Why a VEI-1 Volcano Is One of the World's Deadliest
The Volcanic Explosivity Index measures how violent an eruption is — how much material it throws and how high. On that scale Nyiragongo looks almost harmless: in our data, 17 of its 18 size-rated eruptions are VEI 1, and the lone “VEI 2” entry is just the long 2002–2025 lava-lake episode counted as one event, not a bigger bang. By comparison, Vesuvius in AD 79 was a VEI 5 and Krakatoa in 1883 a VEI 6. So why does Nyiragongo appear on every list of the most dangerous volcanoes on Earth?
Because VEI measures the explosion, and Nyiragongo doesn't kill with explosions. It kills with lava flows— and its lava is unlike almost any other volcano's. Most people picture lava as a slow red glow you could walk away from. That's usually true. It is not true here. The combination of a near-permanent lava lake, a uniquely runny magma and a major city sitting ten kilometres downhill turns a “small” eruption into a catastrophe. The number on the explosivity scale is the wrong number to look at.
The one statistic to quote
Across the 20 eruptions in VolcanoDB's Smithsonian GVP record for Nyiragongo (id 155), 17 of the 18 that carry a VEI rating are VEI 1 — yet the volcano's 1977, 2002 and 2021 lava flows together killed well over 100 people and displaced hundreds of thousands. It is the clearest case in our database of eruption size and eruption danger being two different things.
The Fastest Lava on Earth — and Why
Nyiragongo erupts foidite, specifically a nephelinite-melilitite — an extraordinarily rare, silica-poor rock. Ordinary basalt is around 50% silica; the andesite of an explosive stratovolcano is around 60–65%. Nyiragongo's magma is only about 36–40% silica (Tedesco et al., 2007, Journal of Geophysical Research). Silica is what makes lava thick and sticky, so with so little of it, Nyiragongo's lava has a viscosity closer to warm oil than to the slow-crawling flows most volcanoes produce.
That is why, when fissures opened low on the flanks in 1977 and 2002, the drained lava poured downhill at up to 60 km/h (37 mph) on the upper slopes — the fastest lava flows ever recorded (The Conversation). It's the same fluidity that lets Nyiragongo hold a lava lake at all — runny, gently degassing magma can pool and stay molten for decades, which is why the short global list of persistent lava lakes is dominated by rift and shield volcanoes like Erta Ale, Erebus and Kilauea, not the explosive giants. Nyiragongo sits on the East African Rift, where the continent is tearing apart — the opposite tectonic setting from the Ring of Fire.
A Lava Lake Above a City of a Million
Nyiragongo's main crater is about two kilometres across and, for most of the last century, has held a lava lake — at times the most voluminous known lava lake in recent history, with the molten surface standing hundreds of metres deep inside the crater (it reached roughly 600 metres depth before the 1977 drainage). Ten kilometres to the south sits Goma, which has grown into a city of around a million people on the shore of Lake Kivu. That geography is the whole story: the danger is not that Nyiragongo will explode, but that its lake can empty straight downhill into a dense city with little warning.
There's a second, slower threat beneath the obvious one. Lake Kivu holds enormous volumes of dissolved carbon dioxide and methane in its deep waters. Volcanologists worry that a large lava flow or intrusion entering the lake could, in a worst case, disturb those gas layers — a hazard that has no parallel at most volcanoes. It's a reminder that Nyiragongo's risk profile is about its setting as much as its eruptions.
1977, 2002, 2021: Three Times the Lava Came Down
The pattern that makes Nyiragongo so deadly has played out three times in living memory, and each time it followed the same script — the lake drains through flank fissures and the lava heads for inhabited ground.
- 1977On 10 January the summit lava lake drained in under an hour. The flows moved so fast that there was no outrunning them; at least 50 people were killed in the villages of Kibati and Moniki. It was the event that first revealed how dangerous a “gentle” volcano could be.
- 2002On 17 January lava again drained through fissures and ran directly into Goma, cutting the city in two and flowing on into Lake Kivu. Estimates of homes destroyed ranged from a quarter to three-quarters of the city (~10,000 buildings); the UN cited about 147 deaths, many of them from a petrol-station explosion as lava advanced, while field researchers later put the direct toll nearer 70. Around 400,000 people fled across the border into Rwanda.
- 2021On 22 May fissures opened on the south flank with almost no official warning — the Goma Volcano Observatory had been hampered by a funding and security crisis. Lava stopped about 300 metres short of Goma's airport. At least 32 people died, several while crossing still-hot lava; roughly 415,700 were displaced and water for about 500,000 was cut (UN/ReliefWeb).
| Eruption | VEI | Style | Notes |
|---|---|---|---|
| 2002–2025 CE | 2 | Ongoing lava lake | The refilled summit lava lake and persistent activity recorded as a single long episode. Recorded as VEI 2 because of duration, not because of any large explosion. |
| 2002 CE | 1 | Flank + summit | January 17: lava drained from the lake through flank fissures and ran into Goma, 10 km south, reaching Lake Kivu. |
| 1994–1996 CE | 1 | Summit | Return of lava-lake activity in the summit crater. |
| 1982 CE | 1 | Summit | Lava-lake activity. |
| 1977 CE | 1 | Flank | January 10: the summit lava lake drained in under an hour; lava reached ~60 km/h and killed at least 50 people. |
| 1927–1977 CE | 1 | Summit lava lake | The half-century of near-continuous lava-lake activity that ended with the 1977 drainage. |
| 1920–1921 CE | 1 | Summit | Lava-lake activity observed from the crater rim. |
| 1884 CE | 1 | Summit | Earliest confirmed eruption in our record. |
Source: VolcanoDB analysis of Smithsonian Global Volcanism Program records for Nyiragongo (id 155): 20 recorded eruptions, 17 of 18 VEI-rated are VEI 1. Selected rows shown; see the full data page for the complete history. Its shield neighbour, Nyamuragira, is Africa's most frequently active volcano.
Can Nyiragongo Be Predicted?
This is the part that should worry people most. The 2002 eruption came with some warning; the 2021 one effectively did not. The Goma Volcano Observatory monitors the volcano, but in the period before May 2021 its monitoring had been degraded by a funding crisis — the earthquake and ground-deformation instruments that might have flagged magma on the move were not fully operational. The eruption began as fissures tore open on the flank, and the first most residents knew of it was the glow in the sky.
The science is tractable — fluid-magma systems like this one usually telegraph flank eruptions through swarms of small earthquakes and swelling ground. The obstacle is keeping instruments running and data flowing in a region shaped by armed conflict. For Nyiragongo, the hard problem isn't the geology; it's sustaining the monitoring a city of a million depends on. You can see where it sits among the world's currently restless volcanoes on our active volcanoes tracker and the live erupting-now dashboard.