It’s one of only a handful of active volcanoes in Africa, and one of a kind on Earth: Mt. Ol Doinyo Lengai, the Mountain of God. The moment I heard about this spectacular carbonatite volcano I committed myself to go.“Don’t do it.” My friend advised flatly. “It was the scariest, most horrible climb I’ve ever done. I’d climb Kilimanjaro again, but not that.”

It was a week before I intended to travel to the north of Tanzania to see some more of the country, having spent two months Dar es Salaam for an internship as part of my Master’s studies. I couldn’t wait to see the wonders of the Great Rift Valley, and had visited the National Museum to brush up on my knowledge of the early hominin discoveries that were made there. Of all the possible things to do around Arusha and Moshi however, it was Ol Doinyo Lengai that I was drawn to and began to research.

The volcano is located in the Gregory Rift, south of the alkaline Lake Natron and forms part of the volcanic system of the East African Rift. What makes it so unique is its production of natrocarbonatite lava. Most lavas are rich in silicate minerals, but this carbonatite lava is rich in nyerereite and gregoryite, rare sodium and potassium carbonates. Carbonates are a group of minerals that, most of the time, form in the ocean, coming gently out of solution in water. Due to this composition, when it erupts the lava is only at a temperature of approximately 510° C, and appears black in sunlight, lacking the red glow of higher temperature lavas (e.g. basaltic flows at around 1100° C). It is also much less viscous than silicate lavas, and has been reported to flow faster than a river down the side of the mountain.

What geochemists find so interesting about Ol Doinyo Lengai is that its lavas have almost no silicon in them — none of the familiar silicate minerals found in ordinary lava, such as olivine, pyroxene and feldspar. On this planet, finding an igneous rock without silicon in it is highly unusual. One explanation could be that the carbonatite must be some sort of melted limestone, created as a body of magma rises into a buried carbonate deposit below the volcano. However, geochemical studies have disproved this, due to the high concentrations of rare-earth elements and other constituents, in the carbonatite that have no part in true limestone.

It is thought instead that the carbonatite somehow separates out of silicate magmas by differentiation under the right conditions of temperature, pressure, and chemistry. Such conditions can be met where an old, thick continent begins to crack apart and the rocks beneath it begin to melt, such as in the African Great Rift Valley. Recent research appears to show that the rocks in the mantle beneath Ol Doinyo Lengai are no different from mantle rocks elsewhere. A possible reason for its unique composition that only a tiny amount of partial melting is allowed there, so only the most highly volatile components are able to become magma, eventually yielding carbonatite.

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 Ol Doinyo Lengai is the only known active carbonatite volcano in the world.

Records of eruptions have been maintained since 1883; the last major eruption was in 2008. Prior to this the crater of the volcano was accessible, containing a number of small lava cones, but the explosive eruption created a deep caldera that cannot be reached by climbers.

After a few days in Arusha I found a company that would take me to climb the volcano. We set out early and drove along a badly maintained dirt road into the wilderness; this was not a heavily frequented part of the famous northern tourist circuit. Every so often, we spotted giraffes ambling majestically across the plains in the distance.

We set up camp in the shadow of the volcano as the sun set; we would begin the ascent at 12pm. I tried to get a few hours of sleep before the climb, but I was too excited. At 11pm we set out across the lava covered landscape towards the foot of the volcano. I was accompanied by a guide from the tour company and a local Masai guide. The Masai are a Nilotic ethnic group of semi-nomadic people who occupy southern Kenya and northern Tanzania. Some of their villages are located around the volcano, and their guides have a deep knowledge of the mountain.

In the light of the stars the landscape had an eerie, lunar like quality and all I could see of the mountain was a looming black mass that blocked out the pinpricks of light. We pulled up at a section of the ‘road’ that was impassable, and started to get ready. A head torch, essential for the climb; gators, to help wade through the ash on the lower slopes of the mountain; water and Snickers bars; hiking poles; and plenty of layers – it would be cold at the top. Behind us I could see the headlights of 4x4s struggling across the mottled landscape; we would not be alone on the volcano tonight.

We set off at a punishing pace, gaining elevation quickly. The lower, fertile slopes were covered in elephant grass, but as we ascended, the slope steepened drastically and large crevasses appeared on either side of our path. They were too deep for my torch to penetrate the darkness. With a slope of nearly 45 degrees the climb quickly turned into a scramble, and drifts of ash past my ankles made the going tough. We had to follow the Masai guide closely here, as where the layer of ash was thin it acted like ball bearings on the underlying rock and it was much safer to walk in the thicker layers. The rock was grey and white, and often crumbled with a touch. We soon had to start relying on handholds from the rock to help pull ourselves upwards. Sometimes my torch caught the flash of what I thought was a piece of mica, a reflective mineral which breaks easily in sheets. Looking down, I could see the glimmer of lights from three other climbers far below, and the silhouette of the mountain against the stars.

While it was certainly difficult, I was finding the climb very enjoyable, and was quite surprised when we reached a small platform about four hours in, where the guide said we would rest before the final hour of the ascent, as it was going to be steeper and tougher. It was 4am, so we would sleep for an hour and start out again at five to catch the sunrise at the summit. I sat against the wall of the platform and looked out at the stars, truly a wonderful sight. This was the first time I smelled the rotten eggs odour of sulphur. I put on all of my layers; the wind was really chilling and unfortunately I hadn’t brought a sleeping bag or blanket. I didn’t sleep for the hour but instead spent it shivering; other climbers soon joined us in the hollow – a Russian, two Ukrainians and two Americans, along with their Masai guide. I made no comment on the geopolitics of our group.

When we set off I made sure to get in front with my guide, I was excited to reach the top. It was incredible how he could climb so easily and confidently while the rest of us were scrambling for hand and footholds. The slope was greater than 45 degrees now, and the small amount of climbing experience I have was coming in very handy. The character of the rocks had changed too, now it was a white and grey pavement, studded with large black minerals and more difficult to break. There were lots of loose rocks however and as we you climbed you had to be careful of sending them tumbling on the people below. As we climbed towards the summit, the smell of sulphur got stronger, and some of the pavement was hollow to walk on. Two large white buttresses started to emerge from the gloom; these are called 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. They signalled the summit. We couldn’t go the usual way to the rim of the crater as one of the Masai had recently seen a substantial lava flow and quite a few cracks in the hillside in that area. We turned left after the ‘gates’ and climbed a steeper hill on the side, and finally reached the rim of the crater. At this stage it was very windy and there wasn’t much space from the rim to the crater below. As the sun began to rise I could see we were above the clouds and just how steep the last part of the ascent had been, so unfortunately fear took over my curiosity to see more of the crater. It was however an incredible sight. The far side of the rim had been blasted away from a previous eruption, and numerous cracks were visible, some emitting white steam.

As we descended the clouds receded and the wall of the rift valley was visible far below, running towards the horizon with the plains of the Serengeti beside it. I spent most of the descent on my bum, sliding slowly downwards to keep from falling. Four painful hours later we returned to our starting point. I looked back in awe at the mountain. The Masai people once regarded the mountain as a God, and made sacrifices on its slopes to appease it and deter eruptions. I can fully appreciate why. The composition and eruptive attributes of Ol Doinyo Lengai remain somewhat of a mystery to geologists all over the world, and climbing it was an incredible adventure that I won’t soon forget.

By Gina Kelly

Earth Science Graduate, Trinity College Dublin 2013

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