October 10th, 1996
There are now ten days since the beginning of the eruption, marked by onset of eruption tremor at about 22 h on September 30th. The eruption was subglacial for the initial 30 hours, then broke through the ice. The eruptive activity was largest for the first three days of the eruption, but then decreased markedly on October 3th. Since then no decline in activity is observed on the surface of the glacier although the eruption tremor is slowly diminishing. Occasional explosions yesterday resulted in black ash clouds reaching altitude of 10 000 feet, the eruption plume of vapour and finer ash reached altitude of 13 000 feet. Westerly wind deflected the eruption plume towards the east, the ash fall is still minor and is limited to the Vatnajokull glacier.
The eruption has now melted about 2 km3 of ice and all the meltwater has flowed into the nearby Grimsvotn caldera lake. It is clear that the lake level is high enough to cause a glacier burst, but still it has not happened. The water has to find a subglacial drainage from Grimsvotn to the edge of the Vatnajokull ice cap, a distance of about 50 km. The physical mechanism of the glacier burst triggering is poorly understood, making it difficult to predict the beginning of the burst. It is now clear that the preparation time for the glacier burst is at least few days, the time since the caldera lake was full until now.