Nyiragongo volcano in East Africa: disaster without warning

Earthquakes cannot be predicted.

On the other hand, researchers in a related field of geosciences are far more successful.

In many cases, they can warn of volcanic eruptions quite accurately.

The fact that earthquakes play a decisive role in this may sound ironic at first, but magma movements below a volcano almost always lead to stresses in the earth's crust, which are discharged in earthquakes.

However, the predictability of volcanic eruptions is not a universal law of nature.

Again and again there are cases in which mountains of fire eject lava without giving a single measurable sign.

An international research group has now investigated an extremely dramatic case of a sudden, unexpected eruption of the Nyiragongo volcano in East Africa.

The almost 3500 meter high Nyiragongo in the east of Congo is fed together with its sister volcano Nyamuragira by the magmas of the Albertinian Rift, the western extension of the East African rift system.

Along this large rift in the Earth's crust, the African continent separates from the Indo-Australian lithospheric plate, and Africa breaks apart.

Nyiragongo is a wonderfully cone-shaped stratovolcano less than 20 kilometers north of the Congolese city of Goma.

It is one of the most active volcanoes on earth, its crater is constantly filled with a lava lake.

Millions of cubic meters of glowing lava

At least 34 significant eruptions of Nyiragongo have been reported over the past 140 years.

One of the most momentous eruptions occurred on January 10, 1977, when the rim of the crater collapsed in some places and the lake in its center, filled with millions of cubic meters of glowing lava, emptied within half an hour.

The lava flowed down the southern slopes at a speed of about 60 kilometers per hour and in a few minutes reached the city of Goma.

More than 600 people died.

During an eruption on January 17, 2002, the low-viscosity lava of the Nyiragongo flooded almost a fifth of the city of Goma and part of the neighboring city of Gisenyi in Rwanda.

The lava even reached the north shore of Lake Kivu, where it evaporated large amounts of lake water with a great roar.

Weeks of earthquakes preceded this eruption along the southern flank of the volcano.

In addition, the level of the lava lake in the crater fluctuated by a few tens of meters for days before the eruption.

Because of these warning signs, researchers from the Goma Volcano Observatory sounded the alarm, and the authorities evacuated large parts of the city.

Almost 400,000 people fled their homes and more than 240 people lost their lives.

However, the number would have been much higher if the local civil protection officers had not heeded the researchers' warnings.

However, the volcano behaved completely differently in May 2021. There was also a flank eruption on its southern side, and lava poured out until just before Goma airport.

At least 32 people died in the eruption and hundreds of homes were buried under the lava.

However, there was not a single measurable sign in the weeks and days leading up to this outbreak.

The research group led by Delphine Smittarello from the European Center for Geodynamics and Seismology in Walferdange, Luxembourg, reports in the journal "Nature" (doi: 10.1038/s41586-022-05047-8) that in the weeks and days before there were no special earthquakes or geodetic ones measurable bulges appeared on the flanks of the volcano.

The considerable amount of sulfur dioxide that Nyiragongo emits every day also did not vary.

Instead, seismic activity only increased just under 40 minutes before the start of the more than six-hour eruption.

The tremors all initially occurred under the fissures that opened up along the southern flank of the eruption.

The earthquake sources then traveled more than 25 kilometers to the south and finally reached the cities of Goma and Gisenyi, where they caused considerable damage.

After evaluating the data recorded by the measuring devices installed on the volcano, Smittarello and her colleagues came up with the following picture: The cause of the eruption was a sudden bursting of an underground magma channel.

This channel had practically ruptured due to sustained tension or high temperatures, the magma surging to the surface and emerging as lava.

The bursting of the channel also caused the magma to move further south underground, which was reflected in the numerous earthquakes that followed the eruption.

Luckily, the molten rock did not manage to escape outside of the fissures that had originally been torn open and flood the inner cities of Goma and Gisenyi.

This course of the eruption is in complete contrast to previous ideas, according to which magma rising under a volcano leads to ruptures in magma channels.

Because this ascent is always accompanied by earthquakes, the tremors can indicate an impending eruption.

However, the researchers cannot explain why in May last year on the southern flank of Nyirangongo a magma channel burst without warning and then the magma erupted.