The following article was published in the December 2000 issue of the Newsletter of the Mineralogical Association of Canada. Accompanying photographs are not shown here.

The July 2000 Expedition to the Natrocarbonatite Volcano Oldoinyo Lengai   By ROGER H. MITCHELL, LAKEHEAD U.  

Oldoinyo Lengai, meaning the Mountain of God in the Masai language, lies in the East African Rift Valley near Lake Natron in northern Tanzania. The volcano is the only active natrocarbonatite volcano in the world. Natrocarbonatite is a most unusual volcanic rock consisting of nyerereite [Na₂Ca(CO₃)₂], gregoryite [(Na,K,Ca)₂CO₃, halite, sylvite, fluorite, neighborite [(Na,K)MgF₃], Ba-Na,Ca-carbonates together with accessory Fe-alabandite, ternary K-Fe-sulphides and Mn-Fe-spinels. The absence of silicates, apart from xenocrysts, is a particularly notable characteristic of these rocks. For those of us who study alkaline rocks, Lengai is regarded almost as a holy site, which one must visit at least once in one's lifetime. Unfortunately, the volcano is remote and reaching its lower slopes requires a long and exceedingly uncomfortable journey from the nearest town through the dust bowl of the N'garuka Depression. Dust drifts up to a couple of feet deep are not uncommon. Until recently, visits to the area have also been regarded as dangerous because of the presence of Somali bandits. I first visited Lengai in 1995 and found it so fascinating that I vowed to return. Thus, in July 2000, I was part of a five-person expedition, which spent one week camped out in the crater of the volcano.

Oldoinyo Lengai rises as a "classical" pyroclastic cone of nephelinite from the flat floor of the rift valley adjacent to some spectacular sequences of basaltic lava exposed in the western fault scarp. The top of the mountain at 9,500 feet is reached after a 5,500-foot climb. The climb is deceptive as it is not technically difficult; its deception lies in its apparent ease when viewed from the base. In actuality the climb is a long gruelling slog up a 40-45° slope composed of pyroclastic beds. There are lots of lapilli and progress is commonly two steps up and one back. Major problems are heat at the bottom, cold winds at the top, and the complete lack of water. The top third of the climb is on steeply dipping tuffs and rotten carbonatite between some very deep canyons. It's not recommended for those with balance problems; if you fall you will bounce all the way down to the bottom. Before climbing this mountain, one needs to drink lots and lots of water. The ascent usually commences at about 3 a.m., and follows the western slopes to avoid the morning sun. The ascent is not pleasant: you need to wear a headlamp, have a sure foot, and possess lots of stamina. If you are lucky you will arrive at the summit after about 5-6 hours. Occasionally, some brave tourists attempt the climb but many give up at the halfway point.Others succeed in reaching the summit but do not realise that getting back down on the same day is even more gruelling. Exhaustion, electrolyte imbalance, and hypothermia are common experiences for the unwary. The previous time I went up Lengai on a "day-trip", even some "hardy" geologists gave up and one person had to be practically carried down the mountain.

Our recent expedition planned to stay seven days in the crater. To do this, we required a significant amount of food and water to be carried up the mountain. This was achieved by hiring seventeen local people as porters. Such work is an important source of income for the inhabitants of the nearby Masai village of N'gare Sero. These people are incredibly fit and literally run up the mountain carrying heavy loads wearing sneakers or rubber-tire sandals. Makes us westerners look pretty soft! The party that stayed at the top consisted of a local cook and a guide, myself, two volcano "freaks", a geomorphologist and a student. One of the volcano "freaks" was the organiser of the expedition. His name is Fred Belton, and he is a mathematics instructor at University of Memphis. Although not a geologist, he has a passion for visiting active volcanoes and has become infatuated with Lengai as he can get "really close to the action"!

Many people ask why one should go to Lengai unless one is studying carbonatite volcanism as surely there are more spectacular volcanoes with easier access. The answer lies in the nature of the volcanic activity. The lavas of Lengai are erupted at temperatures of only 550-600°C, in marked contrast to typical basaltic activity. The low temperatures mean one can get really close to the lavas and vents. One can even stand on the lavas while they are-flowing (if you have good boots). One does not require any cumbersome protective clothing.All one needs is a good hat, a thick shirt, and strong jeans. Eruptions are typically not violent pyroclastic events, although these do occur, but consist mainly of highly fluid lavas. These are dark black during eruption and they flow like water due to the low viscosity of the natrocarbonatite. Typically they form thin flows, which cool very quickly. Most flows are about one inch thick unless lava channels or tubes have been formed. The latter may crust over and are dangerous in that one can break through the crust and get a nasty burn, as several people have found out to their distress. However, these unfortunates were wearing sneakers and would not have been injured if they had been wearing strong leather hiking boots. The lavas cool to brown pahoehoe flows. These weather in about 1-2 days to a white material (pirssonite, trona), which decomposes into brown dust in a couple of weeks. This means that the crater is actually a large dust bowl about 1500 ft in diameter. If you can imagine yourself in a lunar-like landscape of active hornitos with blowing dust and clouds, you can get a good idea of what it is like there. At daytime it can be very hot, but at night it is near freezing, windy and damp. The major problems with respect to camping are finding a place to pitch a tent that is not on a fumarole and is not likely to be overrun by a flash flood of lava. The latter quite commonly occur when hornitos collapse, usually without warning. As there is no water for anything but cooking and drinking, one gets incredibly dirty. The fine dust penetrates everything and is really hard on cameras. After seven days of this, we decided that our environment was the very antithesis of Club Med and we decided to call our campsite Club Lengai.

While we camped in the crater, we were fortunate in that activity of some type occurred all of the time. This included gas venting, small amounts of spatter, a very large lava fountain, many small aa-like flows, and copious streams of very fluid lava. One of the exhilarating things about Lengai eruptions for a petrologist is the opportunity to be able to stand inside a system of braided streams of flowing lava in relative safety. At times of quiescence, one could even look down into the vents and see the magma boiling away as bubbles of carbon dioxide broke the surface. Fortunately, volcanic gases are not a great problem in this crater. The bulk of these are carbon dioxide, and sulphur dioxide is only a minor constituent. In addition the strong winds usually prevent gas to build up to dangerous levels.

Mineralogical and petrological studies of Lengai natrocarbonatite are a major part of my current research program. This visit provided many interesting new samples and already I have found in them a couple of new minerals and several species new to Lengai.