after hydrating for a few days (hence white colour)

T51 flow field on July 29, Note steeper cone compared to above image. T49B has also grown significantly.

Erupting Hornito T51 Oldoinyo Lengai, 2002 True hornito with suspended lava blob, Oldoinyo Lengai volcano

T51 cone erupting and emplacing small lava flows

Active lava flows from T51 in 2002 with mini hornito in top right corner

Spectacular Intracrater Natrocarbonatite Eruption Video Taken in July 2000

The video shows various carbonatite eruptions from the recently formed eruptive cone (T51) on the main crater floor in 2000 (July 23, 28-30). The cone was rapidly growing at the time and the difference in size and shape at various stages of the video (chronologically ordered) can be clearly seen. Video includes view into lava lake in cone during pause in eruption, plus cascades of lava spilling out of the cone and associated lava flows.

Major 2007-2008 Eruption

A number of publications cover the 2007-2008 explosive phase and its precursory activity. A good summary is provided by Kervyn et al. 2010 (Bull. Volcanol. 72, p.913-931). In March/April 2006, a significant volume (about 1 million cubic meters) of unusually silicate-rich (around 3%) natrocarbonatite was erupted effusively over a two-week period following degradation of several cone structures in the center of the crater (Kervyn et al. 2008. Bull. Volcanol. 70, p.1069-1086). The lava largely flowed down the W flank creating a significant deposit at the base of the volcano. An 80 meter wide and 20 meter deep irregularly shaped crater was left in the center of the crater floor accounting for about a third of the eruption volume. This event was followed by a year of quiescence, after which, in June 2007, intense natrocarbonatite effusive activity resumed in the pit, rapidly filling it up. Activity dropped again for much of July and August, picking up again on 21 Aug. 2007 according to MODIS (thermal imaging satellite). On August 25, thermal signals indicated lava flows on the E and W flanks. On Sept. 1, extensive flows set fire to large areas of vegetation on the flanks of Lengai. Flow intensity increased further until the onset of the explosive phase.

On Sept. 4, 2007, two explosive events occurred, with the second forming an about 3 km high ash column which was sustained for nearly 12 hours. The eruption continued with a mixture of degassing phases, several hundred meter high jets of ash and other material, low level ash emissions and more powerful vulcanian or sub-plinian events. The most powerful events occurred over two periods. In a several week period spanning the last week of September to mid-October, a number of ash columns reaching 5 km above the crater were documented, then from mid-February to mid-March 2008, the most powerful eruptions were observed, with a maximum column height of 15 km, often accompanied by volcanic lightning. Several pyroclastic flows resulting from minor column collapses were observed on the upper flanks of the volcano. In further violent explosive events on 7-8 April, the south crater was peppered with volcanic bombs, with numerous ones approaching or even surpassing a meter in diammeter. The eruption faded rapidly after this second major explosive period, essentially ending in April 2008.

The crater morphology was significantly changed by the eruption. Before the eruption, the about 400 meter wide crater contained a number of cones built be natrocarbonatitic effusions and spatter. The explosive eruption rapidly built up a pyroclastic cone with its crater on the site of the 2006 effusive event. Eruptions occurred from at least 2 vents therein. By the end of 2007, the flanks of the cone extended to the rim of the 400 m wide crater terrace on all sides. By the end of the eruption, an about 150 meter wide and 130 deep steep sided pit had formed in the center of the pyroclastic cone.

The trigger mechanism of the eruption has been discussed. There was intense seismic activity relating to dyke intrusion at nearby Gelai volcano in July 2007. This stopped about a week before the onset of the explosive phase at Lengai. It has been suggested that the Dyking event may have led to pressure changes in the magma chamber under Lengai (Baer et al. 2008. Earth Planet Sci. Lett. 272, p.339-352). A different mechanism is favoured by Kervyn (Bull. Volcanol. 72, p.913-931 (2010)). It is suggested that the 2006 effusive event, which rapidly erupted the annual average volume for the 25 year effusive phase at Lengai (1983-2006), and was indeed followed by about a years eruptive pause, could have triggered the eruption. Following the eruption of much of the relatively light natrocarbonatite normally occupying the upper magma chamber(s), fresh nephelinite melt appears to have risen into these in the following year. The refilling did not allow sufficient time for full differentiation of a new body of natrocarbonatite, so that following effusive expulsion of a small remaining amount of carbonatite from the top of the magma chamber(s) in June-August 2007, the predominantly nephelinite magma would have reached the surface in September, coinciding with the explosive eruption phase. Indeed the explosively erupted magmas had a silicate content of 25-30 wt %, whilst natrocarbonate tends to have under 0.3 %. The driving force for the explosive eruption has been postulated to be the liberation of substantial amounts of carbon dioxide by decomposition of Na2CO3 and CaCO3, which decompose, releasing CO2 at temperatures of 860 and 825'C, respectively (Mitchell 2009. Contrib. Mineral. Petrol. 158, p.589-598). It is suggested that this process occurred after hot combeite-wollastonite nephelinite (CWN) entering the conduit interacted with solid natrocarbonatite deposits in the crater floor.

The magmas erupted at different phases of the eruption have been analysed in detail (Keller et al. 2010. Bull. Volcanol. 72, p. 893-912). They are classified as carbonated silicate magmas partially evolved from the CWNs with an about 30% natrocarbonatite proportion. From 7-11 wt % of carbon dioxide were found in the eruptates. Carbonatitic activity is largely driven by carbon dioxide and water vapour with only low levels of sulphurous gases. Keller et al. argue that the 2007-2008 magmas, designated as carbonated combeite-wollastonite-melitite nephelinite (carbCWMN), may represent a fractionated CWN approaching the conditions required for immiscible separation.

Unusually, shortly after the 2007-2008 eruption, low level natrocarbonatite effusion has commenced on the base of the pit crater. Following the 1940 and 1966-67 eruptions, no such activity was observed for about 15 years. The source of this natrocarbonatite is unclear.

View inside inactive cone T45

As an interesting little extra, the photos below show the inside of inactive hornito T45 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.

Outised of hornito T45 Inside Hornito, Oldoinyo Lengai Volcano Inside Hornito, Oldoinyo Lengai Volcano

T45 from outside, arrow indicates hole in flank

Top of hornito T45 viewed from inside, July 2004.

Stalagtites inside hornito T45, July 2004.

Fumarolic Deposits

Two types of geothermal deposit can be distinguished at Lengai. Firstly, those from deeper-seated gas sources, and secondly, those from transient (predominantly steam driven) fumaroles resulting from lava being emplaced over damp weathered carbonatite deposits. The latter type as shown in the images immediately below has been studied in detail (Genge et al. 2001. J. Volc. Geotherm. Res. 106, p.111-122). Chemical analysis of the whitish crystalline deposits formed around cracks over fresh flows revealed a composition including approx. 70% (by vol.) thermonatrite (Na2CO3.H2O), 10% aphthitalite (Na,K)2SO4, halite (NaCl) and sylvite (KCl). The deposits are not randomly mixed, individual components crystallize in the order (i) thermonatrite (ii) aphthitalite, (iii) halite masses, and (iv) halite-sylvite masses. The sequence is attributed to progressive leaching of different components from the deposited lava (causing alteration of the lava in particular adjacent to the cracks), and in particular to the gradually dropping temperature of the gases as the flow cools which influences which materials are leached out and how effectively they crystallize at the surface. For example, steam can carry significant amounts of NaCl at temperatures of above 400'C. However, below this temperature the amount carried rapidly drops so that halite crystals will rapidly form (Pitzer and Pabalan, 1986. Geochim. Cosmochim. Acta 50, p.1445-1454).

Genge et al. also suggests that vaporization of meteoric water from underlying deposits explains the observed lifting of the edges of fresh lava deposits from the underlying ground.

Fumarolic deposits of thermonatrite (Na2CO3.H2O), 10% aphthitalite (Na,K)2SO4, halite (NaCl) and sylvite (KCl), Oldoinyo Lengai Solidified Pahoehoe Flow, fumarolic salt deposits, Oldoinyo Lengai Volcano

Fumarolic deposits formed on fresh lava within 24 hours

Fumarolic deposits formed on fresh lava within 24 hours

Natrocarbonatite lava with hydrated spots due to rainfall Sulfur crystals formed in fumarole on Oldoinyo Lengai volcano

Colourful deposits - Lava around them is spotted by hydration of fresh deposit by raindrops

Sulfurous fumarolic deposits (from deeper degassing source)

Visitor Information

Although Lengai is in a little-developed region of Tanzania, many tour operators 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 active crater. 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.

Since the 2007-2008 eruptive phase, visible activity has been confined to the floor of the deep pit crater and is thus not accessible.

The crater area is hazardous since underground chambers full of magma may exist just below the surface. In 2004, numerous members of our expedition spent time standing on an area that collapsed with no warning revealing a lava lake below only hours later. What had appeared to be solid ground turned out to be only about 10-20 cm thick crust. Lava flows may also threaten visitors as the extremely fluid lava can rapidly cover large distances.

Camp in active crater of Oldoinyo Lengai volcano Camp, Oldoinyo Lengai

July 2004 camp - due to proximity to active vents a 24 hour lava watch was performed

View from tent (in foreground on left image) to center of crater, 2004

Ol doinyo Lengai Oldoinyo Lengai and Giraffe

View of Lengai from Engaro Sero village

View with giraffe from near Lake Natron

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