Update, 23 August 23:27 BST: As of this evening, Icelandic experts are reconsidering whether an eruption has begun or not. With no surface changes visible, and no meltwater rushing downriver as of yet, the Icelandic Meteorological Office reports “there are no signs of ongoing volcanic activity”. The aviation alert remains red, “as an imminent eruption cannot be excluded”.

A volcanic eruption has begun near the caldera of Bárðarbunga, the Icelandic Meteorological Office (IMO) announced on 23 August. Officials have raised the area’s aviation colour code to red, signifying that an “eruption is imminent or in progress”.

Halldór Björnsson/Icelandic Meteorological Office

The eruption is taking place beneath 150–400 metres of ice, north and east of the Bárðarbunga caldera. For the past week magma has been rising from the deep and forming a long underground sheet of freshly cooled rock, known as a dyke. The formation of the dyke has been marked by a series of intense earthquakes stretching from Bárðarbunga towards a glacier called Dyngjujökull (see ‘Icelandic volcano shakes ominously’).

Scientists from the IMO and the University of Iceland flew over the eruption today and reported no visible signs at the surface. The eruption was probably detected by seismic stations monitoring the region, as the shaking produced when water interacts with magma and turns to steam has a distinctive energy signature. Since the earthquake swarm began on 16 August, Icelandic scientists have been peppering the region with extra seismic and global-positioning instruments to capture just such an event.

Officials have also moved mobile radar observation stations into place around Bárðarbunga, to monitor any plumes if the volcano starts to emit ash. All airports in Iceland remain open, although airspace of approximately 140 by 100 nautical miles (260 by 185 kilometres) has been closed over the eruption site. If the eruption begins to produce ash, the volcanic ash advisory centre responsible for the region may issue an alert. Those alerts can be monitored here.

How the eruption proceeds will depend on how much magma is forcing its way upward and at what rate. The last eruption in Iceland happened in 2011 at the Grímsvötn volcano and was the most powerful in nearly a century. Like this new one, it took place under the Vatnajökull ice cap, and it broke through the ice to spew ash 20 kilometres high. So far, there is no indication that the new eruption will do anything like that, although the interaction of magma and ice is notoriously unpredictable.