Breaking the seal at GriITlsvotn, Iceland

Of several problems associated with theoretical explanations of the jokul­ hlaups which emerge from the outlet glacier Skeioararj okull of the ice c ap Vatnajokull i n southeast Iceland, the mechanism of flood ini ti atio n i s one that has h itherto 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 character istics. As the con­ di tions for a flood approach, we show that the water divide migrates towards the lake, while at the same time the lake pressure i ncreases. When the hydraulic potential gradient to­ wards the lake is low and the refill ing rate is slow, the s eal will "break" when the catchment boundary reaches the lake, while the lake level is s ti l l below flotation pressure, whereas i f refilling is rapid, flotation can be ach ieved before a flood is initiated. This theory can thus explain why the seal i s normally broken when the lake l evel at Grimsvotn i s still some 60 m below flotation level. In addi tion, we are able to explain why thejokulh laup following the 1996 eruption did not occur until flotation level was achieved, and we s how how the cyeli­ c ity and magnitude ofjokulhlaups can be explained w i thin this theory.