Post-eruption evolution of maar lakes and potential instability: The Lake Pavin case study, French Massif Central

Abstract

Few studies have focused on the post-eruption evolution of maar craters and associated lake level changes, although breaching of the crater rim poses mass-flow and flood hazards long after the initial eruption. Lake Pavin is a maar lake in the French Massif central formed by an eruption c. 6700 years ago. The maar is perched on the edge of a glacially-shaped valley and the rim overlooks the surrounding subdued landscape. The rim is breached to the north by an overflow channel, and to the south the maar has cut into the flank of the Montchal scoria cone and lava flows. Incision of the lake outlet led to the growth of an alluvial fan that pushed the Couze Pavin River towards the north side of the Gelat valley.Two lake terraces within the crater are used to determine stages in the evolution of the lake. The highstand Stage 1 terrace +18 m above the current lake level indicates that the post-eruption lake surface and volume was 15% and 38% larger than the present-day lake, respectively, prior to the 5-7th Century CE. The poorly preserved Stage 2 bench, +4.35 m on the west side of the natural dam, indicates that incision paused between Stage 1 and achievement of present lake level (Stage 3) before the 18th Century. Four units of streamflow deposits (c. 0.087 km2 in area and 0.14–0.18 km3 in volume) that formed the fan at the mouth of the outlet in the Gelat valley have been 14C dated to between the 9th and 16th Century. From the fan sedimentology, we infer that the lake level dropped slowly between highstand Stages 1 and 2, when the outlet cut down through the rim saddle. The lake level drop accelerated between Stage 2 and Stage 3 before the 14th Century, inducing small debris flows. The latter may have resulted from heavy rainstorms, but there is no evidence for catastrophic events. Stepwise downcutting of the north rim saddle was also governed by contrasted properties of the deposits forming the dam. The current lake level has been stable since at least the early 18th Century, but the long-term stability of the dam is at risk.