[10] The mountain itself, a stratovolcano,[11] stands 1,651 metres (5,417 ft) at its highest point, and has a crater three to four kilometres (2 to 2+1⁄2 miles) in diameter, open to the north.

The south face of the mountain was once part of Iceland's coastline, from which, over thousands of years, the sea has retreated some 5 km (3 mi).

The area between the mountain and the present coast is a relatively flat strand, 2–5 km (1–3 mi) wide.

Eyjafjöll is the name of the southern side of the volcanic massif together with the small mountains which form the foot of the volcano.

Hence the southern slopes of the mountain Eyjafjöll result in the sea side strip of land beyond being called Undir Eyjafjöllum.

[15]: 15 The volcano is fed by a magma chamber under the mountain, which in turn derives from the tectonic divergence of the Mid-Atlantic Ridge.

[18] Notably, the ash released from the eruption contained a large fraction of fluoride, which in high doses may damage the bone structure of cattle, horses, sheep and humans.

The eruption also caused some small and medium glacier runs (jökulhlaups) and flooding in nearby rivers Markarfljót and Holtsá [ˈhɔl̥(t)sˌauː].

The eruption columns were shot to considerable heights, with ashfall in both the far north of the country, in Eyjafjörður, and in the southwest, on the peninsula of Seltjarnarnes near Reykjavík.

[18]: 4 In early 1823, the nearby volcano Katla under the Mýrdalsjökull ice cap erupted and at the same time steam columns were seen on the summit of Eyjafjallajökull.

[15]: 16 [20] On 26 February 2010, unusual seismic activity along with rapid expansion of the Earth's crust was registered by the Icelandic Meteorological Office.

[2] The eruption begun on 20 March 2010, about 8 km (5 mi) east of the top crater of the volcano, on Fimmvörðuháls, the high neck between Eyjafjallajökull and the neighbouring icecap, Mýrdalsjökull.

[2] This first eruption, in the form of a 300-meter-long radial fissure vent,[17] did not occur under the glacier and was smaller in scale than had been expected by some geologists.

[24] On 14 April 2010 Eyjafjallajökull resumed erupting after a brief pause, this time from the top crater in the centre of the glacier, causing jökulhlaup (meltwater floods) to rush down the nearby rivers, and requiring 800 people to be evacuated.

Pulsating explosive activity on 17 April 2010 was later understood to be due to periodic clogging/plugging of the conduit associated with the rise and degassing of more magma.

[15]: 80  This second eruption threw volcanic ash several kilometres up in the atmosphere, which led to air travel disruption in northwest Europe for six days from 15 to 21 April 2010.

[28] The London Volcanic Ash Advisory Centre declared the eruption to have stopped on the 23rd of May 2010, but stated that they were continuing to monitor the volcano.

[34][35] On 20 April 2010, Icelandic President Ólafur Grímsson said "the time for Katla to erupt is coming close...we [Iceland] have prepared...it is high time for European governments and airline authorities all over the world to start planning for the eventual Katla eruption".

Emo band The World Is a Beautiful Place & I Am No Longer Afraid to Die released a song on their 2010 EP Formlessness called "Eyjafjallajokull Dance."