According to the Icelandic Meteorological Office, the water level of the Skaftá river at Sveinstindur and electrical conductivity both rose during 16-17 June, indicating a glacial outburst flood (jokulhlaup), originating from [the western Skaftá ice cauldron, about 20 km NW of Grímsvötn]. The jokulhlaup was unconfirmed without visual observations, however. The report warned that hydrogen sulfide released from the floodwater as it drains is particularly potent at the river outlet at the ice margin, where concentrations may reach poisonous levels. The cauldrons drain an average every two years, producing floods of up to 1,500 cubic meters per second.

Geological Summary. Grímsvötn, Iceland's most frequently active volcano in recent history, lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by a 200-m-thick ice shelf, and only the southern rim of the 6 x 8 km caldera is exposed. The geothermal area in the caldera causes frequent jökulhlaups (glacier outburst floods) when melting raises the water level high enough to lift its ice dam. Long NE-SW-trending fissure systems extend from the central volcano. The most prominent of these is the noted Laki (Skaftar) fissure, which extends to the SW and produced the world's largest known historical lava flow in 1783. The 15 km3 basaltic Laki lavas were erupted over 7 months from a 27-km-long fissure system. Extensive crop damage and livestock losses caused a severe famine that resulted in the loss of one-fifth of the population of Iceland.

Source: Icelandic Meteorological Office (IMO)