Thordarhyrna

Thordarhyrna

Thordarhyrna

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Thordarhyrna (Icelandic: Þórðarhyrna [ˈθourðarˌhɪ(r)tna] ⓘ) is one of seven[3] subglacial volcanoes beneath the Vatnajokull glacier in Iceland. It is a paired active central volcano with Grímsvötn, and can be classified as part of the Grímsvötn-Laki volcanic system, with common fissure swarms to the south.[4]

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Geography

Thordarhyrna central volcano is 15 km (9.3 mi) in diameter,[5] and located north-east of the Síðujökull outlet glacier in a line that goes through the Háabunga ice cap feature of Vatnajokull on the way to the Grímsvötn caldera.[6] To the immediate south-east of Thordarhyrna the outlet glacier has been called Djúpárjökull and to its east the outlet glacier has been called Grænalónsjökull.[7] The central volcano Hamarinn of the Bárðarbunga volcanic system is to the north-east.

Most of the volcano is covered with between 100–600 m (330–1,970 ft) of ice and its highest point is about 1,650 m (5,410 ft). [4][1] It is to the south-east of a higher ice covered ridge connecting it with Grímsvötn called Háabunga which reaches a height of 1,742 m (5,715 ft). The eastern flank of the volcano has an ice free area with a height of 1,355 m (4,446 ft) called Pálsfjall.[6]

Eruptions

The Thordarhyrna central volcano definitely erupted in a VEI 4 event, on the 28th of May 1903.[2]. This was associated with Grímsvötn activity, with the whole dual eruption period of activity extending from December 1902 to January 1904.[2] No other eruption can be assigned with certainty to Thordarhyrna.[8] Similar phreatomagmatic eruptions would erupt disruptive tephra.[9]

The following eruptions are likely to have involved the Thordarhyrna central volcano:

  • 15 August 1887 - 1889 with a VEI 2 event[2]
  • 4 February 1823 with a VEI 2 event[2]
  • 1753 with jökulhlaups in the rivers to the south-west side of Thordarhyrna.[8]

An eruption south of Thordarhyrna in 3550 BC ± 500 years poured out 150,000,000 m3 (5.3×109 cu ft) of lava in the area of Bergvatnsárhraun at the southern edge of Vatnajokull (Djúpárjökull) closest to Thordarhyrna.[10]

Jökulhlaups could occur on either the Skeiðarársandur outwash plain or affecting the Djúpá, Hverfisfljót, or Skaftá rivers.[11]

Geology

There is both a mechanical interaction encouraging dyke propagation between Thordarhyrna,[12] a stratovolcano,[13] and Grímsvötn, despite these volcanoes being relatively far apart,[14] and a close chemical affinity in their lavas.[15] Both volcanoes are part of the Eastern volcanic zone of Iceland, are over the Iceland mantle plume,[16] and are related to two associated parallel fissure systems with crater rows extending to the south east; the 25 km (16 mi) long Laki–Grímsvötn fissure system and the 30 km (19 mi) long Rauðhólar-Eldgígur fissure system.[15][17] The 1783 Laki fissure eruption crater row is the location of the most significant effusive eruption in the last 1000 years in Iceland.

A fault runs (N 35° W) from Thordarhyrna towards Hamarinn, and separates two different tectonic regions. The southern topography has north-east to south-west long linear hyaloclastic ridges, while to the north-east, there are the central volcanoes of Hamarinn, Bárðarbunga, and Grímsvötn.[18] These last are tectonically at the north-east corner of the Hreppar microplate.

Mostly samples from the Thordarhyrna central volcano are tholeiitic basalt.[5] However samples from the nunataks poking through the ice have yielded basaltic andesite, dacite or rhyolite specimens consistent with more mature lavas.[19]

Near Pálsfjall in the north-west part of the central volcano, there is a small geothermal area.[20]

See also

References

  1. [1]
    Guðmundsson & Larsen 2019, Central Volcano.
  2. [2]
    "Thordarhyrna". Global Volcanism Program. Smithsonian Institution. Retrieved 31 March 2024.: Eruptive History 
  3. [3]
    Andrew 2008, Figure 2.8.
  4. [4]
    Guðmundsson & Larsen 2019, Short Description.
  5. [5]
    Guðmundsson & Larsen 2019, Detailed Description:1. Geological setting and tectonic context.
  6. [6]
    "National land survey of Iceland (Kortasja)". Retrieved 30 March 2024.
  7. [7]
    Magnússon et al. 2005, fig. 3.
  8. [8]
    Guðmundsson & Larsen 2019, Detailed Description:4. Eruption history and pattern.
  9. [9]
    Guðmundsson & Larsen 2019, Detailed Description:7. Erupted material & Grain size distribution.
  10. [10]
    "Grimsvötn". Global Volcanism Program. Smithsonian Institution. Retrieved 31 March 2024.: Eruptive History 
  11. [11]
    Guðmundsson & Larsen 2019, Detailed Description:8. Volcanic hazards.
  12. [12]
    Gudmundsson & Andrew 2007, 4. Mechanical Interaction.
  13. [13]
    "Thordarhyrna". Global Volcanism Program. Smithsonian Institution. Retrieved 31 March 2024.
  14. [14]
    Andrew & Gudmundsson 2008, 7. Discussion and conclusions.
  15. [15]
    Manning & Thirlwall 2014, p. 958.
  16. [16]
    Andrew & Gudmundsson 2008, 4. Mechanical interaction.
  17. [17]
    Guðmundsson & Larsen 2019, Detailed Description:2. Morphology and topography.
  18. [18]
    Bjornsson 1986, p. 15.
  19. [19]
    Gudmundsson, M. T.; Larsen, G. (2019). "Grímsvötn Alternative name: Grímsvötn-Laki". Retrieved 1 April 2024.: Detailed Description:1. Geological setting and tectonic context 
  20. [20]
    Guðmundsson & Larsen 2019, Detailed Description:5. Characteristics during non-eruptive periods.

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