[10][11] The subglacial eruption fissure is to be found in the northwest corner of Vatnajökull ice cap more or less halfway between the central volcanoes Bárðarbunga and Grímsvötn.

[13] Parts of two volcanic zones of Iceland are placed under Vatnajökull, i.e. the very active East Volcanic Zone (connected to rifting at the divergent plate boundary in Iceland[13]), responsible for the highest number of eruptions after deglaciation[14] and with the mantle plume probably under Bárðarbunga, i.e. under Vatnajökull.

"[16] There is also the much less active Öraefi Volcanic Belt, a flank zone mostly under the eastern part of Vatnajökull.

[13][17] It is thought that due to climate change, Vatnajökull has lost about 10% of its mass since the end of the 19th century.

In particular a Mw5.6 event took place on 29 September in the northern part of the Bárðarbunga caldera and its aftershock sequence propagated over the next two days in a linear fashion towards Grímsvötn.

[8] Another possibility is that Bárðarbunga magma entered a portion the magmatic system of Grímsvötn and started the eruption by this intrusion.

[20] The Gjálp eruption took place at a some kilometers long known fissure under 550–700 m (1,800–2,300 ft) of glacier ice within Vatnajökull.

[6] In the beginning, a 2–4 km (1.2–2.5 mi) long N–S trending depression was formed above the fissure, with time three ice cauldrons were built at each end and in the middle,[1] but the eruption concentrated later on one of them where a 200–300 m (660–980 ft) wide crater came to light.

[6] The subglacial channels were easily recognized, because continuous melting caused by the hot water from the eruption site initiated the formation of depressions on the ice surface.

[9]: 33  Basaltic andesite from a 1887 eruption had been previously attributed to the Grímsvötn volcanic system and had very similar composition.

[9]: 62 In the beginning, scientists presumed that the eruption would be followed immediately by a big jökulhlaup (a sort of a meltwater tsunami including large blocks of ice and a high quantity of sediment).

But it took some time to fill the subglacial lake of Grímsvötn in such a manner that the ice wall holding it back would break.

[21][22] In the end, the water sprang up from under the glacier edge and the flood covered most of Skeiðarársandur glacial outwash plain, destroying on its way large parts of the main road Hringvegur including two bridges and some communication installations.