| Torfajökull | |
| Elevation M: | 1,281 |
| Map: | Iceland |
| Map Size: | 200 |
| Label Position: | right |
| Location: | Iceland |
| Coordinates: | 63.9167°N -19.1667°W |
| Type: | Stratovolcano |
| Age: | Pleistocene |
| Last Eruption: | March 1477 |
Torfajökull (Icelandic for "Torfi's glacier"; pronounced as /is/) is a rhyolitic stratovolcano, with a large caldera (central volcano) capped by a glacier of the same name and associated with a complex of subglacial volcanoes. Torfajökull last erupted in 1477 and consists of the largest area of silicic extrusive rocks in Iceland. This is now known to be due to a VEI 5 eruption 55,000 years ago.
The volcano is located north of Mýrdalsjökull and south of Þórisvatn Lake, Iceland. To its south-west is the volcano and glacier of Tindfjallajökull and almost directly to its west is the volcano of Hekla. Adjacent to the southern edge of its glacier of Torfajökull it has a peak of but the south-eastern caldera margin also extends to the glacier of Kaldaklofsjökull which is on the western slopes of a peak called Háskerðingur that is high.[1] Laufafell dome at is at the north-western edge of the Torfajökull volcanic system and almost halfway between Hekla and the glacier of Torfajökull.
The volcano's eruption around 870, a combined bimodal eruption (rhyolite-basalt) with additional input from a Bárðarbunga-Veiðivötn volcanic system dyke,[2] has left a thin layer of easily recognized mixed tephra all over Iceland, the Settlement Layer or Landnámslag.[3] This layer makes it possible to determine the exact dates of many archeological finds by tephrochronology, and such have been dated in the The Settlement Exhibition, Reykjavík City Museum to before 877 ± 2 CE. There was another bimodal eruption in March 1477.
Amongst Icelandic volcanoes Torfajökull has a unique position at the intersection of the rift zone that is the extension of the Mid-Atlantic Ridge and the South Iceland seismic zone transform zone that connects to the Reykjanes Peninsula/ridge.[4] The central volcano, is a rhyolitic plateau above the surrounding tholeiitic basalts with initial formation at least 384,000 years ago.[5] As well as containing the largest geothermal system in Iceland at, it has a caldera, with of rhyolitic exposed extrusives, which is the largest extent of such rocks in Iceland.[6] The largest volume of rhyolite, being was erupted as the Þórsmörk ignimbrite and widespread North Atlantic and Greenland II-RHY-1 tephra layer about 55,000 years ago.[7]
Within the area of the rhyolitic caldera there are younger extrusives that involve basaltic magma mixing events by lateral propagation, from the fissure swarm of Bárðarbunga's Veidivötn volcanic system.[4] The postglacial rhyolites were produced by partial melts of previously intruded mafic basalts that started forming between 17,000 and 62,000 years ago.[2] It is known from elsewhere in Iceland that the melting of previous hydrated basaltic crust can be rapid over periods perhaps of 8000 years.[2] That the last three (not just two) of these,[2] erupted simultaneous with the Veidivötn tholeiitic basalts along single, continuous fissures, indicates that the magma plumbing systems of the Torfajökull and southern Bárðarbunga volcanic systems are presently tectonically linked.[2] Older rhyolites from west Torfajökull, arose from melts sourced from the transitional alkali basalts that are abundant in the South Iceland seismic zone and did not involve basalt sourcing and presumed intrusions from the Bárðarbunga volcanic system.[2]
In the area of highest temperature geothermal activity of more than, there is an area of low-frequency earthquakes.[4] An area of high-frequency earthquakes (4–10 Hz with magnitude less than 3) is in the western caldera, beneath the most recent eruptive sites, is believed to be related to brittle failure ofthe volcanic edifice.[4] The western caldera is deflating vertically by about /year and there is evidence for a spherical diameter magma chamber at depth.[4] Seismic studies have also detected structures between depth consistent with cold dikes along the north–east caldera border, and beyond the caldera, to its south–west and east, there are anomalies consistent with the presence of warm magma bodies.[4]
The last four eruptions have been separated by about 940 years.[5] The largest eruption known at about 55,400 years ago had a VEI of 5 or magnitude of 5.9.[8]
The 1477 eruption involved the Laugahraun basaltic flow within the northern caldera and the Námshraun, Stútshraun (Norðurnámshraun), Frostastaðahraun, and Ljótipollur basalt flows to the north of Torfajökull's caldera boundary by up to about .[2] The eruptive fissure is at least long extending to the north.
The 877 eruption is associated with the Bláhylur basalt explosion crater, which is located to the west of the later Ljótipollur flow and its fissure. On the other side of the caldera the 877 intrusion erupted at the west edge of the caldera rim the Hrafntinnuhraun flow.[2] This eruption has a VEI of 3, with the Hrafntinnuhraun lava having a volume of and tephra to a volume of being erupted.[5]
The younger part of the Dómadalshraun (Dómadalur) lava flows erupted about 150 CE west of the 1477 Namshraun flow,[2] and has an area of and volume about . To the north the