In part one I took a close look at the large scale forces that drive volcanism in Africa, at continental rifting in its different stages and the types of magma chemistry that form because of it. I also discussed the origin of the forces that created Nyiragongo and Virunga in the first place. In part two I gave a broad overview of Nyiragongo, how it actually works as a volcano and the behaviour of its activity. I discussed how it got its steep cone despite being fluid and effusive. Here in part three I am getting into even more interesting things, as I will have a look at one of the things that makes Nyiragongo so famous: its magma chemistry.
A quick look at magmatic alkalinity
In magma composition Nyiragongo is almost as alkaline as you can get among silicate magmas. But what is alkalinity in igneous geology? The name is used to describe rocks that contain one or more of the alkali elements: lithium, sodium, potassium, and a few less common ones such as rubidium, with odd atomic numbers and which lose their outer electron rather easily. They are strongly reactive, especially with water, and are never found as pure elements. When a magma is alkaline it contains a lot of sodium, potassium in its mineral SiO2 molecular chains. The more sodium/potassium-rich a magma is, the more alkaline it is. Nyiragongo’s magmas are in other words full of alkali metal ions in the silicate chains rather than ”normal” silica oxides. These alkali metal oxides are mainly sodium oxides and potassium oxides, with sodium silicate oxides being the most common.
Although mineral compounds that have alkaline elements in then are quite common on the Earth’s surface, most magmas are not very rich in these elements. Low silica magmas that are mildly alkaline rich are called alkali basalt and these are quite common. Even more alkaline compositions are called basanites which are much less common. Nyiragongo is almost as alkaline as you can get. They are classified as ”ultra alkaline nephelinites”. The mineral nepheline is an alkali mineral that has chemical bonds with the element sodium in the form of sodium oxide.
Alkaline magmas also have their own evolved silica-rich equivalents. Phonolites and trachytes can be seen as alkali-rich andesites and dacites. Phonolitic Mount Erebus is an example of a volcano that produces very alkaline, silica-rich lava. In the photo below I plotted Nyiragongo’s composition at the red point at arrow, they are ”ultra – alkaline”. They also enrich strongly in other elements than only sodium and potassium. The more deficient a magma is in SiO2, the more concentrated the other elements become in the magma as well, not just sodium. CO2 content also rises rapidly with high alkalinity. The blue area on the left in the diagram are alkaline magmas, while the yellow to the right covers more ”normal” subalkaline magmas.
Alkaline magmas also have their own differentiation like basanite > tephrite > tephriphonolite > phonotephrite > phonolite, meaning that an ultrabasic magma like basanite can differentiate and turn into a phonolite which is the alkaline version of a ryholite. Non-alkaline magmas evolve this way as they get old and stale: tholeiite basalt > basaltic andesite > andesite > dacite > rhyolite. In Erebus a basanite turns into a phonolite as it cools and crystallizes. Mildly alkaline basaltic magmas are common, but basanites, tephrites, phonolites and nephelinites and melilitites are very uncommon. The more alkaline a magma is, the rarer it gets.
The most highly alkaline magmas are nephelinites, melilitites (and kimberlites) and they are also the most rare. They are supercharged with CO2 and enriched in other elements as well like magnesium oxides and ferrous oxides. We can place alkaline unevolved magmas from most common to most uncommon like this: alkaline basalt – basanite – nephelinite – melilitite, with melilitite the most uncommon. The two magmas, melilitite and nephelinite, are SiO2 undersaturated and they don’t really evolve into SiO2-rich forms in stale magma chambers, because there is a lack of SiO2 in these magmas.
As explained, removing the silica from a magma solution will increase the other elements on its expense, and that means that magmas that are as silica undersaturated as Nyiragongo are rich in stuff like phosphorus, potassium and calcium, much richer in that than ”normal” basalts. This means that Nyiragongo’s magmas are extremely fertile when they weather into soil. Africas highly alkaline volcanism are a major reason for the high productivity of African soils and biospheres.
What strange Nephelinite magmas are:
a look at Nyiragongo’s magma composition
Nyiragongo is famous for its very low silica composition. The magmas are indeed very unusual. Here I will try to explain Nyiragongo’s lava composition. I myself also have an interest in highly silica-undersaturated rare magma compositions, and a rock from this volcano is a dream for my rock collection. Nyiragongo’s lavas are the rarest extrusive rocks on Earth together with melilitites, that is one reason why this volcano is so interesting. It is impossible to write an article about this volcano without having a look on its bizarre magma composition.
Nyiragongo has the lowest silica content of all currently erupting silicate magmas on the planet. Magmas are classified according to silicate content. High silicate magmas with 70% silica are felsic. They are sometimes called acidic-like ryholites and ryhodacites. Magmas with a bit lower silicate content, 68% to 60% are called intermediate magmas, such as dacites and andesites. Low silica magmas, 46% to 50%, are called ”mafic” or ”basic”. Such magmas are often basaltic. Rare magmas with even lower silica, 36% to 45% Sio2, are called ultrabasic or ultramafic. ”Ultrabasic” magmas are also known as foiditic magmas, such as nephelinites, melililitites and leucites. ”Ultramafic” are rich in magnesium and iron, like prehistoric komatiites. Ultramafic rocks can have 50% silica and are rich in magnesium. Nyiragongo on the other hand is is ”ultrabasic”, very low in silica and is quite low in magnesium and iron as well. In short, to make it clear for the Volcanocafe readers, ultramafic and ultrabasic magmas are not the same thing. A real ultramafic magma has a very high magnesium content, and the silica content is not important for that classification. A real ultrabasic magma for classification has a very low silica content, below 42%.
Most other low silica volcanoes on Earth have magmas with silicate contents in the range of 46% to 50%. But Nyiragongo produces very silicate-undersaturated magmas with silicate contents as low as 36%. Nyiragongo’s magmas are in a group called ”foidites”, rich in sodium and alkaline minerals. Nyiragongo’s magmas are much more silica undersaturated and richer in nepheline than ordinary alkaline basalts and that is the difference from more normal mildly alkali basalts. Nyiragongo’s magma series are uncommon on Earths surface. Nyiragongo is at current the only major active wholly nephelinitic volcano. Nyiragongo’s magma is mostly made of nepheline, leucite and clinopyroxene minerals.
Nephelinite magmas are the peak of silica undersaturation and enrichment of alkaline minerals. This magma is placed in the far left upper section of geochemical TAS diagram. Nyiragongo’s magmas are full of Na2O + K2O rich minerals. The cloudy grey mineral nepheline is what gives Nyiragongo’s magmas the word ”nephelinite” and it is heavily involved in this magma composition. Nyiragongo’s magmas are the product of very small amounts of melting. Perhaps less than 3% of the whole mantle material composition has to melt to form a nephelinite magma, that is why they are so uncommon magmas. Perhaps much less than one thousand of a % of Earth’s surface extrusive rocks are foiditic. like Nyiragongos nephelinites rocks produced by very small amounts of melting in the mantle.
Ultra alkaline volcanism like Nyiragongo is mostly found in slow spreading continental rifts or in weak oceanic hotspots, where partial melting is small. Most nephelinitic volcanism is limited to monogenetic fields because of the small melt rates in the mantle, so Nyiragongo is unusually large and productive for being a nephelinite volcano. It is unknown why that is, perhaps feeding from a large area of small amounts of partial melting.
Nyiramuragira and Nyiragongo are noteworthy for their volcanic productivity despite their alkalinity. The reason the Virunga area is able to produce these large amounts of nephelinites and basanites is because the mantle under Virunga is probably melting far too deep to produce ”normal basalt lavas”. Instead we get large amounts of alkalines if you have melting very deep under high pressure. Nyiragongo is notably more alkaline and SiO2 undersaturated than her sister, so Nyiragongo has the deepest melting source among Virunga’s central volcanoes, perhaps well below 100 kilometers, making nephelinites together with melilitites one of the very deepest magmas sourced from partial melting. Nyiragongo’s nephelinites are low in SiO2 but they are as rock groundmass perhaps not as dark as highly mafic basalts.
Nephelinite is the extrusive form of ijolite magma while ijolite is the coarse grained plutonic version of a nephelinite. Both of these contains around 36% to 38% silica and have identical composition. Ijolite is an ultrabasic alkaline plutonic rock. Nyiragongo’s magma chamber complex is probably encased in ijolite complexes. This rock is a window into Nyiragongo’s depths. Ijolite is a rare plutonic rock only found in a few outcrops in the world such as the Alnö complex in Sweden. These rocks are the ”Roots of Nyiragongo”. Ijolites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes sills they are almost exclusively associated with (extinct) continental rift-related tectonic settings. They can also be found in Canary Islands.
Nyiragongo haves only 36% SiO2 so its easy for a person who is new to geology to believe it is an ultramafic lava.. But Nyiragongo is also quite low in iron and magnesium, and it is an ultrabasic rock, not ultramafic. Ultrabasic magmas haves very low silica but are also low in magnesium and iron. As an example komatiites are true ultramafic rocks which have much higher silica contents of 50% than Nyiragongo’s super-alkaline nephelinite. Nephelinites and melilitites can also have magnesium content in the ultramafic range, since their high alkalinity increases other elements at the expense of removal of SiO2 in the melt. Other true ultrabasic magmas are ultra low silica melilitites that are even more alkaline and silica undersaturated than a nephelinite. A slightly more SiO2 rich ultrabasic magma is basanite that is slightly higher in silicon and less rich in nepheline than a nephelinite is. Nyiragongo’s magmas are mostly made of alkaline foid minerals, formed by the smallest amounts of partial melting in the mantle. Many other nephelinites seem to emerge as quite cool and crystalline melts because of their low temperatures as a product of small amounts of partial melting. Lengai is mostly made of the same Nyiragongo rocks but estimated to have been erupted as crystalline mush at 900 C. Today Lengai produces carbonatites. Little is known about how other nephelinite eruptions behave outside Nyiragongo, but because they are product of such little partial melting many of them probably erupt as cool viscous Strombolian flows, but hot highly fluid examples too exist if they rise quickly from the mantle as they should do with their extreme CO2 content. Many nephelinite rocks haves large leucite crystal porphyry that can be seen in some of Nyiragongo specimens, but Nyiragongo rocks from recent times are mostly fine grained with crystals only visible with microscopes. Nephelinites are extremely gas rich compared to basalts, and most monogenetic examples are explosion maars, while rare long lived nephelinitic volcanic centers build up steep tephra stratocones, formed by paroxymal fountains and plinian eruptions. Nephelinite extrusive rocks are often found as small dykes, monogenetic cones, tuff cones and maars.
Nyiragongo’s lavas are known to have very low viscosity, which means it flows very easily, and that may have to do with Nyiragongo being much higher in temperatures than most other nephelinitic melts. Nyiragongo’s lava lake temperatures are in the range of 1100 degrees C and that combined with the very low silica content makes a very low viscosity magmatic melt. The current silicate content of Nyiragongo is in the range of 36.5 % with some earlier deposits going up in 37.6%. In summary Nyiragongo’s nephelinites are down at 36% silicon dioxide and most other more normal mafic basaltic lavas are 50 to 46% silica. The very low silica content means little polymerisation in Nyiragongo’s highly alkaline silica undersaturated melts.
Nephelinitic volcanism is very rare indeed and occurs in minimal amounts in places with continental rifting or in oceanic islands, or continental field hotspots. They prefer a deep lithosphere and small amounts of melting. The magma composition can be found outside Africa, in places like, the older postshield hawaiian islands that erupt their last dregs after they left the hotspot, in the Canary Islands, in the Eifel volcanic field, Ingakslugwat hills, Northern Cordilleran Volcanic Province and the late stage volcanics of Bermuda that rivals Nyiragongo is low silica.
Nyiragongos nephelinites are are as I described above a ”Foidite” and that is a whole class of extraordinary rare, very alkaline rich, rare silica undersaturated extrusive rocks. Extrusive rocks that can classify as foidites are nephelinite (Na-rich), leucitite (K-rich) and/or melilitite (>10% bearing of the mineral melilite). The lava currently produced by Nyriagongo is the latter. The difference between these foiditic rocks is which mineral dominates in the magma. A nephelinite is a foid that is dominated by nepheline and a leucitite is a foid magma that is dominated by the mineral leucite and so on. These are all very uncommon rocks, and found in very small amounts in the East African Rift and at other locations. The Nyiragongo cone itself is made of nephelinite but its flank cones haves a melilitite lava composition. The central volcano of Nyiragongo erupts slightly more evolved compositions than its flank cones probably because a shallow storage system.
Nyiragongo’s current (historical) magma is a melilitite – nephelinite, meaning it bears some amount of melilite minerals in the composition, but is not dominated by it. Nepheline is the dominant mineral, giving it the name nephelinite. These are not in any way ultramafic magmas as mentioned above, Nyiragongo is very low in silica but not very high in magnesium and iron either. They are ”ultrabasic” as described above. The minerals in Nyiragongo’s magma forms in a silica poor melting process, where there is not enough silica in the magmatic solution to form normal feldspar and quartz. The magmas of Nyiragongo represent the peak of silica undersaturation and enrichment in alkaline metals and alkaline minerals. The magma chemistry in other words have a lot of their silica bindings with sodium in the mineralogy.
Nyiragongo’s main cone is made of nephelinite lavas and tephras, but its monogenetic flank cones are made of even more silica poor melilitites lavas, that are even more silica undersaturated, so that could indicate that Nyiragongo’s nephelinites are an evolved melt sourced from rising melilititic magmas from deep below.
The most silica poor foidities
I have shown here that Nyiragongo is already an extreme composition, but because Nyiragongo is a well formed magma system, its nephelinites can allow to cool and evolve a little, meaning they are not fully fresh. The most silica-poor nephelinites and melilities have been erupted by monogenetic volcanism and deep eruptions without shallow storage. Older volcanoes like the old German volcanoes Vogelsberg, Urach have erupted lavas with SiO2 well below 30% with some from the Urach complex getting down to almost 20% SiO2. They are the most extremely silica undersaturated magmas on this planet, and sit well below Nyiragongo’s 36% SiO2. All of these magmas are a rarity. Even more silica poor is kimberlite, but it is not closely related to Nyiragongo’s nephelinites. Kimberlites are much more MgO rich than Nyiragongo is. The compositions of the Vogelsberg and Urach nephelinites and melilitites were provided by Hector in his own article links. It surprised me a lot and was very extraordinary that SiO2 based magmas can go that low in SiO2 content. Nyiragongo has a very low SiO2, but magmas like this can be even more silica undersaturated than Nyiragongo’s recent historical compositions. Nyiragongo is perhaps that SiO2 poor 100 kilometers down where it receives its magma from partial melting. The magma mix evolves a bit as it rises up, We can only speculate, but Nyiragongos really deep magmas are probably meliliteits as discussed above.
A CO2 rich…relationship
Magmas that are as very alkaline as Nyiragongo is are extraordinary rich in carbon dioxide, Nyiragongo and Nyiramuragira are among the largest CO2 gas producers among individual volcanoes on this planet. The CO2 production is quite notable for the locals, with hazardous gas pockets in low lying areas that can kill animals and small children. These pockets are called Mazukus or ”evil winds” in the local folk lore, and they are found everywhere around these volcanoes. Lake Kivu have been force-fed CO2 from Nyiragongo and its sister through submarine geothermal springs and now over 400 km3 of CO2 laden water exist in a volatile layer along the lake bottom, in a stable condition as long as nothing disturbs it. Alkaline magmas like that are less polymerized and the chemical makeup of the SiO2 bonds in Nephelinite allows CO2 to more easily to be dissolved into the magma. The magma can take up more CO2 gas then what a normal basaltic magma would do. The very low viscosity of this magma allows the gases to escape very easily when the magma rises towards the surface and decompresses, explaining the very high gas emissions of Nyiragongo. Magmas like Nyiragongo’s can have a CO2 content that is many 10 s to even 100 s of times higher than a normal basalt, and it explains why many ultra alkaline magmas forms explosive maars rather than lava flows despite their low silica content. The Lengai nephelinites that co–exist with its carbonatites have 300 times the CO2 amount compared to a normal mid ocean ridge basalt. In other words Nyiragongo and Lengai are probably the world’s most gas rich open vent volcanoes.
Nephelinite magmas are CO2 rich and seems to have a relationship with carbonatites as nephelinite volcanism and carbonatite volcanism almost always occurs together. That relationship can clearly be seen in the Alnö Igenous Complex, where the plutonic versions of these two extrusives can be seen together in the same outcrops, with carbonatite dykes seen in ijolite. The volcano Lengai in Tanzania is composed of a mix of nephelinite, phonolite and carbonatitic materials. Geologists today think that carbonatite is formed deep down from CO2 rich silicate magmas like nephelinites. Deep down under high pressure, the CO2 gas increases and accumulate until it reach saturation and the CO2 separate into a hot liquid, being of different density the carbonatite and nephelinite cannot mix and the magmatic system can erupt carbonatites. While carbonatite has a relationship with nephelinites, carbonatites have never been seen erupting out of Nyiragongo. The detailed genesis and mineralogy of Carbonatites are so complicated that we will not go further into that in this article series.
The plutonic version of a nephelinite magmas is called a Ijolite, same composition just coarse grained and intrusive. Ijolite is a very uncommon rock, formed by minute amounts of partial melting, and the extrusive form is even rarer on Earth’s surface. These types groups of igneous rocks makes up much less than 1% of Earths surface rocks. Its mostly made of nepheline and pyroxene minerals, the white is nepheline. Another related uncommon superalkaline plutonic rock is called urtite. Ijolites are often found together with carbonatite proving their CO2 rich relationship.
A look at some of the minerals present in Nyiragongo’s magma
Most of Nyiragongo’s nephelinite rocks are composed of nepheline, a cloudy dull grey whitish mineral. nepheline is a bit of a puzzle in many alkaline magmas. Its a silica-undersaturated aluminosilicate, Na3KAl4Si4O16. Nepheline is the major component of Nyiragongo magmas. This mineral is used in glass and ceramic manufacturing and other industries. Nepheline is the dominant feldspathoid mineral in Nyiragongo’s lavas
Another mineral that can be present in a nephelinite is leucite, another highly silica undersaturated alkaline mineral with chemical formula KAlSi2O6. It is silica-undersaturated and is composed of potassium and aluminium. Vesuvius has good examples of large leucite crystals in some of its lava flows.
Another mineral that can be present in Nyiragongo’s magmas is Melilite. The formula for common melilite is (Ca,Na)2(Al,Mg,Fe2+)[(Al,Si)SiO7]. Discovered in 1793 near Rome, it has a yellowish, greenish-brown color. The name derives from the Greek words meli (μέλι) “honey” and lithos (λίθους) “stone”. It occurs in very silica undersatuared rocks and is not a common igneous mineral.
Earlier Nyiragongo eruptions and deposits erupted melilite – leucite/olivine – melilitites lavas which is another extraordinary rare silica undersaturated alkaline rock.
Another mineral is Augite that belongs to the pyroxene family. It has the chemical composition (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. These crystals are white or ash-grey in colour, hence the name suggested by A. G. Werner in 1701, from λευκος, ‘(matt) white’. They are transparent, a little milky and glassy when fresh, albeit with a noticeably subdued ‘subvitreous’ lustre due to the low refractive index,
Nyiragongo is a very Feldspathoid mineral rich composition. These minerals are also known as foid minerals, they resemble feldspars but have a different structure and much lower silica content. They occur in these rare and unusual types of alkaline igneous rocks, and are not found in normal more silica rich subalkaline magmas. Igneous rocks rich in nepheline, leucite, melililite and devoid of silica rich minerals are called as mentioned above ”foids”.
Nyiragongo’s magma has the dominant mineral atoms, silica, sodium, aluminium and oxygen, while a normal Icelandic thoelite basalt from high melting rates haves the atoms iron, magnesium, silica, oxygen, calcium. This shows how different Nyiragongo’s ultra-alkaline magmas are compared to normal subalkaline magmas such as Icelandic and Hawaiian basalts.
The conclusion of looking at Nyiragongos magma composition is that it is a magma that is not very common, and maybe the rarest type of extrusive rock togther with melilitites and kimberlites. It is this ultrabasic compostion that makes this volcano so famous in the petrological world, and it has the record for lowest SiO2 among currently erupting silicate magmas. There are even more extraordinary SiO2 saturation among this nephelinite magma group than in Nyiragongo and the more undersaturation the rarer they get as extrusives.
Now I have explained Nyiragongo’s magma composition and explained how unusual that is compared to most other magma chemistries. In the last part, I will have a look at the behaviour of this lava and we will discuss if it really is more fluid than other low viscosity silicate magmas.
Jesper, July 2022