DHA/95/7
Le volcan Nyiragongo dont l'activite
a repris depuis juin 1994 menace les quelques 200 000 Zarois et les 750
000 refugies
rwandais qui vivent a 15 km de, Goma, et dans les 5 camps de refugies dans
la province du Nord-Kivu, l'Est du Zare.
Le Departement des affaires humanitaires, qui coordonne les activites relatives aux catastrophes naturelles avait depache sur place M. John Tomblin, Chef du Service de l'attenuation des effets des catastrophes ainsi que M. Dario Tedesco, vulcanologue afin d'evaluer les risques presents et futurs d'une eruption et ses consequences sur les populations vulnerables.
Le rapport de M. Tomblin a ete soumis,
Goma, la cellule de crise des Nations Unies cree a cet effet et dirigee
par le
HCR. Il appartiendra a cette cellule de determiner les actions entreprendre
concernant les populations vulnerables.
Ci-joint une version resumee du rapport de M. Tomblin.
Pour de plus amples informations veuillez
contacter au DHA :
Mr. Dandeson Fowler (022) 798 70 49
Mr Nikolai Solomatine (022) 788 34 86
Nyiaragongo: a Leaky Volcano
by John Tomblin
UNDHA, Geneva
11 February 1995
The nature of the problem
Living in Goma, the home of 200,000 Zairians and the temporary home, in nearby camps, of about 750.000 Rwandan refugees, is like living 15 kilometres downstream of a reservoir with a dangerously leaky history. Not a particularly large reservoir, but a very high one - within which the liquid is held by a dam about 1000 metres tall. It is also an unusually thick dam, more than 2 km from front to back at its base, which would be excessive by modern standards for a man-made dam. But it is a natural dam, with a serious weakness: it is made of crumbling volcanic ash and scoria, and deeply fractured ancient lava. This kind of stuff was never intended to hold back the liquid in a reservoir.
Events of 1977
This unusual reservoir overlooking the town of Goma and the surrounding refugee camps is in fact the crater of Nyiragongo Volcano. And when it fills, it is not with water but with molten rock. In 1977, it sprang a spectacular leak for a distance of 12 kilometres.
In the year before, it had filled to within 200 metres of its rim with about 20 million cubic metres of molten lava. For seven months, the natural dam held strain, then it suddenly failed, and disaster struck.
Disaster strikes, but Goma is spared
Probably helped by the shaking from a strongly felt regional earthquake, the red-hot liquid broke its way through the deeply fractured southern flank of the volcano and gushed out like a giant spring 1000 metres below lake level in the crater. Pushed by the enormous pressure from its reservoir far above, the glowing lava spouted down the mountainside, reaching a distance of 12 km in barely 20 minutes, and flooding an area of 15 square kilometres. Advancing at about 40 km per hour, it overturned and burned villages, cars, people and animals. Luckily, there was not enough liquid to reach Goma: the lava flow stopped 2 km short of the airport and 4 km from the outskirts of the town.
The 1977 eruption increased the space inside the crater of Nyiragongo more than five times. After the lava suddenly drained out, the inner walls of the crater fell into the throat of the volcano, and were partly blasted back skywards by violent steam explosions which immediately followed.
The 1982 eruption
The next activity was in 1982, when the lowest 300 metres of the post-1977, 800 metre deep crater was filled by about 70 million m3 of new lava which welted up during 3 months. Over the following 12 years, this quietly and slowly cooled and solidified in the crater bottom. No leakage, because the liquid was at a low level in the reamed-out crater: therefore no problem for Goma.
The present eruption
The latest cycle of activity began in June 1994, and over the past 7 months about 25 million m3 of new liquid have risen into the crater. Happily, because the rate of outpouring onto the 800 metre-wide crater floor has been relatively slow, the lava has spread in successive thin layers across the floor and most of it - at least 90% - has already cooled and solidified. Hence there is at the most only 2 1/2 million m3, and probably much less than this, still liquid and capable of leaking down the mountainside.
What could the future hold in store?
So long as the slow upwelling of new lava onto the crater floor continues at its present relatively slow rate, there will be no cause for alarm. If this activity were to continue in the same style for as long as two years, the crater would fill up with congealed lava. Then the population of Goma would see small-scale but for them harmless spillage over the rim to form small lava flows on the upper slopes. Once again, no cause for alarm in Goma or in the refugee camps.
Trouble would begin if the new lava began to well up much faster into the crater. This is distinctly possible, given that during the most active part of the three-month-long eruption of 1982, molten rock rose into the crater at almost ten times the present (1994 to 1995) rate. If this began now, within a month there would be a lake of 35 million m3 of mainly liquid lava (too big a mass to cool quickly) at an altitude not far below the level that proved to be critical in 1977.
And the volume after one month of rapid input would be almost double the amount that rushed down towards Goma in 1977. Double the volume would mean roughly double area affected: Goma airport and at least part of the present town would be flooded - and not just with a liquid like water.
To have a one-metre-high flood of water swill around you would be a frightening and perhaps bruising experience, but unless you were unlucky, you would probably survive. Not so for lava at 1000o centigrade: a one-metre-high flood would scorch off your clothes and skin and would leave you coated with a half-centimetre layer of quickly-congealing volcanic glass like a fragile, life-size porcelain ornament.
Let us hope that such an apocalypse will never occur, and that with the careful monitoring and emergency planning now being carried out by the Goma authorities and international community, this macabre disaster scenario can be avoided. With the commitment and co-operation of the people involved, and no matter of how badly the volcano behaves, we at UNDHA confidently believe that lives need not be lost.