The subglacial Esjufjöll volcano at the SE part of the Vatnajökull icecap, north of Öraefajökull volcano, consists of the Snaehetta central volcano and a large caldera. Most of the volcano, including the 40 km² caldera, is covered by the icecap, but parts of the SE flank are exposed in NW-SE-trending ridges. Most of the exposed rocks are mildly alkaline basalts, but small amounts of rhyolitic rocks are also present. A large jökulhlaup that came down the Jokulsa a Breidamerkursandi along the coast SE of Vatnajökull in the beginning of September 1927 was accompanied by a sulfur stench, and on one occasion, ash fall on the Breidamerkurjökull considered to have possibly originated from Esjufjöll.