The ice cap covers an area of about 100 square kilometres (39 sq mi), feeding many outlet glaciers. The south end of the mountain was once part of the island's Atlantic coastline, but over thousands of years the sea retreated some 5 kilometres (3.1 mi), with the former coastline now consisting of sheer cliffs with many waterfalls, of which the best known is Skógafoss. In strong winds, the water of the smaller falls can even be blown up the mountain.
The volcano, which has a crater 3–4 kilometres (1.9–2.5 mi) in diameter, erupted in 920, 1612 and again from 1821 to 1823 when it caused a glacial lake outburst flood or jökulhlaup. It has 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, and is ongoing.