Breaking the seal at Grímsvötn, Iceland

Of several problems associa ted wit h theoretica l expla nation s of the jokul­ hla ups which emerge from the outlet gla cie r Skeioarar jokull of the ice ca p Vatna jokull in sou theast Iceland, the mechani sm of flood init iation is one that has hi therto defied expl anat ion . We provide such an expl ana tion based on a car efu l analysis of the cla ssica l Nye-Ro thlisbe rger model; near the subglacial lake Grimsvotn, the hyd raulic potenti al gradie nt is towards the lake, and there is theref ore a catchme nt boun dary under the ice, whose location dep ends on the subglacial mel twater drai nage charac terist ics. As the con ­ dit ions for a flood approa ch, we show that the wat er divi de migra tes towards the lake, while at the sa me time the lake pre ssure in creases. When the hydra ulic potential gradient to­ wards the lake is low and the refillin g rate is sl ow, the se al will "break" whe n the cat chment boun dary reaches the lake, while the la ke level is st ill below flotati on pressu re, whereas if refillin g is rapid, flo tation can be achie ved bef ore a flood is initia ted. This theory can thus explain why the seal is normally broken when the lake le vel at Grimsvotn is still some 60 m below flotation level. In addit ion , we are abl e to explain why thej okulhla up following the 1996 eruption did not occur until flo tation level was achie ved, and we show how the cyeli­ cit y and magnitude ofjoku lhla ups can be explained wit hin this theory.