Have you seen the Mountains of the Moon? There are several ways to answer this question. The most common answer will be, yes, and I loved the movie. Less common would be, yes, I just got my new telescope and it was the first thing I looked at. And the least common response would be, yes, it was brilliant but I got malaria.
The Nile has always fascinated people. It is not a large river (in spite of being the longest river in the world), but it flows in an utterly barren landscape. It is a reliable source of water where everything else is bone dry. The floods are so regular that you can set the calendar by it. The Nile made Egypt habitable. It provided essential water in a complete desert (albeit a few thousand years ago the Sahara was not quite as dry as it is now) and the annual floods fertilized the fields. The word ‘Nile’ is semitic: ‘nahal’ means valley. The Egyptians called the river ‘Ar’, meaning black: this is the colour of its flood sediments. But in such a parched land, where could the water come from? Pliny the Elder thought that the river originated underground. Herodotus thought the river came from the western side of Africa, perhaps assuming a connection to the Niger. Later writers assumed that the Nile started in Ethiopia, and was fed either by the heavy summer rains there or by snow melt. (The sediment indeed mainly comes from the Ethiopian plateau.) They speculated about an unknown, high mountain, in an unexplored area, which was the true source of the Nile. But it was impossible to find this mountain. The Nile valley becomes impassable in south Sudan, where it widens into a wide swamp. Anything south of that remained a true terra incognito. There were stories, but they relied on hear-say.
Ptolemy’s Geographia, written around 150 AD, tells the story. It wasn’t his story: he copied the details from an earlier manuscript, an atlas by the geographer Marinus of Tyre which no longer exists. (Ptolemy did the same with his star atlas, the Almagest, where the data came from Hipparchos and was centuries out of date by Ptolemy’s time — which he did not notice.) Ptolemy was not an explorer, and was interested in math more than in geography. Much of his Geographia is about devising a system of coordinates for mapping the round world – we might call him a geo-mathician. For the data and descriptions he turned to others. Ptolemy wrote: Marinus the Tyrian is known to have found out many things that were not known before. He has searched diligently the works of almost all the historians who preceded him. That was exactly what Ptolemy needed.
According to Ptolemy, Marinus tells a story he had heard from a traveling merchant named Diogenes. (This makes his story third-hand! It was just like an ancient Facebook.) That merchant-traveler was sailing along the East African coast around 110 AD, when he was blown off course and landed in a town called Rhapta. The name has not survived and the location of this town is unknown. Traveling 28 days in-land from there, by accident he discovered the source of the Nile: a pair of lakes fed by snow melt pouring off of what he said the locals called the ‘Lunae Montes’: the Mountains of the Moon. The name was too good not to be true, and Ptolemy’s report, like an ancient twitter, went viral.
The story is confused, and Ptolemy in one place says that Marinus put the lakes near the coast, and elsewhere in his manuscript places them far in-land, based on information from other travelers. The coordinates he gives are pretty much in the centre of Africa. Ptolemy’s map of Africa (which was drawn many centuries later, based on his list of coordinates) shows the Nile flowing from two parallel lakes fed by waters from mountains. On later maps these lakes acquired the names of Zaire lacus (Lake Zaire) and Zaflan lacus (Lake Zaflan). Sometimes they were shown side by side; sometimes they were shown in sequence. But which lakes they were was lost in time. So was the identity of the mountains. Ptolemy at one place describes it as a ‘great snow mountain’, and this is generally thought to be Kilimanjaro. If Diogenes story is true, he may well have meant Kilimanjaro as it is not impossibly far from the coast. More likely seems that Ptolemy or Marinus amalgamated several stories about the Nile, the lakes and the mountain. No one managed to repeat the exploits, and find the lakes and mountains.
The story grew into that of a mysterious mountain range that existed somewhere in Africa. Even in the 18th century, maps of Africa still showed the mythical mountains and their lakes, in various different locations – the African version of ‘Here be dragons’. An example is the Münster map of 1554. Münster, a professor of Hebrew at Basel, was the first mapmaker to print separate maps of the four then known continents. His map contains some dubious features: a one-eyed giant over Nigeria and Cameroon which represents the mythical tribe of the “Monoculi”; a dense forest located in the Sahara Desert, an elephant filling much of southern Africa. The Niger River begins and ends in lakes. Clearly, the map maker used some non-fact-checked resources. Even Madagascar is missing. And he depicts the source of the Nile as two lakes, fed by waters from a small mountain range: the fabled Mountains of the Moon.
The mythical Mountains of the Moon were sometimes shown dissecting the entire continent, sometimes more localized. They were used as an impassable barrier, used to cordon off the parts of the continent that were unknown. Though no one had seen them, they remained an eye-catching legend.
Until the 1850s, when Richard Burton and John Speke traced the origins of the White Nile to Lake Victoria. Kilimanjaro is 300 km from this lake, and no river flows in between. But 200 km to the northwest lies a small but spectacular mountain range, snow-capped and located between two lakes. These are the Rwenzori mountains. Burton and Speke fell out badly about whether Lake Victoria or the Rwenzori Mountains should be considered as the true source of the Nile. We now recognize that the Nile has many sources. But only one mountain range is seen as the real Mountains of the Moon: the Rwenzori’s, the snowy mountains of the tropics.
Although revealed, they still remain unknown. People have heard the old name, but it still sounds mythical and evocative. This is for a reason, as traveling in these mountains can be very difficult. Some of the landscapes are out of this world. But they might as well be on the moon.
The Rwenzori mountains run along the eastern edge of the Albertine rift, in between Lake Albert to the north and Lake Edward to the south. The mountain range is small, only 120 km long and 65 km wide, but it packs a punch. The tallest peak is called Mount Stanley, in an echo of colonial times. At 5109 meters high it is the 3rd highest mountain in Africa, and the highest peak in both Uganda and the Democratic Republic of the Congo (the border runs over the summit). There are 6 separate ranges, with deep gorges in between.
The explorer Stanley gave them the name Ruwenzori; it is based on what he said was the local name. It seems to originate from Swahili, ‘rwe nzururu’ meaning ‘place of snow’. The spelling has recently changed from ‘Ruwenzori’ to ‘Rwenzori’.
There is something funny here. You would expect a mountain range to form where two tectonic plates collide. If no such plates are available, volcanic activity may still throw up decent mountains in unexpected places. But here, neither is the case. There is no plate collision, and although there are fascinating volcanoes here (more about that later), they are not in the mountains. And the impressive height shows that these mountains are young. They are not a remnant of some continental merger a long time ago. The Rwenzori mountains are the highest non-volcanic, non-orogenic mountains in the world! They are mountains without a cause. Perhaps they did come from the moon.
What are these mountains like? The west side of the range is especially steep, rising from Rift valley. The east side is a bit more sedate. The rivers runs in deeply incised, densely vegetated valleys. The views are spectacular, but are rare. The highest peaks are normally hidden in the clouds, and rain is daily. There are tours you can take. The western side is difficult to access: it is located in the DRC with expensive visas, little tourism and dubious security. The eastern, Ugandan side is better equipped. The local population provide guiding services. You would be ill-advised to go on your own — The guide is not a luxury! The tours are mainly for climbers. You need to be fit, able to walk for a week while climbing many kilometers, equipped to deal with rain, mud, hail, and snow. To climb the peaks, crampons are essential. Everything you bring should be water proof. You will come back exhausted, thrilled, wondering what happened to the vistas, and longing for dry warm weather. You will have seen few people other than those in your group. And you will be very grateful to your guide.
In May 2020, many of the trails were damaged in land slides and flooding. The local population has been rebuilding them, waiting for the tourism to restart.
Afterwards, you may want to visit the other attractions in the regions. Chimps in Kibala forest, volcanic lakes near Forth Portal, gorillas on the far side of Lake Edward. There will be many more tourists there – but they won’t have seen what you have seen. They won’t have been to the moon.
The African Rift
Africa has a problem. It has long been losing parts to the Indian Ocean. This problem dates back all the way to Gondwana. When Africa split from Gondwana, it formed the Indian Ocean. This split happened some way east of the current edge of the continent. India remained part of Africa, but some time later it rifted off, and sailed to Asia. The sharp edge where it split off is still visible on the African coast line. Next came a rift further south, allowing Madagascar to move off.
The third rift formed north, and it split off Arabia. Arabia does not have much room to move and has remained close, but deeply non-attached. The split started as a triple junction from a hot spot in Afar, Ethiopia. Since that time, the southerly rift from the triple point has slowly grown its way further south. It has formed the famous African Rift valley. Standing on its edge and peering into the verdant green valley below is quite an experience. It is threatening yet another separation, this time sending Somalia into the Indian Ocean as yet another floating micro continent.
The rift has run into its own problems. Tanzania refused access, and the African rift valley instead developed two branches, going around it on either side. The two are in competition. The eastern branch runs through Kenya, close to Nairobi before petering out. It has some nice volcanism, and it is famous for its soda lakes (think flamingos). The western branch begins in Tanzania, runs along the Uganda-Congo border, through Malawi and ends in Mozambique. It is famous for the Rift lakes. The northern part of the western branch is called the Albertine rift, and this is where the Rwenzori mountains are found.
The rift reality is more complex than a simple story, of course. The African rift is far from complete. Neither the eastern nor the western branch line up well with the Ethiopian rift. The eastern branch goes nowhere. The western branch has gaps and offsets, and is interrupted by the Virunga bulge which contains Africa’s best (but not tallest) volcanoes. There may even be a third branch, off-shore between Africa and Madagascar but this is not as well defined.
What has caused the African Rift valley? Afar was a significant mantle plume. It shows the typical triple-point pattern, and the Ethiopian rift is clearly associated with it. But there is limited evidence that the Afar hot spot is responsible for the entire Rift valley. It is just too far. A separate plume has been postulated to be underneath Kenya and Tanzania, responsible for the southern extension. The uplift of the region into a large bulge shows that there is heat underneath. But there is no obvious triple point. The earlier rifts in East Africa long predate Afar. And there are scars of other, failed rifts across the continent. They seem too numerous to all blame on mantle plumes. And there is a lack of hot spot tracks which would be a clear sign of mantle plumes.
The Virunga volcanism has been argued to be another hot spot. But this story too might be too simple. Once a rift forms, it can allow for upwelling which brings further heat: this could cause the volcanism there. Virunga is just one aspect of mantle warmth across a large area. The volcanics in Kenya sit on a bigger bulge than Virunga and would be a better candidate for the location of a plume. Perhaps this is all a long-lasting thermal anomaly, a lingering remnant of the previous continental splits.
Taking the long view, the African rift is indeed just the final part of the breaking-up of Gondwana. The present is just the current state of the past. That would also be a much more African way of thinking about life. And indeed, some parts of the African rift may be a re-activation of the ancient faults that tore Africa apart in the past. For instance, the third branch between Africa and Madagascar is at the same location where the old rift that rifted Madagascar should be hiding.
The Making of the Mountains of the Moon
The Albertine rift is some 13 million years old. The Rwenzori mountains are much younger. They were uplifted within the past 5 million years. The height is due to their youth, and the ruggedness is caused by the extensive glaciation.
But how did such a small range become uplifted in an area of rifting and subsidence? No continental collisions here! The Rwenzori mountains are almost on the shoulder of the Rift valley, and their origin clearly lies, somehow, in the rift.
The Albertine rift contains a patch work of tectonic faults. The west side of the mountains is bordered by the Bwamba border fault, with a step-over connected by the Lamya fault. The east side is bordered by the Rumi-Wasa fault and the Nyamwamba fault. Other faults run through the mountain range. It is not surprising that this is the most earthquake-prone region in Uganda.
The Rwenzori mountains fill in a substantial area of the Rift between Lake Albert and Lake Edward. Before the mountains formed, the two lakes were joined together. The Rift valley now bends around the mountains, to such a degree that the Bwamba border fault runs on the west side of the Rift valley close to Lake Albert, but becomes the east side along the Rwenzori mountains. Bwamba is the fault that accommodated the uplift – more than 4 km between the bottom of the valley and the highest peaks in the central area of the Rwenzori mountains.
The highest peaks of the Rwenzori mountains are confined to a small region, bordered by the Lamya step-over fault. They seem a bit off-centre. If you draw a line along the ridge of the mountains, the central area is offset to the west. The model is that the entire range was uplifted slowly over the past 5 million years (and possibly starting earlier), but this central area had a rapid separate uplift, relatively recently, and a bit of clockwise rotation.
The whole structure is a horst, where the sinking blocks on either side push up the block in between. This is a common process, but here in a small area, the uplift became extreme. There are various explanations in the literature and a consensus has not yet been reached. Yet another fault has been postulated running through the central range of the Rwenzori mountains, with the accelerated uplift only occuring on the west side of this fault. In this model, the development of this new fault caused the extreme uplift, but only in a small area. Where did the fault come from? Accusing glances are directed at the unassuming Lake George, south of the mountains and outside of the Rift valley. This lake is considered a failed rift, a branch that didn’t make it. The Rwenzori mountains were caught between the two. The interaction caused the mountains to rotate clockwise, strongly enhancing the stress, and up the mountains went.
This is a mountain on the move, reaching for the stars – or at least for the moon.
Volcanoes in the mist
Although the mountains of the moon are not volcanic, there are several volcanic fields around it. Three adjacent fields are located near Fort Portal: The Fort Portal field just north of the town, the small Kasekere field northeast of it, and the larger Ndale-Kasenda Volcanic Field about 20 km to the south of the town.
The Fort Portal field contains around 50 monogenetic tuff cones, nine of which have crater lakes. The largest lake is Kyaninga, some 3 km northeast of the town. Most of the eruptions produced tuff and ash, but the Kalyango cone, west of the town, is the source of a 5-meter thick, 0.3 km2 lava flow. The tuff and lava are carbonitite, consisting for 30% of CaO, 5% MgO, 15% CO2, and mostly less than 20% SiO2. The phosphor content is notable, with 2-3% P2O5. The cones look young and this has been confirmed by C14 dating: the eruptions took place 4000 to 6000 years ago. Fossil wood embedded in one tuff layer has been dated to 4070 ± 120 BP. The cones and craters formed first. One or more explosions produced a 2-meter thick carbonatite tuff layer covering 140 km2; it has a volume of 0.25 km3. The lava flow was the final event in the eruption sequence.
The Ndali-Kasenda Volcanic Field (also called Kyatwe) is about 20 km south of Fort Portal. It contains some 60 steep-walled crater lakes. A typical case is lake Kifuruka, around 5 meters deep, 200 meters across, and surrounded by weathered lava soils. Other lakes are Lake Lyantonde, lake Nyabikere and Lake Nyinambuga. Not all lakes survive as lakes: for instance Kabala swamp is a filled-in lake, 100 meter above the surrounding area. C14 dating of a core taken inside the swamp indicate that it formed 20,000 years ago and is considerable older than the Fort Portal volcanoes.
Immediately south of the Rwenzora mountains lies the largest volcanic area of the region, the Katwe-Kikorongo Volcanic Field. This is a rare example of a volcano on the equator! The northern edge of the field just touches the equator. It lies between Lake Edward and Lake George, covers 200 km2 and includes a large number of craters (78 maars, according to those-who-know, i.e. Volcano World). But the ground is so densely covered with the explosion craters that many have been obliterated by later events. In the densest area, I counted 20 visible craters within 10 km2. Perhaps this is the densest volcanic field in the world! Those-who-know describe the area as resembling ‘the surface of the moon’ – quaintly appropriate, next to the mythical Mountains of the Moon.
The largest crater is 3 km across. There is not much lava, just four small flows with a combined volume of 0.03 km3, within the Kitigata and Kyemengo crater areas. The age is not well determined. Local folklore suggests historical eruptions, but this should be treated with considerable caution. As elsewhere in the region, the main fault on which the craters lie runs SW to NE. The pyroclastics contain carbonate, and the rocks in this region are potassic to ultra-potassic in composition.
Finally, 20 km southeast of Katwe-Kikorongo is the Bunyaruguru volcanic field, immediately to the south of Lake George. It contains more than 130 craters, many only faintly visible but 27 of which still contain lakes.
There are two unusual aspects about the volcanism: the monogenetic, clustered explosions, and the carbonitite. The monogenetic events themselves seem typical for water interacting with a high heat flow, and do not involve any large magma chambers. The heat may percolate up through faults: the faults here trend SW-NE and that is also the line of activity at Fort Portal. The events are mostly phreatic. Actual lava is rare. So that is explainable. But why the carbonitite?
Carbonitite comes from melted carbonate. It is known at over 500 locations on Earth, of which only Ol Doinyo in East Africa is active. It melts at very low temperature, of 500 C. Carbonate contains CO2, so it is very gaseous and as a lava becomes very fluid. It can flow like water. The presence of carbonate can therefore trigger volcanism where other rocks would not. If it weren’t for the carbonate, perhaps neither Ol Doinyo nor the region around Rwenzori would be volcanically active. These are regions of higher heat flow, but not enough for a decent melt.
Carbonitite contains a mixture of Na2O and CaO (together with the CO2). The lava from Ol Doinyo are particularly sodium rich. Sodium rapidly leaches out of the lava and within days to months, and the solid lava becomes dominated by CaO instead. This may have happened at Rwenzori as well.
But where does the carbonitite come from? The obvious explanation is that it is melted limestone. But that does not seem to be the case. Instead, the magma appears to come from the mantle. A plausible model is that it involves subducted oceanic plates, at the transition zone of the upper end deep mantle at a depth of some 400 km. The low melt fraction leaves the silicate rock untouched but melts the carbonates. The melts percolates up to the bottom of the continental crust (or lithosphere). The African rift has broken the thick continent, and this has allowed the material to come up from the bottom of the crust. The low melt fraction implies that there is no strong plume involved, and in fact Uganda lacks the vigorous volcanism found further south and north along the Rift valley. This is true for both branches of the Rift.
An interesting aside is that carbonitite contains half the rare earth deposits in the world! The association between carbonitite and rare earth elements is seen in many places: a good example is Mountain Pass mine in California, at one time the main source of rare-earth elements across the world. The rare earth elements are likely from the same source as the carbonate: marine sediments that are subducted deep below the continent. The rare earth elements readily dissolve into CO2-rich fluids, and come up with these fluids to become part of the continental crust.
Feeding the Nile
Rwenzori’s other local name is Gambaragara, which means (approximately) ‘place of boiling clouds’ . It is often translated as ‘rain maker’. Both names, ‘place of snow’ and ‘place of clouds’ refer to precipitation! The prevailing wind comes from the east, the Indian Ocean. The 5 km high mountain chain forces the air upward. As the air cools, the moisture condenses and the rains begin. Annual rainfall is 2-3 meters. The climate officially has two dry seasons, one around January and the other around July, but even in those months it may still rain on most days. April and November are particularly wet, as the low pressure of the Inter-Tropical Convergence Zone passes the area.
Around the summit the rain falls as snow. The Rwenzori mountains carry the largest glaciers of Africa – in spite of being almost on the equator! Good views of the glaciers are uncommon because of the frequent cloud cover.
The glaciers are now in rapid retreat. In the 19th century they covered 10 km2, but by 1990 only 1.7 km2 was left and by 2003 it was reduced to 1 km2. At that time it was expected that the glaciers would be gone by 2025. Of the 6 glaciated mountains, at the moment only Mount Speke (4890 m) and Stanley (5109 m) still carry glaciers. The glaciers on Mount Speke are small, but the ones on Mount Stanley are still substantial, with the longest (on the Stanley Plateau) over 1 km long. The decline is due to three factors: the recovery from the Little Ice Age since 1850, the change in rainfall, and in recent decades the global warming.
The snow cover makes the Rwenzori mountains identified as the fabled Mountains of the Moon. But are they really the source of the Nile?
Rivers in the Rwenzori mountains are fast and furious, at least high up. The hard rock is impermeable and the never-ending rains runs off immediately. The rivers that run west end up in Lake Edward or in the Semiliki river, both heading for Lake Albert. The rivers on the east side mostly end up in Lake George: this lake gets half of its water from the Rwenzori mountains. Lake George empties into Lake Edward and on into Lake Albert. Lake Albert in total receives some 2.5 billion cubic meters of water per year through these three routes.
Lake Albert flows into the White Nile. But so does Lake Victoria. The total flow rate of the White Nile south of Lake Albert is 33 billion cubic meters per year (half of which is lost to evaporation in the swamps land of South Sudan). The Rwenzori mountains contribute less than 10% of that. And other rivers contribute to Nile, especially the Sobat river and the Blue Nile. Inn total, the Rwenzori mountains contribute about 3-4 per cent of the water in the Nile. The Mountains of the Moon they may be, but they are only a minor source of the Nile.
Between glaciers and savanna
In spite of its mythical status and extraordinary height, this is a small range, no more than 50 km wide. That gives an immense variety in climate over small distances, from the tropical Rift valley to the icy glaciers. The plant diversity has to be seen to be believed.
It begins with African savannah, down in the valley. Above 1000 meters this gives way to rain forest (and boy does it rain!) If you like wild bananas (pretty inedible), this is for you. Above 1500 meters, the grass returns but in an unexpected form. Here grows bamboo (a type of grass, after all) and forms a 30 meter tall forest (Sinarundinaria alpina). Above 3km, where nights may bring frost, there is heather, but not like you know it. The tree heather (Erica kingaensis) grows into ten meter tall, moss-covered trees, as a fairy-tale (but open) forest. And finally, at 3.5 km the alpines rule again, as giants. Giant, 10-meter tall lobelia (Lobelia stuhlmannii) are found here, and even sun flowers (Helichrysum stuhlmannii) and groundsel (succulent daisies: Dendrosenecio adnivalis and Dendrosenecio friesiorum) have evolved into trees in an other-worldly landscape. They grow in the (reportedly) muddiest place on earth where slipping on tussock grass (Carex runssoroensis) can leave you chest-deep in mud. It slowly gives way to a barren landscape with lichen covered rocks, before finally the snow and glaciers rule – 100 km from the equator.
And it is not only the plants. Some animals here are found nowhere else on earth, including the Rwenzori Leopard (Panthera pardus ruwenzorii, black-spotted and now very rare) and the Rwenzori Turaco (Ruwenzoronnis Johnstoni). 19 bird species are endemic.
But in spite of this variety, the number of species is actually significantly smaller than in other national parks, and the alpine region has half the species found on Mount Kenya. The flora has even been described as ‘impoverished’. On the other hand, a record number of those are endemics. This is probably due to the geographical isolation. This mountain range is young, came from nowhere and is not in contact with any similar mountains. Alpine plants cannot migrate easily in Africa. It has been suggested that there were no high altitude plants in Africa until about 20 million years ago when a land bridge formed with Europe. Tropical plants don’t easily evolve to handle frost.
To go where none have gone before
Rwenzori is beautiful, but not easy. The mountains are steep, high and very very wet. Walking here will make you understand the meaning of ‘bogged down’. The rain seems perpetual and vistas either far between or fully absent. Kilimanjaro, a single volcano, is visited by 50 times as many people as this mountain range. It doesn’t help, of course, that the western side is in the DRC, a country not known for tourism facilities. And if the highest non-volcanic , non-collision mountain range in the world where heather grows into trees is not enough, the densest volcano crater collection in the world is just next door. The Mountains of the Moon are more than the source of the Nile. They are out of this world.
Albert, February 2021
And for those with access to BBC content, Part 2 of David Attenborough’s Africa ‘Savanna'(2013) covers the Rwenzori mountains with lots of pictures of clouds. As of writing, this is available on BBC’s iplayer.