Breaking the seal at Grimsvotn, Iceland

Of several problems associated with theoretical explanations of the jokulhlaups which emerge from the outlet glacier Skeioararjokull of the ice cap Vatnajokull in southeast Iceland, the mechanism of flood initiation is one that has hitherto defied explanation. We provide, such an explanation based on a careful analysis of the classical Nye-Rothlisberger model; near the subglacial lake Grimsvotn, the hydraulic potential gradient is towards the lake, and there is therefore a catchment boundary under the ice, whose location depends on the subglacial meltwater drainage characteristics. As the conditions for a flood approach, we show that the water divide migrates towards the lake, while at the same time the lake pressure increases. When the hydraulic potential gradient towards the lake is low and the refilling rate is slow, the seal will "break" when the catchment boundary reaches the lake, while the lake level is still below flotation pressure, whereas if refilling is rapid, flotation can be achieved before a flood is initiated. This theory can thus explain why the seal is normally broken when the lake level at Grimsvotn is still some 60 m below flotation level. In addition, we are able to explain why the jokulhlaup following the 1996 eruption did not occur until flotation level was achieved, and we show how the cyclicity and magnitude of jokulhlaups can be explained within this theory.