Nyiragongo history of activity and nature of its eruptive activity
(See here for Part I)
Nyiragongo’s beautiful steep cone, encased in its unsettling night glow, towers over the densely populated Kivu region that is home to millions of people. The volcanic edifice itself measures around 13 kilometers wide and rises to 3470 meters above lake Kivu, and sometimes is dusted in snow. The volcano has a rift system and flank eruptive deposits that span the entire Goma area and extends under lake Kivu. It is also owner of a lot of flank cinder cones, they are Nyiragongo’s smaller monogenetic children. To the west is the towering Nyiramuragira and to the east are the snow-covered Karisimbi, Visoke and Muhavura. Nyiragongo has around two million persons around it and most of its lower flanks have been turned into agricultural lands. Despite the dense levels of human activity on the slopes and the presence of a large human population, not a lot is known about Nyiragongo’s eruptive history before the 1880’s, other than that its a steep ultrabasic tephra cone that has dangerous lava lake drain-outs.
It is in one of the oldest populated places on the planet, and modern human beings been present there long before Nyiragongo was born. We are as a species way older than both Nyiragongo and Nyiramuragira. That says how young these volcanoes are. Goma became a major city center only quite recently, so the area is still quite unexplored, and the violent human history of the region has prevented research of the area. Still Nyiragongo has quickly made its way into geological research and volcano recordings, thanks in part to the volcanologists Tazieff, and the Kraffts videos and their photography. The modern information era has made Nyiragongo monitoring a much easier job. A description of the geological exploration and the eruptive history follows.
Nyiragongo has no doubt been climbed by the locals many times before recorded history, but the first European who recorded a visit was Gustav Adolf von Götzen (1866 – 1910 ) in the year 1894 when he explored what was 31 years later to become the beautiful Virunga National Park. He was one of the last people of the classical age of exploration in the European imperial age. In 1894 he became the first European to observe the lava lake and confirm its existence. The climb up must have been difficult without a well trekked trail.
A volcanologist who really brought Nyiragongo to the western worlds public eyes was the French Haroun Tazieff, he was born in 1914 and died in 1998 when I was only 3. Tazieff began to explore Nyiragongo in the late 1940’s and published video and still photos from the lava lake that existed from the 1890’s to 1977. The first Tazieff expedition to this volcano occured in 1948. These where major undertakings, taking weeks. The expedition teams lowered themselves hundreds of meters down to the lava lake, getting close enough to touch the lava lake. Over the decades Tazieff undertook numerous expeditions to Nyiragongo for photography, filmmaking and science as well, including the first color films of its activity in the 1960’s showing his geology students the activity. Tazieff became very familiar with Nyiragongo and he wrote his memories in the book “Nyiragongo The Forbidden volcano” that was published in 1979. Tazieff was the first to bring back color films and photography from Nyiragongo. He was also perhaps the first the bring volcanology into the public mind in Europe, perhaps pioneering that even before the famous Maurice and Katia Krafft that Tazieff was friends with. The Kraftts visited Nyiragongo in 1973, 1977 and in the 1980s. Tazieff was the person who made Nyiragongo famous through public media. The Kraffts discovered Nyiragongo through Tazieff’s old films.
During the modern era Nyiragongo has become an unusual tourist attraction. It has been visited by quite a lot of tourists, as a major attraction of the spectacular Virunga National Park. Tourism to Nyiragongo began rather early and possibly because of Haroun Tazieff who had made Nyiragongo famous in the western world with his popular science videos and books. The experience involves hiking to the summit lava lake and spending a night at the cabin huts on the crater rim. A visit to Nyiragongo can look like the following video, a good explanation of exactly what happens if you try to visit Nyiragongo.
Historical activity from Nyiragongo has been dominated by an open conduit situation with the circulating magma columns visible as Nyiragongo’s famous lava lakes with their cycles of draining and refilling. The 1977 and 2021 flank eruptions are examples of lava lake destruction when the magma column gets too tall and bursts the sides of the volcano. Nyiragongo belongs to the group of constantly active volcanoes, but in historical times it has not been very productive. It has just been open and constantly active with circulating lava lakes.
Most of the activity that you see on Nyiragongo involves Hawaiian style activity in the summit crater in the form of lava lakes or temporary fountain cones. Both styles of activity generally fills the inner summit caldera with vent lava flows or in form of lava lake overflows. Eruptive activity can last decades or even centuries with such open vent conditions, but in general not a lot of material is produced. Nyiragongo’s open vent is sourced from a deep and large storage region, with gas-rich magma rising quickly from source. Nyiragongo is significantly less productive than her larger and less alkaline basanitic sister volcano Nyiramuragira, but both are more or less constantly active. Nyiramuragira erupts as much lava as all other African volcanoes combined, while Nyiragongo acts more like an open lava gas vent.
As I mentioned there are different types of Nyiragongo activity while being the same lava.
Open lava lakes are the most famous type of activity from this volcano. There been two open conduited lava lake cycles in historical times up to this writing, one from the 1890’s to 1977 and one from 2003 to 2021, each one ending in catastrophic drainage through a flank rift. Like all lava lakes, Nyiragongo’s lake is basically a magma chamber pillar visible in the throat of the volcano. The rise of the lava in the crater fills the crater with thin sheets of lava overflows. Each stack builds a platform around the lava lake that keeps rising higher. The taller the magma column gets the more chance there is for it to catastrophically burst out through the volcanos flank. The rise of lava lake magma columns can take decades. It took nearly 20 years for the 2003 – 2021 lake to get to bursting point. Lava lakes are long lived and can last decades, or even longer than our lifetime.
Nyiragongo vents large amounts of gases during a lava lake phase. A huge steam plume often hovers over the volcano bringing acid rain. The birth of the 2003 – 2021 lava lake had a sulfur output of 50 000 tons a day! This lasted for quite a long time. After a few years the sulfur output dropped and became steady at 6000 to 8000 tons a day.
During this type of activity magma circulates constantly between the surface and the deep magma system, as hot fresh incoming magma rises and cooler degassed magma sinks back into the conduit. A lava lake may act very much like a lava lamp with its convective currents. A massive heat influx and constant supply is needed to keep the whole thing running and to keep it from solidifying. This explains why lava lakes are so rare on Earth. Nyiragongo together with Kilauea’s summit contain the worlds largest circulating lava lakes, with the 2003 – 2021 summit lake reached 260 m wide before it was destroyed in Nyiragongo’s 2021 eruption.
Another type of Nyiragongo activity is temporary caldera fillings. This occurs when the magma supply is lower, and is not enough to maintain a circulating magma column. Such type of activity forms lava fountains, spatter cones and rootless lava ponds. It is common after a major lava lake drain-out if the lava lake column fails to reform. The 1980’s and 1994 up to 1996 and the post 2021 caldera eruption are good examples of such kind of activity. This type of activity fills up the caldera as well. Lava is delivered to the crater surface but it is not circulating during this type of activity. Such activity can also happen together with open lava lakes as co- vents in the caldera floor. This kind of activity at Nyiragongo is very similar to Vesuvius early 1900’s effusive activity that filled its cone with pahoehoe lava, and such activity can last for years at a slow pace.
As placid as everyday typical summit Nyiragongo activity may be, it remains a very dangerous volcano, close to a large human population. A flank eruption and even a cinder cone could pop up right in the streets of Goma or even on the Kivu lake bottom. That would be the worst scenario, potentially setting off a catastrophic limmic eruption. Luckily for the people of Goma, flank eruptions are a relatively infrequent phenomena of Nyiragongo with only two examples in historical times, but it will only get worse the larger Goma grows and as the city is expanding towards the volcano itself. This brings us to the most dangerous and feared type of activity at Nyiragongo, flank eruptions.
Flank eruptions are caused either by a lava lake (magma column) that gets too tall and heavy and which bursts the side, or it can be caused by tectonic rifting that ruptures the edifice. The flank eruptions of Nyiragongo are superfast because of the pressure from the magma column and only last an hour or two. They are generally quite small in volume, but their speed and eruption rate are one of the fastest for any historical effusive eruption. The 1997, 2002 and 2021 rift eruptions all had fatalities, despite the small volume in these eruptions. In 2002 one vent just beside Goma airport allowed lava to to flow right through the city. Flank eruptions often exploits crustal weaknesses in Nyiragongo’s rift, and significant amounts magma that are involved in a flank eruption may be intruded in the rift and never erupt during a drain of the magma column inside. Not all flank eruptions been a fast affair. On Nyiragongo’s right side a large sparsely vegetated tube feed pahoehoe flow can be seen. Such long lived slow eruptions have covered the Goma area before a few 100 years ago. It would be a disaster in slow motion if it happens again. Formation of flank cones as mentioned are just as worrisome as the flank rift eruptions that involve the central magma storage, because a cone can pop up anywhere in Goma.
Not so long ago (a few 100 years perhaps) Nyiragongo’s summit caved in and the current summit caldera formed. This was because of collapse of a shallow magma body, either through a Plinian paroxysmal eruption, or more likely effusive drainage into the rift. It must have been an impressive terrible event if anyone was there to see it, releasing a few cubic kilometers of magma from the edifice storage system, dwarfing historical rift eruptions in scale. The true nature of the formation of the summit caldera have not been researched much, but the volcano was much taller before, with recent summit lava flows cut off by the caved-in summit. Nyiragongo today displays effusive Hawaiian activity, and has moved away from gas rich pyroclastic violent eruptions.
Very little is known about Nyiragongo’s deep interior, but according to geological investigations it probably has at least two magma chambers connected by an open pipe-like pathway. Geological investigations have found signs of a shallow and deep storage at Nyiragongo. One shallow chamber is just under the edifice and a much larger deeper one is at around 20 km depth. The magma chamber storage explains the slightly evolved nature of Nyiragongo’s ultrabasic nephelinite magma compared to monogenetic nephelinites. The summit caldera of Nyiragongo must have formed by the drainage of a previous large shallow chamber and that happened very recently, knowing how young looking the caldera is. The magma conduit pipe in Nyiragongo is deep and is probably going down 100 km or more where it connects to the deep partial melting region. The magma supply is constant but perhaps not extremely large, and if it is large a lot is probably going into rift storage deep down rather than coming up. A surprisingly large amount of magma is likely resting below Nyiragongo and Nyiramuragira, a bit like Iceland’s deep rifts storage, with Nyiragongo having the deepest magma source among all of Virunga’s central volcanoes.
One of very few volcanoes where lava is an instant threat
This is one of very few places where lava poses an instant threat. Almost no volcanoes have killer lava flows, even if many people think that the lava is the biggest hazard. Nyiragongo’s main hazards are its steep slopes and associated lava lake column ruptures as will later be described for the 1977 event. Nyiragongo and Hualalai are perhaps the world’s two most hazardous effusive mafic volcanoes. Their steep slopes and fast eruption rates as well as very low viscosity pose a great threat. Close to the vents the lava is estimated to move at perhaps 100 km an hour. As it flows down to communities it goes from running to fast walking speed. Nyiragongo is the only volcano so far where lava flows have directly killed 100s of persons and indirectly caused many more deaths during the last 100 years. The precise numbers of persons killed by Nyiragongo will never be known. No where else in the world does a steep sided stratovolcano host such a large lake and magma of such fluid low silicate magma composition and pose such a hazard to such a large population. Close to flank vents during a lava lake drainage you could never escape the lava. You would have to run from them lower down and the lava may move at walking speed over flat ground. The behaviour of this lava will be described in detail in the later parts of this series.
The 2002 eruption that had fissures erupting within the city has the most recorded fatalities: over 200 died and thousands of structures where destroyed. That is likely the most of any of Nyiragongo’s lava flows drain-outs.
Nyiragongo is problematic and dangerous volcano that is in great need of better monitoring. The region is dangerous too and suffers from many issues. Goma is growing fast and buildings are constructed on the volcano’s fissure fracture systems. That is a very problematic development that was discussed in Albert’s article on this volcano.
Steep slopes despite very fluid lavas. How Nyiragongo got it shape
Nyiragongo’s steep slopes and fluid lava confuse many people, and they wonder how that is even possible. But it is not as strange as it seems. Here we will have a look at a very simple solution for that mystery.
Despite having extremely silica-poor and fluid lavas Nyiragongo is steep. Nyiragongo’s steep slopes together with fast eruption rates are the reasons why the lava flows so quickly downhill. The steepness has to do with earlier eruptive behaviour. The whole Nyiragongo complex is ultrabasic, so something else rather than an earlier composition change is responsible for the steep shape.
Nyiragongo’s steep shape was most likely formed by violent lava fountains and their pyroclastic tephra fall. High fountains earlier in Nyiragongo build a huge pyroclastic mound… think something like Puu’Oo during 1983-1986 or the major Etna fountaining paroxysms of 2015. Short-lived overflows from a high-standing lava lake may also been responsible for forming a steep pile. Nyiragongo’s magmas are very gas rich, so this could perhaps have happened at earlier times with a less open pathway, or a quick rise of gas rich magma through a narrow conduit. Many other steep mafic stratovolcanoes may also have mostly formed form high fountains caused by gas-rich magma.
Volcanoes that produce fluid lavas can get very steep if the main vents produce mostly tall fountains and therefore a lot of tephra materials. There are many tall steep mafic volcanoes like Shishaldin, Pavlof, Villaricca, Stromboli, Etna, Pacaya and a few others that have been formed by paroxysmal fountain tephras and short basaltic flows. In fact it seems that the most perfect steep volcanic cones are paradoxically formed by mafic fountains from open vent cones like the examples above here. High silica lava flows forms lumpy dome volcanoes instead.
Nyiragongo is a very young volcanic cone with most of the upper steep parts only a few thousand years old. The change to mostly effusive lava lakes may have to do with changes of how the magma system degasses and how magmatic volatiles are allowed to accumulate. Lava channels are common on the upper slopes, suggesting past high lava lake stands. Before Nyiragongo got its caldera it was probably a steep tephra cone like Shishaldin, Villaricca and Pavlof, formed by high lava fountains. Nyiragongo’s lava are very gas rich, and ideal to build a large lava fountain cone like that, as mentioned above. Nyiragongo is now too open to produce tall fountains, as gases are allowed to escape. This idea on the formation of the cone is the most likely since the whole cone is indeed ultrabasic, and other nephelinitic polygenetic volcanoes are steep fountain cones.
Not long ago the volcano’s upper cone collapsed (because of collapse of a large shallow chamber ), forming the current caldera that fills and drains in lava lake cycles. The caldera allows us to read the eruptive history in its walls. Most of the cone’s interior is composed of tephra pyroclasts. Nyiragongo’s caldera walls are mostly made of brown oxidised tephra and ash. Its only the very upper parts of the cone deposits that have lava lake overflow deposits, thin stacks of lava overflows from high-standing lava lakes that drape the steep tephra cone. The lava drapery likely protected Nyiragongo’s perfect pyroclastic cone from the tropical rain erosion. The change in Nyiragongo’s eruption behaviour must have happened very recently, perhaps only a thousand or even only a few 100 years ago.
In short, Nyiragongo is an ultrabasic fountain tephra cone, whose summit has caved in, and that now displays effusive lava lake behaviour and dangerous drain-outs.
Having had a look how Nyiragongo behaves, and why it looks as it does, we can now have a look at its magma composition and the behaviour of its lava, which I will look at at the next part of this series…
Jesper Sandberg July 2022