Eyjafjallajökull consists of a volcano completely covered by an ice cap. The mountain itself, a stratovolcano, stands 1,651 metres at its highest point, and has a crater 3–4 kilometres in diameter, open to the north. The crater rim has three main peaks, being (clockwise from the north-east) Guðnasteinn, 1,500 metres (approx), Hámundur, 1,651 metres and Goðasteinn, 1,497 metres. The south face of the mountain was once part of Iceland’s Atlantic coastline, from which, over thousands of years, the sea has retreated some 5 kilometres. The former coastline now consists of sheer cliffs with many waterfalls, of which the best known is Skógafoss.
The volcano is fed by a magma chamber under the mountain, which in turn derives from the tectonic divergence of the Mid-Atlantic Ridge. It is part of a chain of volcanoes stretching across Iceland. Its nearest active neighbours are Katla, to the northeast, and Eldfell, on Heimaey, to the southwest. The volcano is thought to be related to Katla geologically, in that eruptions of Eyjafjallajökull have generally been followed by eruptions of Katla.
The Eyjafjallajökull volcano erupted in 920, 1612 and again from 1821 to 1823 when it caused a glacial lake outburst flood or jökulhlaup. It erupted twice in 2010 — on 20 March and in April/May. The March event forced a brief evacuation of around 500 local people, but the 14 April eruption was ten to twenty times more powerful and caused substantial disruption to air traffic across Europe. It caused the cancellation of thousands of flights across and to Iceland.