THE PUY-DE-GRAVENOIRE AND ITS FLOWS, IN THE AGGLOMERATION OF CLERMONT-FERRAND (FRENCH MASSIF-CENTRAL) - AN UNUSUAL PATTERN OF DEBRIS AVALANCHE, TRIGGERED BY A STROMBOLIAN ERUPTION IN A PERIGLACIAL CLIMATE

The Puy de Gravenoire is a cinder cone built up during Upper Pleistocene on the western fault line scarp of the Limagne rift. It tops the city of Clermont-Ferrand by 400 m. The eruption triggered, after the beginning of activity, a debris avalanche < 0,05 km3 in volume, which spread over a 5 x 0,7 km surface in the southern suburbs. The following activity rebuilt the cinder cone and overflowed a lot of lavas which overlaid the avalanche deposit. The lava displays a homogenous composition of potassic trachybasalt, subaphyric. 4 km away from the source, the debris avalanche deposit consists of still recognizable panels (block facies) which show an outset of fragmentation and plastic deformation. They are partly pulled out of the volcano's bedrock (granitic basement, old porphyritic basalt, sediments from the edge of the graben), and partly from the volcano itself (initial phreatomagmatic products, scoriaceous ejecta, massive basalt). The basalt of the massive megablocks is the same one as the overlying lava flow; it was still hot enough to bake the rapping granitic clayey sands and gravelly mudstones. The features of mixing and mutual injection among lava and sediment show that avalanche and lava flow were simultaneously moving. This proves that the avalanche is really synchronous with the eruption. The avalanche scar is obvious in a quarry which worked the cinder cone. Ten attempts at dating the Gravenoire lava flows were carried out by TL and ESR methods. In spite of relatively dispersed results, we can rate the age of eruption at around 60 000 years. At this time (Wurm lower Pleniglacial) the region was subject to a periglacial climate, which could explain the abundance of loose material on the Limagne fault scarp, probably congealed by a permafrost. The heat produced by the eruption likely melted locally the permafrost, while the split bedrock was shaken by the initial explosions ; this curtly freed the material and triggered the collapse. The debris avalanche from Puy de Gravenoire covered an area on which 30 000 people are now living. This example should attract attention on a type of hazard that has so far been ignored, susceptible to affect any inhabited country in a periglacial environment, even related to a small volcano during an usually slightly dangerous eruptive event. South America seems specially concerned.