This is the last part of my article series, and it will be about the most complicated stuff: a look at the behaviour of Nyiragongo’s lava and how fluid it really is. In part three we had a look at how unusual its low silica magma chemistry is, and how this led to the famous media phenomena of Nyiragongo having “the fastest and most fluid lava on the planet”. In the last part of my Nyiragongo series we will have a look to see if that really is true.
The behaviour of Nyiragongo’s lava depends on many factors. The behaviour of viscosity is called rheology and that is a very complicated field of mathematical science, and especially for beginners. So in part 4 I will have a quick peek at the rheology of Nyiragongo’s lava. Rheology is a field that I am not an expert in either, so I will explain very simply below what determine viscosity in magmas. The behaviour of “ultrabasic” alkaline magmas is not well known, since they are so rare, that is why it is worth having a whole article on this
Nyiragongo viscosity discussion: how fluid is it really?
Before I discusses Nyiragongo’s viscosity it is worth to take a look at what determines magmatic viscosity. Knowing the factors that determines magmatic viscosity, makes it more clear to understand how complicated the question is that I am discussing in this article.
The main factors that affect magmatic viscosity
Silicate contents: Low silicate magmas like basalts are in principle always much much more fluid than the highest silicate magmas. Silica forms long silica chain polymers in the magmatic melt and that thickens up things. Low silicate magmas with little polymerization erupt fluidly and especially so if the temperature is very high. High silica magma can be nearly solid if the temperature is low when it erupts. Low silica content stops these polymers from making the magma sticky. Nyiragongo is the record holder for the lowest silica magmas among active silicate volcanoes.
Higher temperatures significantly helps to break down these silicate polymers in the magma. An andesite melt at 1100 C will be very much more mobile than an andesite at 880 C. A basalt at 1240 C will be very much less polymerized and liquid than a basalt at 1090 C. In basaltic activity, temperature determines viscosity and determines if you get hawaiian or strombolian activity.
Crystal Content also affects the magmatic viscosity. Most magmas are a mix of glass melt and crystals. A hot crystal-poor melt is smooth and flows easier without stress. A crystal-rich magma can become almost like liquid concrete or wet sand. Crystals thickens things up, and causes rough lava flow surfaces. A crystal-rich magma can be compared to liquid ice cream with a lot of hard chocolate pieces in it, while a crystal-poor magma is a warm chocolate ice cream where everything is liquid. Nyiragongo is generally poor in crystals, so rises freshly from depth.
Alkalinity (sodium, potassium content in minerals) in the melt helps too to lower the viscosity since alkaline minerals have lower/ less SiO2 polymerization chain bonds than subakaline minerals.
Water: Magmatic water lowers both viscosity and melting points. It loosens up the silicate polymer bonds as well as lowering the temperature at which magmas remain liquid.
As we learnt in part three, Nyiragongo’s odd composition gives it many good scores in having low viscosity when it comes to the factors above here. Nephelinites are generally very low viscosity magmas. But exactly how low Nyiragongo’s viscosity is, is rather complicated. I will now dive into that complicated question, after having a look at what determines viscosity in magmas.
The 1977 example lava flood The 1977 eruption is a famous example. In 1977 Nyiragongo burst open, and very fluid, extremely fast moving lava poured down the flanks. Within minutes it got to settlements kilometers away. Eyewitness suggest it flowing at between 70 km/h and 100 km/h at the upper slopes. The flow passed the forests at such a speed that it did not set fire to the trees and even left some thicker leaves with a thin glassy layer. Some pahoehoes (smooth lava flows) close to the vents were only thin paper-like sheets of dark grey glass, at places less than a centimeter thick. Numerous persons, mostly the elderly or children, could not escape from the flows. Exact numbers of victims cannot not be confirmed. Although the official count was 74, it is assumed that maybe 400 people have died. The nephelinite flood covered persons and animals with a thin glassy carapace. A whole herd of forest elephants were all killed by the lava flood and encased in dark grey glass. Their hollow casts were later broken into to reveal their lava moulds. These elephant casts where photographed by Maurice and Katia Krafft. The trunk and feet were all visible in the lava moulds as well as their calcified bones.
Around 20 million cubic meters of very degassed magma from the upper lava lake conduit erupted inside of an hour, after which the eruption stopped instantly. A similar spectacle happened in 2002 and recently in 2021 when the magma lake column drained once again because of pressure in the magma column. The 1977 eruption is infamous and still reverberates through history as the first eruption to really show Nyiragongo’s hazards. The famous volcanologists Maurice and Katia Krafft arrived just 2 days after this eruption happened. They took almost all the existing color and black and white photos from the fresh flows at the eruption sites.
The 1977 eruption can be thought of as a hole in a dam, resulting in a catastrophic drain-out. The lavas where extremely fast near the events and very fluid, but the magmatic liquid cooled and crystallized as it flowed down. The very high eruption rates strongly favours Aa lava flows (crinkly and rubble-like) and Aa lava forms indeed the majority of the flow fields of 1977, 2002 and the 2021 eruption. There may have been turbulent lava flow close to the vents, meaning there where lots of vortices in the flow, rather than everything flowing smoothly in the same direction. Turbulent flow regime is reserved for only the most fluid lavas, and was probably last seen in the huge surges in lava flow rate in Fissure 8 during the 2018 Leilani eruption
The 1977 eruption has been described as a typical Nyiragongo flank eruption, but the volcano can do other stuff as well. These historical lava floods 1977, 2002 and 2021 are events with extreme eruption rates because of the pressure of the high-standing lava lake magma column. The pressure from that pillar of lava and the volcano’s steep slopes is why these eruptions flows so fast. This mechanism makes Nyiragongo flank eruptions probably the most dangerous effusive eruptions on Earth. Nyiragongo’s steep slopes makes it a very different phenomena than gentle sloped Kilaūea.
The paper sheet thin lava flows near the 1977, 2002, and 2021 flank vents were also a result of the lava being very degassed and not frothy and erupting very rapidly. Lava which is low in gas forms much less thick deposits then gas-rich lavas. The 1977 lava was a degassed lava from the lava lake column, and resulted in deposits that probably appeared more fluid than they really where. But such fluid features near vents could not be produced if the lavas where very viscous. No volcanologists were on site when these fissures erupted. We will never know the viscosity of these near vent conditions of Nyiragongo eruptions. We can only do the stuff in laboratory.
The viscosity and rheology of Nyiragongo have been not extensively researched. The measurements that have been done in the lava lakes have showed a very low viscosity, that is in line with the lowest measurements from Hawaii. Temperature measurements are poorly recorded but seem to be a bit above 1100 C range from the summit. Viscosity of lava has a lot to do with temperatures as well as silicate contents. Many other alkaline magmas with low MgO contents and very low silicate undersaturated magmatic melts emerge at low temperatures and erupt as cold strombolian eruptions. An example of that is Oahu’s sugarloaf nephelinitic flow that emerged as a cold viscous crystal mush. Nyiragongo and Nyiramuragira may own their low viscosity to a combination of their exceedingly low silicate contents and relatively high temperatures.
As described in the introduction, temperatures can affect magma a lot even if their silicate contents are relatively high. For example, most normal non-alkaline andesites, dacites and rhyolites emerge at relatively low temperatures at around 860 C and some as low as 700 C. That is one reason they are so viscous. The low temperatures makes active silica chain polymerisation which clogs up the melts. The world’s hottest andesite is erupted by West Mata Volcano that erupts ultramafic high-silica boninite andesites at 1320 degrees C. When submarines filmed that eruption it flowed like fluid basaltic pillow lavas and channels. The same high magnesium andesite eruption on land with the same temperatures would be as fluid as Fagradalshraun near the vent despite its 60% silica. In ancient komatiite lavas, that are very hot, all silica polymerisation was broken down because of high temperatures. This created a melt viscosity as low as water, showing how efficient high temperatures are in breaking the SiO2 chains.
High temperatures also affect evolved magmas. Some rare rhyolite magmas that emerge at 1000 C can form highly mobile obsidian block flows that can move many kilometers. Fissure 17 in Hawaii 2018 erupted a very hot 1100 C andesite pocket that flowed just as easily as an Etna basalt, while Soufriere Hills erupted similar andesites at 820 C forming very blocky lava domes. Crystals also increase viscosity in cooler magmas when high melting point minerals freeze out forming a melt that is a lot like melted ice cream but with lots of hard chocolate pieces in it. Now we learnt that very high (rare) temperatures can affect even some higher silicious magmas.
So what about ultra-low silica magmas and Nyiragongo? Low silica magmas are generally the most fluid, as they erupt at lower silicate contents and higher temperatures than their more silicious relatives. Nyiragongo with the lowest silicate contents of all silicate magmas and relatively high eruption temperatures should have the lowest silicate polymerisation and therefore the lowest viscosity among silicate based melts. Measurements suggest it is one of the lowest, quite a bit more fluid than really warm honey in some lab experiments. That agrees with the flow structures during the 1977 eruption. But there are other hot basaltic contesters with low viscosity.
Nyiragongo’s magmatic origin with very small amounts of partial melting may give it a much lower eruption temperature than really hot basaltic eruptions. Kilaūea above the powerful Hawaiian hotspot challenges Nyiragongo with some the lowest measured viscosity silicate magmas on Earth. Kilaūea produces hot thoelite basaltic magmas from large amounts of partial melting. Kilaūea’s silicate content are much higher than Nyiragongo’s but the temperatures are also really high, sometimes in range of 1250 degrees at Halema’uma’u. As mentioned, high temperatures helps to break down the silica polymerisation resulting in measured Kilaūea summit viscosities that are in range of many of Nyiragongo’s measurements. But many of Kilaūea’s lavas especially at the flanks at the Puu Oo vent have been a bit more viscous than the Nyiragongo measurements.
Nyiragongos ultrabasic alkaline magmas are perhaps the least polymerized on modern Earth. Nyiragongo’s lava would flow more easily than Kilaūea at the same temperatures. Most viscosity measurements of Nyiragongo lavas are a bit lower than typical fluid basaltic lavas and very much lower than higher silicate magmas. Many experiments been performed in laboratory furnaces, including some for Nyiragongo. Samples from Nyiragongo, Nyiramuragira were crushed and melted and stirred in furnaces. Similar experiments been done with rocks from other volcanoes. The Nyiragongo nephelinites where the most fluid when the magmatic furnace melts were at the same temperatures. Nyiragongo was easier to stir at lower temperatures than basalts at the same temperatures.
But it is also clear to me that very high temperature basaltic lavas in Hawaii and Iceland can have the same low viscosities. The lowest viscosities of all modern modern magmas could be a hot picrite basalt, since alkaline nephelinites are erupted typically at low temperatures. At high temperatures in many lab experiment basalts have been among the most fluid of all melts. Nyiragongo has generally been the most liquid at laboratory experiments. Nyiragongo’s rare mineral composition kept it much less polymerized than other samples.
The viscosity that have been measured in nature at Nyiragongo have been among the very lowest of all silicate magmas on the planet, with some of Kilaūea’s summit lavas being just as low. Sometimes temperatures is just as important as silicate content in determining a magma’s viscosity, especially for melts of basic and intermediate composition. Its worth noticing that the theoretical calculations for Nyiragongo’s melt viscosities have suggested viscosity of around 1 Pascal-second for Nyiragongo’s lava, but real world measurements in the lava lakes resulted in higher values at around 20 to 50 Pa-s. That is in line with the very lowest measurements from Halemaumau in Kilauea (20 Pa-s). The lowest theoretical eruption viscosity for Kilauea has been around 10 Pa-s with some hotter flows from Kilauea’s depths even much lower. Values below 10 Pa-s have also been suggested for some Nyiragongo flows. These figures for these two volcanoes are very low compared to almost any other silicate magma. Kilaūea’s summit can probably display viscosity below 10 Pa-s as well. Most other active basalt lava flows have viscosities in range of a few 100’s of Pa-s to over 10 000. And many more silicic lavas have viscosity that are much higher than that. 10 Pa-s is about same as lukewarm honey. The very high density of lavas makes them look more fluid than they really are. But the paper-thin sheets of lava spatter that can be found in photos with both Kilaūea and Nyiragongo’s lava lake walls may suggest viscosities below 10 Pa-s, making these two volcanoes perhaps capable of having lava a bit more fluid than honey.
The few laboratory furnace experiments that have been done on Nyiragongo-type rocks shows very low polymerization. The 2003 Jacques Durieux summit lava lake samples shows no crystal formations at all and only gas bubbles. Such pure liquidus Nyiragongo magmatic melts have been simulated under laboratory furnaces and shown to have very low viscosities. The pure melt viscosity for Nyiragongo is around 1220 C, the temperature needed to melt all mineral components. Most of the other Nyiragongo samples I have been reading about have been a bit below this full liquidus temperature, with tiny white crystals of nepheline present – as mentioned, like melted ice cream with some small hard chocolate bits in. A magma below its liquids temperature will be more viscous than its full melt temperature, although being very silica undersaturated clearly helps to keep its viscosity low even below the full liquidus temperatures.
Nyiragongo’s strong silica undersaturation allows it to move quickly even at sub-liquidus temperatures according to the lab experiments above. That is the reason why Nyiragongo lava flows were able to flow may many kilometers in the 1977, 2002 and 2021 flank eruptions without loosing a lot of speed, because such a strongly silica undersaturated lava, does not saturate a lot with silica as it cools and crystallizes. In other words a more crystalline nephelinitic flow does not have a notable higher SiO2 content in its glass melt than a fresher crystal poor one (although increased crystallization raises viscosity). It is this that explains why Nyiragongo’s fissure feed Aa flows can race forward at terrible speeds even if they are undercooled and crystal rich, because the remaining glass melt are SiO2 poor as well. Because of that the lava was able to flow great distances before significantly cooling and succumbing to the rheological barriers of crystallization and cooling.
Such behaviour is quite different from more normal basaltic lavas, that rapidly clog up as they cool and crystallize and their SiO2 saturate as the lava flows loose heat.
Both Nyiragongo and Nyiramuragira silicate rocks (nephelinite and basanite ) have been melted in furnace experiments and their viscosities compared. Nyiragongo is a bit more fluid even if it has undercooled conditions, thanks to its lower polymerization. The viscosity of both volcanoes are low but in line with hot hawaiian lavas. Nyiragongo’s viscosities been slightly lower in experiments with batches from both at the same temperatures, but the difference is very small.
Nyiragongo remained less crystallized at lower temperatures than Nyiramuragira in these experiments. Nyiramuragira’s lavas also had higher total liquidus temperatures than Nyiragongo’s composition in furnace experiments. Nyiramuragira is richer in silica and poorer in nephelinite and other sodium bearing minerals than Nyiragongo is. Most other research on past nephelinite volcanism in Lengai and post shield flows in Hawaii have found that the flows where viscous and crystal rich, so Nyiragongo is perhaps unusually hot for being a nephelinitic melt.
It is a question thats hard to answer
The very interesting question that I am looking at in the text is ”is Nyiragongo the most fluid silicate based magma”? Magmatic fluidity has a lot of different factors that have to be taken into account. Lava is an incredibly strange fluid, it is a mix of molten minerals. Viscosity goes down with increasing temperature and lower silica content. A combination of both would be best. Most alkaline magmas seen in historical times appear to have lower temperatures than subalkaline magmas, and erupt from shallow, cool storage as viscous strombolian eruptions. However, many older, preserved, highly alkaline eruption sites are purely monogenetic. These may have erupted without shallow storage, in which case they could have erupted at high temperatures as direct mantle eruptions, with low viscosity.
Two historical volcano eruptions that had very hot alkaline magmas more than 1140 C are Hualalai and La Palma. Both of Hualalai 1800’s eruptions, produced very hot silica-poor alkaline basalt with 42% SiO2 and 1140 C. The 2021 basanite from la Palma has very low silica content combined with a very high temperature close to 1200 C. The viscosities in the latest geological reports of the deep basanite magmas from la Palma are incredibly low, as low as the very lowest viscosities from Nyiragongo and Hawaii, and lower than almost any historical magma. But Nyiragongo also has that combination of high temperatures and very low SiO2 content. Nyiragongo does have the record among frequently erupting silicate volcanoes as having the lowest SiO2 content, and Nyiragongo’s temperature is high enough to push the viscosity down to one of the lowest among any silicate magma. Nephelinites and basanites have the specs of having the lowest viscosity if they are combined with a high eruption temperatures. (Basanite is slightly less alkaline and little more SiO2 rich than Nyiragongo’s foidites). They are both very rare magmas.
Conclusions and final thoughts about Nyiragongo’s viscosity
It is difficulty to know if Nyiragongo is the most fluid silicate based magma. While the silica content is very low, there are lots of other factors that determine viscosity, like temperature. Magmas are highly complex fluids that do not really have a single melting point.
Nyiragongo does have very low viscosity, one of the lowest measured viscosities of all silicate magmas on the planet. But there have not been enough studies on Nyiragongo on this question in my opinion. No volcanologists have been close to the erupting flank vents to measure viscosity.
The research that was done in the 2003-2021 summit lava lake is inconclusive. While very low in viscosity, it has not been lower than hot basaltic lakes in Hawaii. The extremely fluid-looking flows of near vents of 1977, 2002 and 2021 have a lot to do with very high eruption rates as well as with low viscosity. The volcano does have the lowest silicate content among terrestrial silicate lavas and it is the least polymerized lava at any given temperatures. But really hot basaltic lavas in Iceland and Hawaii can have the same low viscosity because of their very high temperatures. The fluid glassy watery look of the 2002 and 1977 near vent flows have a lot to do with that they where degassed and not frothy rather than being insanely runny, they where runny but perhaps not more so than other fluid lavas.
Nyiragongo does have the lowest amount of silica polymerization at any given temperature because of its very low silicate content, meaning that a normal basalt and nephelinite at same temperature, would have the nephelinitic melt as the most fluid. The magmas of this volcano also has remarkable mobility even at undercooled conditions (below full liquidus point) because of their low silica content.
Nyiragongo’s viscosity is low, but the myth that it is as fluid as water, is clearly just a myth. It is mainly created by the very fast eruptive rates of this volcano’s flank drain-outs that are one of the fastest effusive eruptions on Earth in terms of effusion rates. Its the steep slopes and fast eruption rates that creates the lava flow speeds together with a low viscosity, and many other fluid mafic lavas would also be just as dangerous if they erupted in the same conditions.
I have spent hours, hours on youtube videos and looking at photography comparing Nyiragongo with different fluid lavas mainly with Hawaii and mostly I cannot see any difference at all. Hawaii’s lavas appear a little hotter and brighter and much more shiny in cooled surfaces, but the viscosity looks just the same. The two lavas do look very different. Nyiragongo lacks the shiny aluminium looking skins of basaltic flows because its silica content is so low it does not form a nice glass up on air cooling. Nyiragongo lava lakes and lava rivers have a glossy look on its sun exposed surfaces, instead of Hawaii’s and Fagradalshraun’s shiny crusts. The lava lava lakes also look radically different, with Nyiragongo’s nephelinitic lake having numerous small degassing spattering points, and a strongly fissured glossy crust as normal convective regime, while Hawaiian basalt lakes haves a shiny aluminium looking crust with a few large plates and a few degassing points. This certainly has to do with Nyiragongo’s composition and different gas content. In some videos I seen of Kilauea’s summit lava lakes, it sometimes actually looked more fluid than Nyiragongo’s lava to my eyes. But this could also be because Hawaii’s lava is more “glassy”.
Nyiragongo’s lack of polymerization because its low silica makes it unable to form Pele’s hair. In other words, Nyiragongo’s composition is totally bizarre if you compare it to the basalt lava from Holuhraun or Hawaii. In theory Nyiragongo should be more fluid, with its much lower polymerization at any given temperatures, but Nyiragongo’s viscosity has a lot to do with temperature and how differentiated it is as well.
A very important thing when looking at Nyiragongo’s viscosity is that it is a well formed volcanic system and with a well built magma chamber storage, that allows Nyiragongo’s magmas to cool. The very slightly evolved nature of Nyiragongo’s nephelinites (not 100% fresh from mantle) maybe increases viscosity to the range of very fluid basaltic lavas. A monogenetic example of this magma composition could be even a bit more silica poor and primitive than Nyiragongo’s own nephelinites.
Temperatures are very good as well at removing that silicate polymerization, and thats why many normal basaltic lavas can be exceptionally fluid despite their higher SiO2 if they are hot enough.
But a truly primitive nephelinite / melilinite, like the extraordinary magmas of Vogelsberg, Urach that goes to below 30% Sio2, will be much more fluid than basalts, if they erupted at same temperatures as normal basalts.
But to end the discussion, Nyiragongo is almost certainly not more fluid than Halemaumau or Fagradalshraun’s basalts, that are both hotter and more silica rich than Nyiragongo’s. In other words the viscosity is probably the same (although Nyiragongo could be a bit lower in viscosity) but its impossible to say really. Nyiragongo’s nephelinite is one of the planets most fluid silicate magmas, but it is hard to say if it has a lower viscosity than a really hot basalt. The contest of which is the most fluid currently active silicate based magma/lava ends with a draw… and that ends this series.
Photographed by Patrick Aventurier during the 2003 sample expedition
This article series is mostly based on what I learnt myself about volcanism, Nyiragongo and lava rheology and is written in my own words which is the whole point, but some scientific sources that support my materials can be found below here.
Fledgling Mantle Plume May Be Cause Of African Volcano’s Unique Lava https://www.sciencedaily.com/releases/2009/03/090313110733.htm
Petrogenesis of Basanitic to Tholeiitic Volcanic Rocks from the Miocene Vogelsberg, Central Germany https://academic.oup.com/petrology/article/44/3/569/1576323
Thermo-rheological magma control on the impact of highly fluid lava flows at Mt. Nyiragongo https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006GL028459
Dynamics of the Mount Nyiragongo lava lake https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JB010895
January 2002 volcano-tectonic eruption of Nyiragongo volcano, Democratic Republic of Congo https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006JB004762
The rheology of crystallizing basaltic lavas
from Nyiragongo and Nyamuragira volcanoes, D.R.C. http://jvolcanica.org › ojs › article › download
Jesper Sandberg, July 2022