The glacial outburst flood, or jökulhlaup, which started when the ice sheet in the Grímsvötn volcano beneath Vatnajökull glacier began to melt 11 days ago, is predicted to reach its peak on Sunday. At time of writing, the Icelandic Road and Coastal Administration does not believe that the runoff will affect traffic on Route 1 in South Iceland, nor that roads will need to be closed.
As of Friday morning, the Met Office reported that Grímsvötn ice sheet had sunk over 27m [89 ft] and was flooding the Gígjukvísl river at a rate of 1600 m3/s [56503 f3/s]. The electrical conductivity of the river, which is an indicator of how much geothermal meltwater it has taken on, has also been increasing and was measured above 464 µS/cm on Friday. The gas concentrations along the perimeter of the glacier have been measured at higher than normal levels, but do not currently pose a danger.
In the past, eruptions at Grímsvötn have begun following a glacial outburst flood. Per the Met Office, “[t]he loss of the water from Grímsvötn lake reduces the pressure on top of the volcano and this can allow an eruption to begin.” This happened in 1922, 1934, and later, in 2004, when an eruption beginning three days after a flood began. In that instance, there were a series of earthquakes before the eruption. But no earthquakes have been measured around the volcano at present.