Extending south from the Vatnajökull ice cap, Öræfajökull is Iceland's highest mountain at 2110 m. Its actual height depends on the season and how deep the snow is - the highest peak, Hvannadalshnjúkur, is topped by ice. It is thickest in springtime and thinnest in the autumn. The land at the foot of the mountain is approximately 100 m above sea level.

Öræfajökull is a stratovolcano - shaped like a cone with its top cut off - with a large ice-filled caldera, 600-700 m deep. its ice cap reaches down to 1000 m above sea level, but several cliffs on the peaks around the caldera rim are generally free of ice. This stratovolcano is believed to the second largest of its type in Europe - only Etna on the island of Sicily is larger. The diameter of the mountain at its foot is approximately 20 km. It has a base area of almost 400 km² and a volume of 370 km³.

Black desert and glaciers

Two vast expanses of volcanic sand deposits came into being after the large eruption in 1362. They lie on each side of Öræfi, a settlement at the foot of the mountain that had to be abandoned due to ash falls when it was left standing in an area that looked like a black desert.

The rhyolite peak Hvannadalshnjúkur rises 300 m above the compacted snow field in the ice-filled caldera. Other rhyolite peaks exist but the mountains under the ice are probably for the most part tuff. The caldera is approximately 5 km long and covers an area of 12 km².

Accumulation on the glacier is a little less than 10 m per year and the average annual precipitation is close to 5000 mm, the highest in Iceland. A number of small outlet glaciers extend down the furrowed flanks of the mountain from an altitude of approximately 1800 m down to its roots. They include Falljökull, Kvíárjökull, Fjallsjökull and Svínafellsjökull.

Eruptions

This stratovolcano has erupted twice in recorded history, in 1362 and in 1727. Geologists believe that the earlier eruption was the largest pumice eruption in the recorded history of Iceland, although there was an even larger one about 2800 years ago. In 1326, ten billion cubic metres of volcanic ash were emitted (or 10 km3, which is equivalent to 2.5 km³ of solid rock). Devastating floods followed the eruption, sweeping away a large number of farms down on the coast. In addition to the deluge, pyroclastic flows and ash fall caused the abandonment of other farms. Thick, light-coloured pumice layers from this eruption are a common feature in the surrounding district.

The second eruption began in August 1727 and lasted for almost a year. The volcano was at its most active during the first three days. The ash fall was so great that it was impossible to distinguish night from day. However, fewer people and livestock were killed than in the earlier eruption, and no farms were destroyed. Farmhouses had been built on higher ground than previously, and the total ash fall was actually less. The main flood flowed along the courses of the Rivers Sandfell and Hof where evidence of the deluge is still visible.

Travel on the ice cap

Early documents provide evidence of travel over the Vatnajökull ice cap from north to south and vice versa, but there are no descriptions of ventures onto Öræfajökull until 1794 when Sveinn Pálsson trekked up onto the mountains from Kvískerjar. During his trip, he discovered the reason for the formation of glaciers, probably the first person to do so in the world, and their movement as a viscous material that creeps forward due to the effects of gravity. He also noticed ash layers in the glacier where it broke from the edge of the summit on its way down, which helped him draw the correct conclusions.

Eruption changed the landscape

Háalda is a bulky glacial sediment ridge between Sandfell and Hof. The composition of the sediment indicates that it was deposited by a fast-flowing glacial burst (jökulhlaup), partly from under the Kotárjökull glacier. The ridge was formed during a huge flood carrying rocky granules, volcanic ash and icebergs during the 1727 eruption under Öræfajökull glacier. A prominent depression (jökulker) in the mountainside is the imprint of an iceberg. This phenomenon is typical of a dead-ice landscape.