Torfajökull (Icelandic for "Torfi's glacier"; Icelandic pronunciation: [ˈtʰɔrvaˌjœːkʏtl̥] ⓘ) 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 large eruption 55,000 years ago.

Geography

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 1,199 m (3,934 ft) 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 1,281 m (4,203 ft) high.[1][a] Laufafell dome at 1,164 m (3,819 ft) is at the north-western edge of the Torfajökull volcanic system and always half way between Hekla and the glacier of Torfajökull.

Volcano

The volcano's eruption around 870, a combined bimodal eruption (rhyolite-basalt) with additional input from a Bárðarbunga-Veiðivötn volcanic system dyke,[5]: 388  has left a thin layer of easily recognized mixed tephra all over Iceland, the Settlement Layer or Landnámslag.[6]: 129  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 871 ± 2 CE. There was another bimodal eruption in March 1477.

Geology

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.[7]: 2920  The central volcano, is a rhyolitic plateau 600 m (2,000 ft) above the surrounding tholeiitic basalts with initial formation at least 384,000 years ago.[8] As well as containing the largest geothermal system in Iceland at 150 km2 (58 sq mi), it has a 18 km × 12 km (11.2 mi × 7.5 mi) caldera, with 450 km2 (170 sq mi) of rhyolitic exposed extrusives, which is the largest extent of such rocks in Iceland.[2] The largest volume of rhyolite, being 25 km3 (6.0 cu mi) was erupted as the Þórsmörk ignimbrite and widespread North Atlantic and Greenland II-RHY-1 tephra layer about 55,000 years ago.[9][b]

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.[7]: 2921  The postglacial rhyolites were produced by partial melts of previously intruded mafic basalts that started forming between 17,000 and 62,000 years ago.[5]: 395–6  It is known from elsewhere in Iceland that the melting of previous hydrated basaltic crust can be rapid over periods perhaps of 8000 years.[5]: 388  That the last three (not just two) of these,[5]: 389  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.[5]: 388  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.[5]: 388 

Seismic Activity

In the area of highest temperature geothermal activity of more than 340°C, there is an area of low-frequency earthquakes.[7]: 2921 . 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 of the volcanic edifice.[7]: 2921  The western caldera is deflating vertically by about 12 mm (0.47 in)/year and there is evidence for a spherical 4 km (2.5 mi) diameter magma chamber at 8 km (5.0 mi) depth.[7]: 2921  Seismic studies have also detected structures between 1.5–6 km (0.93–3.73 mi) 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.[7]: 2938 

Eruptions

The last four eruptions have been separated by about 940 years.[8]

The 1477 eruption involved the Laugahraun basaltic flow within the northern caldera and the Namshraun, Stútshraun, Frostastadahraun, and Ljótipollur basalt flows to the north of Torfajökull's caldera boundary by up to about 5 km (3.1 mi).[5]: 388  The eruptive fissure is at least 40 km (25 mi) long extending to the north.

The 871 eruption is associated with the Bláhylur basalt explosion crater, which is located 2 km (1.2 mi) to the west of the later Ljótipollur flow and its fissure. On the other side of the caldera the 871 intrusion erupted at the west edge of the caldera rim the Hrafntinnuhraun flow.[5]: 388  This eruption has a VEI of 3,[4] with the Hrafntinnuhraun lava having a volume of 0.18 km3 (0.043 cu mi) and tephra to a volume of 0.4 km3 (0.096 cu mi) being erupted.[8]

The younger part of the Domadalshraun (Dómadalur) lava flows erupted about 150 CE west of the 1477 Namshraun flow,[5]: 388  and has an area of 6 km2 (2.3 sq mi) and volume about 0.1 km3 (0.024 cu mi). To the north the {[cvt|60|km2}} Tjörvi lava was erupted simultaneously but from the Bárðarbunga Veidivötn fissure swam.[8]

The older Domadalshraun to its south of about 3100 BP is about 4 km (2.5 mi) west of the 1477 Namshraun flow.[5]: 388 

The Markarfljöt domes formed about 3500 BP and are in the western central volcano area.[5]: 388 

Just to the west of Laugahraun, and just outside the caldera margin is the Haölduhraun lava flow of about 6500 BP.[5]{{rp|388}

The Laufafell basalt lavas in the in the western central volcano area erupted about 6800 BP and are close to the Laufafell domes.[5]: 388 

The Domadalshraun lava flow is dated to about 7000 BP and is about 2 km (1.2 mi) north of the Haölduhraun flow.[5]: 388 

Just to the east of the Hrafntinnuhraun flow from the 871 eruption is the Slettahraun lava flow that erupted about 8000 BP and east of that the Hrafntinnusker flow of about 7500 BP,[5]: 388  which had an area of 9 km2 (3.5 sq mi) and volume up to 0.4 km3 (0.096 cu mi).[8]

The Þórsmörk (Thorsmork) ignimbrite and widespread North Atlantic and Greenland II-RHY-1 tephra layer of 55,380 ± 2367 yr b2k[9] Other ages determined 40Ar/39Ar dating are 51.3 ± 4.2 ka and 55.6 ± 4.8 ka.[9][10] This eruption had previously assigned to Tindfjöll (Tindfjallajökull) to the south in the 1980s as the Þórsmörk ignimbrite is to the east of Tindfjallajökull but the composition of other Tindfjöll eruptives later studied is different. The Þórsmörk ignimbrite had covered some of the sides of Tindfjöll.[9]

The Rauðfossafjöll tuya at the western aspects of the Torfajökull volcano is dated at 67,000 ± 9,000 years ago.[10]

Glaciers

The two glaciers, Torfajökull and Kaldaklofsjökull, that cover the south–eastern portions of the central volcano are regressing. In 1945 Torfajökull was 16 km2 (6.2 sq mi) in area, in 1999 11 km2 (4.2 sq mi), and by 2019 it was down to 8.1 km2 (3.1 sq mi).[11][12] It has lost 64 % of its maximum mapped area.[11] Kaldaklofsjökull, to the west of Torfajökull has regressed even more being by 2019, 79 % of its past maximum area at only 1.6 km2 (0.62 sq mi).[11]

Naming

According to legend, the glacier is named for Torfi Jónsson í Klofa, an Icelandic historical figure. When the plague arrived in Iceland in 1493, Torfi fled with his family and his belongings into the highlands and settled in a valley surrounded by the glacier.[13]

According to another legend, the glacier is named for Torfi, a farm worker at a nearby farm. Torfi eloped with the farmer's daughter and fled to the glacier.[14]

See also

Notes

  1. ^ a b This article when reviewed as a stub gave an unreferenced height of 1,259 m (4,131 ft) which does not agree with any identified original source but is close to the current height of Kaldaklofsfjöll. So to provide best article historic consistency the height of Háskerðingur as determined by modern survey methods at 1,281 m (4,203 ft) was chosen. Traditional survey height of Háskerðingur was 1,278 m (4,193 ft). The peak near the southern edge of the Torfajökull glacier was 1,190 m (3,900 ft) by traditional survey, but is 1,199 m (3,934 ft) on modern survey and one to the north-east of the glacier had been by traditional survey assigned a height of 1,175 m (3,855 ft) but on modern survey is 1,192 m (3,911 ft).[1] The heights quoted in the volcano literature of 1,190 m (3,900 ft)[3] and 1,280 m (4,200 ft)[4] which are usually trusted sources, may thus be explained.
  2. ^ All the previous timings based on lava samples were off by 22,000 years, and events assigned to about 70,000 years ago before 2019 are still in the recent literature. This is because feldspar crystals yield older ages of 77 ± 6 ka than other techniques of which the currently most accurate time is 55.4 ± 2.5 ka.[9]

References

  1. ^ a b c "National Land Survey of Iceland-Mapviewer (Kortasja-Landmælingar Íslands)". Retrieved 26 May 2024.
  2. ^ a b Sæmundsson, Kristján; Larsen, Gudrún (2019). "Catalogue of Icelandic Volcanoes:Torfajökull". Retrieved 26 May 2024.: Detailed Description:1. Geological setting and tectonic context 
  3. ^ Sæmundsson, Kristján; Larsen, Gudrún (2019). "Catalogue of Icelandic Volcanoes:Torfajökull". Retrieved 26 May 2024.: Central Volcano 
  4. ^ a b "Torfajökull". Global Volcanism Program. Smithsonian Institution.
  5. ^ a b c d e f g h i j k l m n o Zellmer, G.F.; Rubin, K.H.; Grönvold, K.; Jurado-Chichay, Z. (2008). "On the recent bimodal magmatic processes and their rates in the Torfajökull–Veidivötn area, Iceland". Earth and Planetary Science Letters. 269 (3–4): 387–397. doi:10.1016/j.epsl.2008.02.026.
  6. ^ Boygle, J. (1999). "Variability of tephra in lake and catchment sediments, Svínavatn, Iceland". Global and Planetary Change. 21 (1): 129-149. Bibcode:1999GPC....21..129B. doi:10.1016/S0921-8181(99)00011-9.
  7. ^ a b c d e f Martins, J.E.; Ruigrok, E.; Draganov, D.; Hooper, A.; Hanssen, R.F.; White, R.S.; Soosalu, H. (2019). "Imaging Torfajökull's magmatic plumbing system with seismic interferometry and phase velocity surface wave tomography". Journal of Geophysical Research: Solid Earth. 124 (3): 2920–2940.
  8. ^ a b c d e Sæmundsson, Kristján; Larsen, Gudrún (2019). "Catalogue of Icelandic Volcanoes:Torfajökull". Retrieved 26 May 2024.: Detailed Description:4. Eruption history and pattern 
  9. ^ a b c d e Moles, J.D.; McGarvie, D.; Stevenson, J.A.; Sherlock, S.C.; Abbott, P.M.; Jenner, F.E.; Halton, A.M. (2019). "Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland)". Geology. 47 (6): 577–580. doi:10.1130/G46004.1.
  10. ^ a b Guillou, H.; Scao, V.; Nomade, S.; Van Vliet-Lanoë, B.; Liorzou, C.; Guðmundsson, Á. (2019). "40Ar/39Ar dating of the Thorsmork ignimbrite and Icelandic sub-glacial rhyolites". Quaternary Science Reviews. 209: 52–62. Bibcode:2019QSRv..209...52G. doi:10.1016/j.quascirev.2019.02.014.: Conclusions 
  11. ^ a b c d Hannesdóttir, H.; Sigurðsson, O.; Þrastarson, R.H.; Guðmundsson, S.; Belart, J.M.; Pálsson, F.; Magnusson, E.; Víkingsson, S.; Kaldal, I.; Jóhannesson, T. (2020). "A national glacier inventory and variations in glacier extent in Iceland from the Little Ice Age maximum to 2019". Jökull. 12: 1–34. doi:10.33799/jokull2020.70.001.: Table 2. 
  12. ^ Miodońska, Alicja. Assessing evolution of ice caps in Suðurland, Iceland, in years 1986 - 2014, using multispectral satellite imagery: Masters Thesis (Thesis). Lund, Sweden: Lund University. pp. 1–110.: 20 
  13. ^ "Sagnir af Torfa í Klofa". Archived from the original on 23 January 2023.
  14. ^ "Torfajökull". Archived from the original on 15 July 2011.

External links