Les phénomènes physiques essentiels liés au gel, les structures périglaciaires qui en résultent et leur signification climatique

L'auteur examine les effets des trois mécanismes physiques les plus importants se produisant dans le sol sous l'action du gel : I. Le retrait thermique. Après avoir rappelé comment se forment les fentes de gel, l'auteur distingue les fentes de gel à remplissage de glace, les fentes de gel à remplissage de sable et de glace et les fentes de gel à remplissage uniquement minéral. Les caractères propres à chacune de ces structures sont définis. II. La formation de glace de ségrégation. Après un rappel de l'explication classique de l'apparition de glace de ségrégation, les multiples conséquences de ce phénomène sont examinées : congélifluxion, dessication en profondeur, triage granulométrique, cailloux dressés. III. L'augmentation de volume de l’eau se transformant en glace. Le diagramme des phases de H2O permet de bien comprendre trois phénomènes périglaciaires importants : a. les buttes de glace d'injection pour lesquelles trois origines distinctes sont rappelées, b. les involutions périglaciaires dont une genèse possible par congélation d'eau capillaire comprise au sein du sol est établie par des expériences de laboratoire, c. la gélifraction, mécanisme dans lequel le gel de l'eau capillaire joue aussi, selon l'auteur, un rôle fondamental.The author examines the effects of three physical phenomena produced in the ground by freezing: I. Thermal contraction. After summarizing the physical laws accounting for the appearence of frost cracks, the author groups them into three categories: a. frost wedges filled with ice, b. frost wedges filled with sand and ice, forming in the active layer and in the permafrost, (the sand wedges of Pewe), c. frost wedges with a purely mineral filling, appearing in the active layer or in areas without permafrost. The characteristics of each of these types of frost wedge are enumerated in the hope that they may be recognized in the fossil form. The author insists that all frost wedges do not necessarily imply the existence of permafrost at the time of their formation. II. The formation of segregated ice. Following a short account of the accepted hypothesis of the origin of segregated ice, several consequences are considered: a. the uplift of the soil by frost, b. its supersaturation on thawing, (producing congelifluction), c. its dessication at depth, (of interest in that it produces small scale polygonal cracks), d. size sorting, (the movement of fines in the same direction as the frost wave, and of coarser material in the opposite direction), e. the change in the orientation of pebbles moved by frost, (the origin of verticalstones). The appearance of all these phenomena by no means requires the existence of permafrost. III. The increase in the volume of water as it changes to ice. The phase diagram of H2O enables us to understand three important periglacial phenomena: a. mounds of injection ice which may form not only in an open system (Alaska type)and in a closed system (Mackenzie type), but also in a temporary closed system produced each winter. Pingos indicate the existence of permafrost but the climatic conditions under which they form vary considerably. b. periglacial involutions. After summarizing the various theories that have been proposed for their origin, a new explanation for cryoturbations which affect beds of widely differing grain size is offered: the plastic deformation of already frozen ground by the freezing of capillary water. This hypothesis, based on laboratory experiments which consisted mainly in the measurement of pressure within the soil as it freezes, means that periglacial involutions are not necessarily an indication of the presence of permafrost. c. gelifraction, for which the author stresses the fundamental importance of the freezing of capillary water in the fissures in the rock. This takes place below 0 °C when the fissure is already closed by ice formed at a higher temperature