Breaking the seal at Grimsvotn, Iceland

Of several problems associated with theoretical explanations of the jokulhl aups which emerge from the outlet glacier Skeioararjokull of the ice cap Va tnajokull in southeast Iceland, the mechanism of flood initiation is one th at has hitherto defied explanation. We provide, such an explanation based o n a careful analysis of the classical Nye-Rothlisberger model; near the sub glacial lake Grimsvotn, the hydraulic potential gradient is towards the lak e, and there is therefore a catchment boundary under the ice, whose locatio n depends on the subglacial meltwater drainage characteristics. As the cond itions 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 hyd raulic 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 refillin g is rapid, flotation can be achieved before a flood is initiated. This the ory 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 ab le to explain why the jokulhlaup following the 1996 eruption did not occur until flotation level was achieved, and we show how the cyclicity and magni tude of jokulhlaups can be explained within this theory.