Large rockslide‐debris avalanches, resulting from flank collapses that shape volcanoes and mountains on Earth and other object of the solar system, are rapid and dangerous gravity‐driven granular flows that travel abnormal distances. During the last 50 years, numerous physical models have been put forward to explain their extreme mobility. The principal models are based on fluidization, lubrication, or dynamic disintegration. However, these processes remain poorly constrained. To identify precisely the transport mechanisms during debris avalanches, we examined morphometric (fractal dimension and circularity), grain size, and exoscopic characteristics of the various types of particles (clasts and matrix) from volcanic debris avalanche deposits of La Réunion Island (Indian Ocean). From these data we demonstrate for the first time that syn‐transport dynamic disintegration continuously operates with the increasing runout distance from the source down to a grinding limit of 500 µm. Below this limit, the particle size reduction exclusively results from their attrition by frictional interactions. Consequently, the exceptional mobility of debris avalanches may be explained by the combined effect of elastic energy release during the dynamic disintegration of the larger clasts and frictional reduction within the matrix due to interactions between the finer particles.

Article en ligne sur le site de l'éditeur. Disponible sur demande auprès des auteurs pour usage strictement privé.International audienceFlint has been widely used during prehistoric times and the question of determining the origin of specific pieces is recurrent in archaeological discussions. We have run a Scanning Electron Microscope examination of the surfaces of both natural flints from geological deposits and flaked artefacts excavated from prehistoric sites in the Massif central (France). These samples exhibit mineralogical textures resulting from genomorphic and phenomorphic processes, dispersed alteration in the form of erosion as well as the dissolution and migration of more or less mobile elements. Rocks used to manufacture artefacts at Sainte-Anne I (Haute-Loire, France) and Payre (Ardèche, France) show various kinds of alteration related to the range of phases of transport and deposition to which they were subjected beginning with their first exposure in a geological context and following through to the post-depositional phase, (ie, following abandonment as artefacts in an archaeological site). Later phenomenon do not however completely obliterate stigma imprinted prior to their prehistoric collection, because the main late-phase change that occur on flat surfaces prior to their collection is preserved in the hollows of remnant natural surfaces on archaeological tools. With the Scanning Electron Microscope, five distinct morphologies representative of different environments have been characterised by establishing associations between the different forms and by plotting their regional distribution.Les silex ont été largement utilisés par l'homme pendant la Préhistoire et la question de leur provenance est récurrente dans la démarche archéologique. Un examen au MEB a été conduit sur des surfaces de silex naturels et d'objets taillés provenant de gîtes et de sites préhistoriques du Massif central. Ces échantillons présentent des textures minéralogiques découlant de processus génomorphiques et phénomorphiques : l'altération s'est développée, sous la forme d'érosion, de dissolution et de migration d'éléments plus ou moins mobiles. Les roches ayant servi à la fabrication des outillages de Sainte-Anne I (Haute-Loire, France) et de Payre (Ardèche, France) montrent des formes variées contrôlées par les différentes phases de transport et de résidence des silex, de la mise à l'affleurement à la phase post-dépositionnelle, c'est-à-dire après l'abandon de l'objet dans le site archéologique. Les phénomènes tardifs n'oblitèrent cependant pas complètement les stigmates antérieurs à la collecte préhistorique car la phase tardive principale enregistrée sur les surfaces planes des silex avant collecte est préservée dans les creux des surfaces restées naturelle sur les objets archéologiques. Cinq morphologies distinctes représentatives de milieux différents ont été caractérisées à l'aide du MEB. Des critères d'association de formes et de leur distribution distinguent ces types