The subglacial Esjufjöll volcano at the SE part of the Vatnajökull icecap, north of Öraefajökull, consists of the Snaehetta central volcano and a large caldera. Most of the volcano, including the 40 km2 caldera, is covered by ice, but parts of the SE flank are exposed in NW-SE 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 odor, and one ashfall event on the Breidamerkurjökull possibly originated from Esjufjöll. Although Holocene eruptions have not been confirmed, earthquake swarms that could indicate magma movements were detected in October 2002.