THE PUY-DE-GRAVENOIRE AND ITS FLOWS, IN T HE 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 Pleistoce ne on the western fault line scarp of the Limagne rift. It tops the ci ty of Clermont-Ferrand by 400 m. The eruption triggered, after the beg inning of activity, a debris avalanche < 0,05 km3 in volume, which spr ead over a 5 x 0,7 km surface in the southern suburbs. The following a ctivity rebuilt the cinder cone and overflowed a lot of lavas which ov erlaid the avalanche deposit. The lava displays a homogenous compositi on of potassic trachybasalt, subaphyric. 4 km away from the source, th e debris avalanche deposit consists of still recognizable panels (bloc k facies) which show an outset of fragmentation and plastic deformatio n. They are partly pulled out of the volcano's bedrock (granitic basem ent, old porphyritic basalt, sediments from the edge of the graben), a nd partly from the volcano itself (initial phreatomagmatic products, s coriaceous ejecta, massive basalt). The basalt of the massive megabloc ks 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 t hat the avalanche is really synchronous with the eruption. The avalanc he scar is obvious in a quarry which worked the cinder cone. Ten attem pts at dating the Gravenoire lava flows were carried out by TL and ESR methods. In spite of relatively dispersed results, we can rate the ag e of eruption at around 60 000 years. At this time (Wurm lower Plenigl acial) the region was subject to a periglacial climate, which could ex plain the abundance of loose material on the Limagne fault scarp, prob ably congealed by a permafrost. The heat produced by the eruption like ly melted locally the permafrost, while the split bedrock was shaken b y the initial explosions ; this curtly freed the material and triggere d the collapse. The debris avalanche from Puy de Gravenoire covered an area on which 30 000 people are now living. This example should attra ct attention on a type of hazard that has so far been ignored, suscept ible to affect any inhabited country in a periglacial environment, eve n related to a small volcano during an usually slightly dangerous erup tive event. South America seems specially concerned.