Esjufjöll

The subglacial Esjufjöll volcano is located in the south-east part of the Vatnajökull icecap. Esjufjöll is a strict nature reserve (IUCN category Ia).

Geography
The recognisable mountains associated with the volcano are nunataks within Vatnajökull. These are located about 20 km to the north of Öræfajökull volcano and about 20 km north-west of the Jökulsárlón glacial lake. The highest part of volcano is an ice-covered ridge called Snæhetta, whose assigned height is lower than previous figures, at 1745 m. The mountain range to this ridge's south consists of four nunatak mountain massifs called Vesturbjörg, Skálabjörg, Esjubjörg and Austurbjörg, in a north-west to south-east alignment, with three valleys in between them. In the second half of the 20th century, a number of small nunataks appeared as the more southern ice cover decreased; and two of the largest are named Bræðrasker and Kárasker.

The glaciers flowing around them are feeding the Breiðamerkurjökull, the most south-easterly part of the Vatnajökull icecap, immediately adjacent to Jökulsárlón, which it feeds. Breiðamerkurjökull, which had in 2010 an area of 906 km2, covers the volcano. There are three main glacial catchments, with the western arm draining the north western slopes of the Öræfajökull volcano, being separated from the central arm by the Mávabyggðir (Fingurbjörg), 1439 m, and Kárasker nunataks. To its west the central arm is associated with the Vesturbjörg and Skálabjörg nunataks and the eastern arm is associated with the Esjubjörg and Austurbjörg nunataks.

Volcanism
Esjufjöll is a volcanic system and is part of Iceland's Öræfi volcanic belt. Also part of this zone are Öræfajökull and Snæfell (north-east of Vatnajökull).

The volcanic system consists of the Snæhetta central volcano with an assumed large caldera, but this has not yet been mapped well. Most of the volcano, including the 40 km2 caldera, is covered by the icecap. The latest data is suggestive of a volcano that may be 20 km in diameter and its northern part is underlain by a dense intrusive body. Satellite imaging suggests a caldera that may be 5 x. Vesturbjörg, Skálabjörg, Esjubjörg and Austurbjörg constitute the south-eastern flank of this caldera. Most of the exposed rocks are mildly alkaline basalts, but to the east there are rhyolitic rocks.

In the beginning of September 1927, a glacial flood (jökulhlaup) came down the Jökulsá á Breiðamerkursandi (a short river from Breiðamerkurjökull to the sea). The glacier run was accompanied by a sulfur stench. On one occasion, ash fall on the Breiðamerkurjökull was thought to have possibly originated from Esjufjöll. Although Holocene eruptions from Esjufjöll have not been confirmed, earthquake swarms that could indicate magma movements were detected in October 2002, and again in October 2010.

Mountaineering
Ari Trausti Guðmundsson proposed a mountaineering tour crossing Breiðamerkurjökull and up on Lyngbrekkutindur.