Nyiragongo is one of my favourite volcanoes and one of the most peculiar and most unusual of all active volcanoes on this planet. It’s also a very problematic volcano, close to a huge population center that is rapidly growing, and there is a lot of risks associated with its volcanism as well as with human-caused political issues and other hazards. I first got to know Nyiragongo as a 6 year old from the TV reports of the catastrophic 2002 eruption, when the magma column burst out into Goma city. I remember lots of people running from fast-moving a’a flows, and the burned shells of cars and busses floating in the river of molten rock. I remember hot fingers of pahoehoe dripping into the Kivu lake on the low quality TV image of the early 2000s. I developed an interest in Nyiragongo and its peculiarities. It is an interesting volcano in many ways. For many many years, I was completely alone with my volcano addiction, but with the invention of Volcanocafe Blog it is finally possible to put my volcano thoughts permanently on the internet. This will be my first post for Volcanocafe. Mt Nyiragongo is very unusual in many ways which is one reason to write about it.
Nyiragongo has a very unusual magmatic composition, basically one of the rarest magmas on the planet, and that is another reason that I write this article: I have an interest in highly ultrabasic magmatism.
Nyiragongo is also a very beautiful volcanic cone, eye stunning. That is also why I write about this volcano: Nyiragongo must be what most people imagine a volcano should be like – deadly gas pockets, hot fast lava, a lava lake, and a beautiful steep conical profile rising out of the jungle. Its fiery glow casts an eery shine over the night jungle far far away in a ”dark, poorly known” mysterious land. It is as stereotypical as a volcano can be on Earth. It is just like every child imagines what an active volcano would look like. This volcano is infamous for its fast-flowing lava and its unusual composition and is claimed to have the lowest viscosity of all silica-based lavas. In this article, I will have a close look at that. I myself have always been impressed by Nyiragongo’s perfect look, and perhaps only Shishaldin, Pavlof and Villaricca are even more perfect and stereotypical as steep mafic composition volcanic cones. Most people imagine volcanoes to look like Nyiragongo and the other examples, but that is clearly not the case with most volcanoes.
Nyiragongo is a relatively unknown volcano, and actually, despite frequent geological investigations since late 2000 it is not very well-investigated, but it is fun trying to write an article about a relatively unknown volcano. In other words, it is a very difficult volcano to write about. In this article, I will have a look at Nyiragongo and its peculiarities, like its viscosity and why it is so steep despite its composition and general geology.
Nyiragongo’s geological setting and a look at African continental rifting in general
Nyiragongo is a continental rift volcano, like a majority of the African volcanoes are. Here I will give a summary of what powers Nyiragongo. The volcano is in the slow-spreading Albertine Rift, the western arm of the great African rift system where Nyiragongo and Nyiramuragira form a very active volcanic center. Continental Rifting is divergent tectonics in the interior of large continents. There are many continental rifts in the world, some are very active and some are almost passive spreaders. Some are boosted by mantle plumes while other rifts are not. Most of the Albertine Rift is quite passive, with little volcanism. In places, the spreading has formed very deep almost Baikal-like rift lakes, as there is a lack of volcanic in-fillings. When you spread apart the continental lithosphere the warm asthenosphere will move upwards, since it is generally above its melting point it will undergo decompression melting and generate rift magmas. In the Albertine Rift, it seems to be relatively passive, but when we come to Virunga area, we have intense volcanic activity at only Nyiragongo and Nyiramuragira.
The Virunga volcanoes are Africa’s most active volcanoes and are the only place in Africa other than Erta Ale that are intensely and frequently active. The intense local activity in Virunga has given many geologists the idea that there is a local hotspot in that part of the Albertine Rift, an emergent mantle plume that just started to decompression-melt. There is a large local round uplift that has elevated the whole area to just over 1 kilometers above sea level. This ”lithospheric dome” seems to confirm the mantle plume hotspot idea in the Virunga area.
Volcanism in Virunga became intense only very recently. The Nyiragongo and Nyiramuragira volcanoes just started forming in the late Pleistocene. Nyiramuragira is thought to be 20 000 years old and Nyiragongo is between 15,000 and 12,000 years old. The young age of these two volcanoes makes the region very productive volcanically, perhaps the third most productive in the world after Iceland and Hawaii. The two volcanoes are built on a much older metamorphic basement and perhaps emerged as two islands in the deep rift lakes of Africa. Another theory from geologists is that the two volcanoes are fed by an arm from the African Superplume, the large mantle plume body that governs the East African Rift far east of Nyiragongo. The plume found its way up in the younger orogenic seams between the cratons, and initiated volcanism. Virunga magmas are all highly alkaline and many like Nyiragongo are very silica undersaturated. Such magmas form deep down by small amounts of partial melting of the mantle rock, so perhaps the emergent mantle plume is heating rock in the region which has just started to melt. In many geological textbooks, classic examples of continental rifting and break-up start with the arrival of a large mantle plume, doming of the lithosphere and an initial flood basalt. That may be the case with Pangea break-ups and the arrival of the Afar Plume far to the north of Virunga. But at Virunga it seems that the plume has only recently arrived, well after the Albertine continental rifting started 10 million years ago since the high levels of magma production at Virunga only started very recently.
Continental Rift Volcanism produces magma compositions according to the depths of melting, and the amount of melting in the mantle. It all depends on what stage the rift is in and how advanced it is in the process of becoming a real ocean. It also depends on if there is a mantle plume involved, and how thick the lithosphere is. Alkaline magmas like alkali basalts and the even more alkaline basanites, nephelinites require increasingly smaller amounts of melting deeper down the more alkaline these lavas are. Africa’s Rift System has a whole variety of magmas because of that. The rising magma also interacts with the continental crust that contaminates the mantle melts. Continental Rifting is very complex in magmatism and has a lot more magmatic variety than true oceanic ridge volcanism because of the thick continental crust.
Many continental rifts do not evolve into true oceans. The East African rift system has a chance of becoming a new ocean breaking the continent, but the future of the western part with the great rift lakes is much more uncertain. The rifting in Africa is trying to find its way between the old stable cratons. These are indeed stiff bulky behemoths to push aside, Nyiragongo’s rift is stuck between the Congo Craton and Tanzania Craton, a challenge for the Albertine Rift that is still far from being an ocean.
The geologically recent valley-forming rifting started around 25 million years ago in East Africa. Exactly what started it is unknown but is generally agreed to be convection currents in the mantle and the melting associated with the African superplume and other plumes that likely exist in the rift. There are maybe many mantle plume heads in the African rifts, even if most studies on this generally agree on a single large plume under Africa.
The Afar plume and the Virunga plume are the two sources of Africa’s most active rift volcanoes. In my mind, they look like two separate mantle plume upwellings, but they could be just places where the African Superplume sneakily found a way up between the cratons, in the younger seams between them. I myself believe that Nyiragongo and its sister feed from a separate emerging mantle plume in the Albertine Rift.
Continental Rifts are thought to be driven by convective currents in the mantle as well as extra heat from mantle plumes if they are present in a rift. Around 30 million years ago the Rift system experienced its first continental flood basalt that formed the Ethiopian Highlands. That was the result of the arrival of the Afar Plume whose remains still feed Erta Ale and other Afar volcanoes today.
In the East African Rift, the magma chemistry follows the depth of melting as the ABC figure below shows. In the northern-most parts of the rift in Ethiopia with thin continental crust, spreading has gone far and melting is shallow and amounts of melt are large, volcanoes like Erta Ale and Hayli Gubbi produce relatively normal oceanic tholeiitic basalts which is the same magma you find in Hawaii and Iceland and mid-ocean ridges. As we go south into Ethiopia the melting in the asthenosphere gets deeper, producing smaller amounts of melting. The amount of sodium and potassium increases in the magma chemistry and volcanoes like Korath range, Silali, and Olkaria are a lot more alkaline than Erta Ale to the north. The continental rifting has already gone far enough to form a well-formed rift valley and allowed the rise of quite normal basaltic lavas, but alkalinity dominates. When we travel south to Kenya, Tanzania, and Uganda, still with a clear rift valley well into Kenya, the melting plume asthenosphere runs into really thick crust and it’s partially blocked by the Tanzanian craton. Here the magmas get really strange as melting is deep down. Kenya and Tanzania are thick-crusted enough to stop the spreading from expanding southwards as it runs into the Tanzanian craton. Here you find extremely alkaline volcanoes like the snow-covered giants of Kilimanjaro, Mount Kenya, and Mount Elgon that grow into giants on the slow-moving thick crust.
The largest area of the most silica-undersaturated extrusives (lavas) in Africa occurs west of the Kenya Rift. There exist large amounts of volcanic extrusives made up of nephelinite and carbonatite lavas, and some have built some large (now inactive) volcanoes.
These large (very sleepy) central nephelinite/carbonatite volcanoes include Mount Elgon, Kadam and Napak. The extrusives of these three volcanoes consist mostly of nephelinites and melanephelinites, among which are olivine- and melilite-bearing varieties. More evolved phonolites and trachybasalts, or trachyandesites also occur among the earlier flows. These volcanoes are largely built of pyroclastics, mainly as coarse agglomerates, which are mostly composed of fragments of gas-rich nephelinite. Many smaller polygenetic volcanoes with nephelinites and carbonatites occur in that area such as Moroto, Yelele, Toror, Ruri Hills, Homa Montain, and Kisingiri. Volcanic activity in this area has long died off. It seems that Tanzania is today the most likely location for future nephelinite eruptions in the world outside Nyiragongo, where we have a very thick lithosphere with a melting asthenosphere deep down. This is the area of Ol Doinyo Lengai and other nearby less active nephelinite and carbonatite volcanoes, both polygenetic and monogenetic. But Nyiragongo in Virunga is at the moment the only active erupting Nephelinite volcano
Carbonatite volcanism appears as well at Lengai. Other sites with previous carbonatite volcanism can be found in the thicker crusted parts of the rift system. Nyiragongo chemistry lavas can also be found in many volcanoes around Kilimanjaro and Lengai. Nyiragongos own rift, the Albertine, stops just north of the Rwenzori Mountains above Lake Albert when it runs into the behemoth that is the Congo Craton. The most unusual magma composition in Africa is found in Kenya and Uganda as well as Congo. The mantle has difficulty melting here and melts in small amounts deep down giving rise to highly alkaline sodium-rich silica-undersaturated lavas and even carbonatites like the the Ndali-Kasenda Volcanic Field and Fort Portal volcanic field. The nature of Nyiragongo’s silica-undersaturated magmas I will discuss much further down in my article.
The future of the Albertine Rift is still uncertain as to how far continental rifting will go in this area, but Virunga as discussed above seems to have an emerging mantle plume. The high levels of neodymium and strontium in Nyiragongo’s magmas are identical to ancient asteroids, which suggests that the partial melting that feeds Virunga is occuring very deep down, well below the continental crust. Other great rift volcanoes with quite similar chemistry and setting as the Virunga volcanics are Lengai which is partly made of Nyiragongo-type magmas. Others are Visoke, Meru, Homa Mountain, Bufumbira volcanic field, Katunga tuff maar, Bunyaruguru volcanic field, Katawe Kikorongo volcanic field and Fort Portal volcanic field. These examples are rift volcanism on thick crust with a setting similar to figure A in this graphic https://www.nature.com/articles/s41467-021-27166-y/figures/3 . They all have highly unusual magmatism and we are going to look one such chemistry much later in this post.
At the start (A) melting is deep down and in small amounts. This produces the ultra alkaline magmas such as nephelinites, basanites, phonolites and even carbonatites that we see in Nyiragongo, Lengai, Kilimanjaro and others in slow rifts with thick lithosphere. Virunga Volcanoes and Kilimanjaro area are in this early stage of highly alkaline rifting under a deep lithosphere.
At B continental rifting and thinning of the lithosphere continues, and the asthenosphere moves closer to the surface, which results in more melting and less alkaline magmas. Shallow magma chambers form evolved melts. Alkaline basalts and evolved trachytes are common. This is typical of most of the African rift north of Kenya. A whole variety of volcanoes forms depending on supply and crustal residence in the magma chambers.
At C, the asthenosphere is getting very close to the surface, boosted by a plume head. Melting now becomes very extensive indeed and that forms normal oceanic tholeiitic basalts. Volcanism is common and often intense. Such places are Erta Ale on the Afar Triangle. At this stage alkaline volcanism has completely vanished, replaced by normal basalt, Andesite, dacite and rhyolite, because melting is extensive in the asthenosphere. The process of replacing continental lithosphere with oceanic quickly begins at this stage of rifting. Voluminous basaltic eruptions happen if a plume is involved at this stage.
In conclusion, Nyiragongo and Virunga volcanics represent the very early stage of continental rifting over a thick semi-rigid lithosphere with a mantle plume involved. Now I have explained to the Volcanocafe readers what large-scale tectonic forces are behind Nyiragongo, it is time to take a closer look at the volcano itself and its magma in the next part.
Jesper Sandberg, July 2022