“Oldoinyo Lengai” means “The Mountain of God” in the Maasai language. The summit of this strato-volcano is 2962 metres above sea level, and affords direct views into the caldera of Tanzania’s only officially-certified active volcano, and the world’s only carbonatite volcano; records of eruptions have been maintained since 1883, the largest of which deposited ash 100 kilometres away in Loliondo on the Kenyan border to the north west.
The lava of Oldoinyo Lengai is carbonatite-based, rich in nyerereite and gregoryite. These sodium and potassium carbonates mean that the lava erupts at relatively low temperatures, around 500-600 °C, giving the lava a black/muddy appearance in sunlight, as opposed to the reddish hue of the higher-temperatures (1100°C) of the silicate/basaltic lavas. Carbonatite lava is the most fluid lava in the world, much less viscous than silicate lavas, and is often more fluid than water. The minerals formed by this lava are very unstable in earth’s atmosphere, and rapidly turn from black to grey/white in colour when exposed to moisture. The resultant rock formations are extremely friable, creating a shifting landscape of hard pavements alternating with knee-deep ash. The resulting scenery is different from any other volcano, as Oldoinyo Lengai is the only known active carbonatite volcano in the world.
The closest vehicular access is from the north; a dirt track from Lake Natron crosses the Eastern Rift Valley floor, fording several arroyos before climbing a slight incline and reaching a flat area large enough for vehicles to turn. At the foot of the mountain there is abundant elephant grass, some reaching over 2 metres in height. Ascending diagonally to the base of the first slope, the vegetation reduces dramatically, and soon ceases altogether.
On the lower slopes, the scree is extremely deep in places, and walking over it is as slippery as climbing a sand dune. The ash is comprised of very fine particles, and moving up these slopes can be very taxing. The route across the lower slopes is punctuated by crevasses that can reach 6 metres in depth; the edges of the crevasses are extremely weak, and it is advisable to stay well back. The route passes from pavement to pavement; with short descents into shallow crevasses before resuming the upward climb.
The inclination of Oldoinyo Lengai is approximately 30° on the lower slopes; as the route ascends, the shifting scree is left behind, and the firm pavement studded with small stones can offer good grip. However, the inclination is steadily increasing to 45°, and the rocks are not always securely embedded, making the footing treacherous.
The top of the north-western flank of Oldoinyo Lengai is marked by the presence of the ‘Pearly Gates’; two large white towers of lava that erupted in the Holocene period and have resisted weathering and erosion by more recent eruptions. The thermals that ascend the mountain during the day, and katabatic winds that descend at night, can create ‘dust-devils’ in the mouth of the gates. From the viewpoint of the route up the mountain, the active ash pit is in line with the mouth of the gates; it is important to distinguish between these ‘dust-devils’ and any significant gaseous release from the caldera that may herald a fresh eruption, as the fluidity of the carbonatite lava means that it can flow downhill faster than a river, especially as the ‘Pearly Gates’ represent the path of the most recent lava eruption. Colour can be used to indicate the constituents of the dust devils; steam is white, the dust devils are grey-brown, and the rotten egg-smelling Hydrogen Sulphides are yellow-brown. If you are in any doubt, or if you feel any tremors, or significant steam emission from the caldera, it is advisable to retreat from the mountain as soon as possible.
Climbing through the Pearly Gates, the route takes a sharp left turn behind the northern most pillar to reach a slope ascending to the lip of the caldera. This represents the only solid path up to the summit; the rest of the slope is soft scree, with fist-sized rocks that break away under the slightest pressure. The path follows the lip of the caldera, whose internal slope is almost vertical, and offers no hand-holds. The caldera descends approximately 20 metres before a series of terraces and perforated by flues interrupt the view of the floor of the caldera. The path tracks around 1/3rd of the caldera, before angling down towards the flat floor between the caldera and the summit, which represents the floor of the old caldera to the south. The route veers right to follow the lip of the old caldera to the summit, where views of Ngorongoro Highlands, Lake Natron, Mount Gelai, Mount Kitumbeine and the Ngorongoro Escarpment surround you. Witnessing sunrise here is a breath-taking sight, and well worth the scrapes and bruises gained during the ascent.
Descending this mountain can take as long as the ascent for inexperienced climbers, so it is important to ensure that you have enough water and energy snacks for 12 hours. The descent begins by back-tracking down the lip of the old caldera, across the floor, and back up to the lip of the ash pit, before tracing back to the ridge line that connects with the Gates. Any attempt to traverse the western face of Oldoinyo Lengai from the flat floor of the old caldera to the slope above the Gates is extremely ill-advised due to the shifting scree and the lack of solid rock. The initial part of the slope is quite solid, but halfway across the scree becomes too loose to support your bodyweight, and there is no safe path up to the lip of the ash-pit; the path behind you has been weakened by your passage, and the path down to the pavement between the gates is too risky to contemplate as there is nothing to break a fall to valley floor; a rock released from here takes several minutes to descend the 6km slope. A fall here would likely be fatal.
Once the connection to the slope above the gates and the lip of the ash pit has been reached, descent is made by having the maximum amount of contact with the slope, which often means sitting down, and moving one limb at a time until the slope meets the Gates.
A ski-pole in one hand and a fist-sized rock in the other are useful, as the pole can be used on the slope below you, and the rock grips well on the slope above, protecting the palms of your hands against the friction from the lava. The same ‘three-point’ technique is used on the pavement that lies between the Gates until the soft scree of the lower slopes has been reached. Here the speed of the descent increases, as the soft scree behaves almost like deep powder snow, but it can place a strain on knees tired from the ascent; it is very important to follow the guide precisely here, as some paths are deep with scree, but others only have a thin layer over hard pavement, where the ash acts like ball-bearings, and the edges of the gulleys often have razor-sharp projections at mid-shin height. Furthermore, some stratified layers of ash are solid enough to bear weight, others are loose and will break away from the mountain, hazardous to those descending below you, and a few crumble to ash when touched; selecting suitable holds is very tiresome, as sometimes the holds only crumble and break away when full body weight is applied.
The nature of the geology of this unique mountain means the landscape is constantly changing. The sparse vegetation means the slippery scree slopes extend almost the entire length of the route, interspersed with random patches of hard pavement. Each eruption can completely alter the route taken, and the challenges faced, meaning that no two climbs are the same.