| Prestahnúkur | |
| Elevation M: | 1223 |
| Translation: | peak of the priests |
| Language: | Icelandic |
| Location: | Iceland |
| Type: | Subglacial |
| Last Eruption: | about 900 CE |
The peak Prestahnúkur (pronounced as /is/) with a height of, is in the Western Volcanic Zone to the west of the Highlands of Iceland to the west of Langjökull glacier, or to be more specific, to the west of Geitlandsjökull glacier, a part of the Langjökull. The volcano Prestahnúkur includes the terrain under the Geitlandsjökull glacier continuous to the peak and also includes fissure fields to its north and south.
The central volcano Prestahnúkur consists of a rhyolite tuya that was formed entirely under ice, and has been dated as 89 ± 24, but another dating is about 60 ka. Two vents have been found. At the base of the northern flank is a geothermal area, with the rhyolite and hot springs suggesting that a long-lived magma chamber is or has been present. There has been intermittent seismic activity, mostly in an area about wide centred at the central volcano, but as no dyke intrusions have been observed, so it is unproven that the volcano is active.
In 2009 geologists in their research of earthquake events in the area showed that volcanic fissures lie in direction southwest-northeast and reach under the glaciers Þórisjökull and Geitlandsjökull-Langjökull. The Prestahnúkur fissure swarm has subsequently been defined as extending both to the north and south of the central volcano with a total length between and width up to . To the north the Hallmundarhraun lava flow is believed to have most likely erupted sometime between 782 and 860 CE and extends well beyond Eiríksjökull. The Jökulkrókur lava to its west is between 1800 to 4500 years and one of the Geitlandshraun lava flows to its south-east has been dated to 8900 years BP. The other Geitlandshraun lava flow is under ice just north of the central volcano and is undated, but older. The Ok shield volcano rises to a height of to the east-nor-east of the central volcano. To the south-east of the central volcano some of the fissure swarm is now covered by lava from the Skjaldbreiður shield volcano which is in a neighbouring volcanic system. The southern extent of the fissure swarm is unclear and may overlap with that of the Hengill volcanic system. Intraglacial volcanic formations in the fissure swarm include tuyas produced by fissure eruptions through single vents and tindars. Apart from the Hrúðurkarlar tindars which cut obliquely across the swarm, the tindars parallel the fissure swarm at a strike of about 30°.
The central volcanoes rhyolite has for Iceland a high silica content of about 77%, and must have come from a single batch of magma over a relatively short time span of less than 20 years. It was for some time a popular exploited construction material, partly for export. But the mine has been closed. The fissure swarm lavas are basaltic olivine tholeiite.
The name means "peak of the priests".
The origin of the name was an expedition of two priests into the highlands in the 17th century. It was seen as quite an enterprise at the time when they went into this region. They explored especially a valley behind the Þórisjökull called Þórisdalur, which had a bad reputation in sagas and folk stories because it was believed to be haunted by ghosts and that lawless people would be living there. As is clear today, they found nothing of the sort, but they were regarded as heroes when they came back from this expedition.
The highland road Kaldadalsvegur is situated not far from the mountain and it is possible to access the mountain by a bad jeep track and climb it.