: 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

• Main Authors: De Goër De Herve, Alain, Camus, Guy, Miallier, Didier, Sanzelle, Serge, Falguères, Christophe, Fain, Jean, Montret, Michèle, Pilleyre, Thierry
• Other Authors: Laboratoire de Géographie Physique et Environnementale (GEOLAB), Université Blaise Pascal - Clermont-Ferrand 2 (UBP)-Université Clermont Auvergne 2017-2020 (UCA 2017-2020 )-Université Clermont Auvergne (UCA)-Institut Sciences de l'Homme et de la Société (IR SHS UNILIM), Université de Limoges (UNILIM)-Université de Limoges (UNILIM)-Centre National de la Recherche Scientifique (CNRS), Laboratoire de Physique Corpusculaire - Clermont-Ferrand (LPC), Université Blaise Pascal - Clermont-Ferrand 2 (UBP)-Institut National de Physique Nucléaire et de Physique des Particules du CNRS (IN2P3)-Centre National de la Recherche Scientifique (CNRS), Institut de Paléontologie Humaine (IPH), Fondation I.P.H-Centre National de la Recherche Scientifique (CNRS), L' homme préhistorique : son évolution, son milieu, ses activités, Muséum national d'Histoire naturelle (MNHN)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: French
• Published: HAL CCSD 1993
• Subjects: Chaîne des Puys; Limagne; French Massif central; Debris avalanche; Strombolian eruption; Trachybasalt; TL dating; Permafrost; Volcanic hazard; Trachybasalte; Aléa volcanique; Massif central français; Avalanche de débris; Eruption strombolienne; Datation TL; Pergélisol; geo; envir; Freed; Gravenoire; permafrost; pergélisol
• Online Access: https://halshs.archives-ouvertes.fr/halshs-00406479/file/Gravenoire.pdf; https://halshs.archives-ouvertes.fr/halshs-00406479

The Puy de Gravenoire is a cinder cone built up during Upper Pleistocène 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 faciès) which show an outset of fragmentation and plastic deformation. They are partly pulled out of the volcano's bedrock (granitic basement, old porphyritic basait, sédiments from thé edge of thé 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 cône. 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 (Würm 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 thé éruption 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 ...