This description on the Grímsvötn eruption comes from Dr Olgeir Sigmarsson, an Icelandic volcanologist who is Director of Research, CNRS at the Laboratoire Magmas et Volcans, Observatoire de Physique du Globe de Clermont-Ferrand. He also works at the Institute of Earth Sciences, University of Iceland. His research is on geochemistry: genesis, chronology and evolution of magma. Dr Sigmarsson has been collecting tephra samples from the glacier in the field near to the Grimsvötn volcano and the below contains information from his Icelandic colleagues.
The Grimsvötn eruption 2011 has stopped for the time being. It started around 19:00 last Saturday (21 May) and it’s eruption column rose to approximately 17 km during the first night. It thus penetrated into the stratosphere, observed for the first time for an eruption from Grimsvötn volcano. Grimsvötn is a complex caldera structure covered by the largest ice-cap in Europe, namely Vatnajökull. The high-temperature area melts the floating ice on the subglacial lake that occasionally is lifted from the bedrock by the water mass, emptying the lake and creating glacier bursts, better known as jökulhlaup. Such pressure release generates more vigorous boiling in the geothermal system and sometimes may act as a trigger for an eruption (such as in the 2004 eruption). Fortunately the last jökulhlaup occurred last autumn, leaving little water in the subglacial caldera lake.
The actual eruption emitted approximately 10-20 tons/sec of tephra during the first day but declined rapidly and on the 24th May the column rose only to 3 km height during occasional explosions. On the 4th day, only vapour explosions with little solid material were observed at the crater close to the south wall of the caldera. Tephra fall was noticed all over Iceland (with the exception of the Westfjord peninsula) with grain-size having approximately 10% finer than 10 micronmeters and 50% finer than 50 micronmeters. Based on preliminary measured tephra thickness, the volume is crudely estimated as ½ cubic km of freshly fallen tephra.
Seismicity and deformation indicate a shallow magma source and the magma is basaltic of quartz-normatve tholeiite composition similar to all historic tephra. However, since the Laki eruption, increased concentrations of incompatible elements have increased with time suggesting a closed-system behaviour with minimal deeper input of more primitive magma. The fact the actual eruption was very explosive in the beginning and rose above the tropopause indicate either (1) a more evolved basaltic magma than before or (2) a deeper gas-rich magma entering into the plumbing system beneath Grimsvötn caldera. The first possibility would suggest that the volcano is evolving towards more evolved magma with higher gas content
and potential high explosivity, whereas the second possibility suggests a changing feeding system with renewal of fresh basalt from depth. These two possibilities can be distinguished by precise trace element analysis of the actual tephra.