1996 eruption of Gjálp

Volcanic eruption in Iceland May 2021 1996 eruption of Gjálp Skeidara.jpg Gjálp eruption: Jökulhlaup 1996 over Skeiðarársandur, the piedmont glacier Skeiðarárjökull and Öræfajökull in the background yes center 285 450 -17.5 64.55 9 [,,,,,,] Geological map of subglacial Gjálp ridge (violet outline). Shading shows: #cecc90subglacial terrain above 1100 m,#e0eda0seismically active areas between 1995 and 2007,#dbb252 calderas,#d9cf90central volcanoes and #fae8c0fissure swarms. Clicking on the image enables fll window and mouse-over with more detail. 30 September 1996 13 October 1996 Gjálp Subglacial fissure eruption Western Vatnajökull 64 32 00 N 17 25 00 W title,inlineJarosch A. Gudmundsson M.T. Högnadóttir T. Axelsson G. 2008 Progressive cooling of the hyaloclastite ridge at Gjálp, Iceland, 1996–2005 Journal of Volcanology and Geothermal Research 170 3–4 218–229 10.1016/j.jvolgeores.2007.10.012 2008JVGR..170..218J 3373010 Grímsvötn Jökulhlaup over Skeiðarársandur, Hringvegur partially destroyed thumb|250px|Vatnajökull: In the western part to be seen are the Grímsvötn caldera (dark half-moon indentation), to the north of it Gjálp, and to the north of Gjálp the completely subglacial caldera of Bárðarbunga thumb|250px|Icelandic rift zones and Vatnajökull: East Volcanic Zone (no. 4) crossing its western part Between 30 September and 13 October 1996, an Icelandic hyaloclastite ridge named Gjálp (is ˈcaul̥p) erupted. Located under the Vatnajökull glacier shield, its present form resulted from the eruptions, and it is likely part of the Grímsvötn volcanic system.Snæbjörn Guðmundsson: Vegavísir um jarðfræði Íslands. Reykjavík 2015, p. 280-281See also 373010 Grímsvötn:Eruptive history Retrieved 29 August 2020. However, not all scientists are of this opinion, as seismic studies are consistent with a 10 km lateral dike intrusion at about 5 km depth from Bárðarbunga being the trigger event. This does not exclude a shallower secondary intrusion from Grímsvötn being important in the subaerial eruption itself.Konstantinou K.I. Utami I.W. Giannopoulos D Sokos E. 2019 A reappraisal of seismicity recorded during the 1996 Gjálp eruption, Iceland, in light of the 2014–2015 Bárðarbunga–Holuhraun lateral dike intrusion Pure and Applied Geophysics 177 6 2579–2595 10.1007/s00024-019-02387-x 2019PApGe.177.2579KDespite the extensive study the precise sequence of events during the eruption has not been conclusively determined as well as assignment to volcanic system. Several authorities have speculated on the following sequence of events given evolving volcanology theory in the last decade:62 Deep basaltic primary intrusion from Bárðarbunga system on 29 September 1996 This intercepted a maturing Grímsvötn magma pocket (that may have had some active Grímsvötn basalt magma input at the time) Which was triggered into eruption 30 September 1996 onwards Another alternative explanation for the observations could be: Tectonic interaction along a fault that propagated from Bárðarbunga towards Grímsvötn This intercepted an almost primed maturing Grímsvötn magma pocket (that may have had some active Grímsvötn basalt magma input at the time) Which was triggered into eruption 30 September 1996 onwards Less consistent with the compositional studies evidence: Subglacial eruption at north western part of Bárðarbunga Caldera on 29 September 1996 Deep basaltic Bárðarbunga intrusion on far side of Bárðarbunga system into maturing shallower magma pocket shared with Grímsvötn system Which was triggered into eruption 30 September 1996 onwards Inconsistent with current compositional and seismic evidence base: Maturing shallow magma pocket in either Bárðarbunga volcanic system or its Loki-Fögrufjöll subsystem (best location data on 192 peri-eruption seismic events with good location solutions only assigned two to Loki Ridge, but perhaps there is a magma pocket under the Loki Ridge) Bárðarbunga Caldera priming event 29 September 1996 Bárðarbunga volcanic system triggered into subaerial eruption 30 September 1996 onwards Importance The eruption was of importance, because it was for the first time that a subglacial eruption under a thick ice cover as well as the connected jökulhlaup could be observed and analyzed by modern techniques.https://link.springer.com/article/10.1007%2Fs00445-003-0295-9 Magnús T. Gudmundsson Freysteinn Sigmundsson Helgi Björnsson Thordís Högnadóttir The 1996 eruption at Gjálp, Vatnajkull ice cap, Iceland: efficiency of heat transfer, ice deformation and subglacial water pressure Bulletin of Volcanology 2004 66 1 46–65 10.1007/s00445-003-0295-9 2004BVol...66...46G subscriptionSee also: https://eprints.lancs.ac.uk/id/eprint/620/1/Tuffen_et_al_Ann_Glac_2006.pdf Hugh Tuffen D.W. McGarvie Gilbert J.S. Will subglacial rhyolite eruptions be explosive or intrusive? Some insights from analytical models Annals of Glaciology 2007 45 1 87–94 30 August 2020 10.3189/172756407782282534 2007AnGla..45...87T Geography Eruption location The subglacial eruption fissure is to be found in the northwest corner of Vatnajökull ice cap more or less halfway between the central volcanoes Bárðarbunga and Grímsvötn.https://www.cambridge.org/core/services/aop-cambridge-core/content/view/847378B0EB5F48B6EF3A1C950BB2CD92/S0022143000212239a.pdf/div-class-title-glacier-volcano-interactions-deduced-by-sar-interferometry-div.pdf Björnsson H. Rott H. Gudmundsson S. Fischer A. Siegel A. Gudmundsson M.T. 2001 Glacier–volcano interactions deduced by SAR interferometry Journal of Glaciology 47 156 58–70 8 August 2020 10.3189/172756501781832520 2001JGlac..47...58B 3 May 2019 https://web.archive.org/web/20190503044701/https://www.cambridge.org/core/services/aop-cambridge-core/content/view/847378B0EB5F48B6EF3A1C950BB2CD92/S0022143000212239a.pdf/div-class-title-glacier-volcano-interactions-deduced-by-sar-interferometry-div.pdf bot: unknown It is also to the west of the Hamarinn central volcano of the Bárðarbunga volcanic system which has the Loki Ridge extending west–east which has been assigned historically to the Loki-Fögrufjöll volcano.Björnsson H. Einarsson P. 1990 Volcanoes beneath Vatnajökull, Iceland: Evidence from radio echo-sounding, earthquakes and jökulhlaups Jökull 40 147–168 10.33799/jokull1990.40.147 https://jokull.jorfi.is/articles/jokull1990.40/jokull1990.40.147.pdf live https://web.archive.org/web/20230320235442/https://jokull.jorfi.is/articles/jokull1990.40/jokull1990.40.147.pdf 20 March 2023 25 March 2024155 Vatnajökull ice cap The Vatnajökull glacier which covered the location at time of eruption had a thickness of 500 – 600 m. In other places the glacier shield can have thicknesses of up to 900 m. Vatnajökull covered an area of 8.2 km2 in 1996,https://www.academia.edu/2424905 P. Einarsson Bryndis Brandsdottir Magnus Tumi Gudmundsson Helgi Bjornsson Karl Gronvold Freysteinn Sigmundsson Center of Icelandic Hotspot experiences Volcanic Unrest Eos, Transactions American Geophysical Union 78 35 2 September 1997 29 August 2020 10.1029/97EO00237 1997EOSTr..78..369E 369–375 but it is retreating and measured just 8.1 km2 in 2007.https://www.researchgate.net/publication/228616358 C. Pagli Sigmundsson F. Lund B. Sturkell E. Geirsson H. Einarsson P. Árnadóttir T. Hreinsdóttir S. 2007 Glacio-isostatic deformation around the Vatnajökull ice cap, Iceland, induced by recent climate warming: GPS observations and finite element modeling Journal of Geophysical Research: Solid Earth 112 B8 2006JB004421 10.1029/2006JB004421 2007JGRB..112.8405P 8 August 2020 11568/500513 free The glacier is temperate, lies in lower elevations and is therefore sensible to climatic changes. As a consequence it has been advancing and retreating since the Weichselian glaciation. Its last advance took place during the so-called Little Ice Age from the 13th to the end of the 19th century and since then it is retreating. Parts of two volcanic zones of Iceland are placed under Vatnajökull, i.e. the very active East Volcanic Zone (connected to rifting at the divergent plate boundary in Iceland), responsible for the highest number of eruptions after deglaciationhttps://www.researchgate.net/publication/228467883 Thorvaldur Thordarson Ármann Höskuldsson Postglacial volcanism in Iceland Jökull 58 2008 197–228 10.33799/jokull2008.58.197 24 March 2024 and with the mantle plume probably under Bárðarbunga, i.e. under Vatnajökull.Hildur María Friðriksdóttir Landris á Vatnajökulssvæðinu metið með GPS landmælingum. BS ritgerð Jarðvísindadeild Háskóli Íslands. Leiðbeinendur Sigrún Hreinsdóttir, Erik Sturkell. 2017 24 March 2024 https://skemman.is/bitstream/1946/18617/1/Landris%20%C3%A1%20Vatnaj%C3%B6kulssv%C3%A6%C3%B0inu%20meti%C3%B0%20me%C3%B0%20GPS%20landm%C3%A6lingum.pdf is "More than 80 eruptions occurred during the last 800 years in Vatnajökull."https://www.academia.edu/2424904 Helgi Björnsson Subglacial lakes and jökulhlaups in Iceland Global and Planetary Change 35 2002 3–4 255–271 31 August 2020 10.1016/S0921-8181(02)00130-3 2003GPC....35..255B There is also the much less active Öraefi Volcanic Belt, a flank zone mostly under the eastern part of Vatnajökull.See also: https://www.researchgate.net/publication/253235655 Helgi Björnsson Páll Einarsson Volcanoes beneath Vatnajökull, Iceland. Evidence from radio echo sounding, earthquakes and jökulhlaups Jökull 40 1990 147–168 10.33799/jokull1990.40.147 8 August 2020 It is thought that due to climate change, Vatnajökull has lost about 10% of its mass since the end of the 19th century. Measurements showed an accentuated and even accelerating rate of glacio-isostatic uplift. This could lead to increased magma production (so called decompression melt production), because the "pot lid" formed by the glaciers and their weight will be absent in the future, and eruption frequency could increase as a consequence.See eg.: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgrb.50273 Schmidt P. Lund B. Hieronymus C. Maclennan J. Árnadóttir T. Pagli C. Effects of present-day deglaciation in Iceland on mantle melt production rates Journal of Geophysical Research: Solid Earth 118 7 3366–3379 10.1002/jgrb.50273 2013 2013JGRB..118.3366S 4 September 2020 11568/500303 free The region of the Gjálp fissures is part of this active East Volcanic Zone under Vatnajökull. Geology thumb|250px|This kind of eruption, but under a glacier shield thumb|250px|Ice cauldrons merging to form an ice canyon at the glacier covered Colombian volcano Nevado del Tolima thumb|250px|Similar built hyaloclastite ridges (tindars) of the Bárðarbunga-Veiðivötn volcanic system not far from Landmannalaugar The Gjálp eruption formed in about two weeks a subglacial hyaloclastite ridge, also called tindar by some geologists, in a zone of known former eruptions. Predominantly basaltic andesite was erupted to a volume of 0.45 km3 DRE. The eruption in 1996 Precursors and possible connection between volcanic systems Some large earthquakes (M5+) had taken place in the central volcano Bárðarbunga just before the eruption and proved to be precursors of the eruptive events. In particular a w 5.6 yes event took place on 29 September in the northern part of the Bárðarbunga caldera and its aftershock sequence propagated over the next two days in a linear fashion towards Grímsvötn.zh A reappraisal of seismicity recorded during the 1996 Gjalp eruption in Iceland using modern seismological methods. PhD dissertation National Central University, Taiwan (國立中央大學) 2018 Utami I.W. http://ir.lib.ncu.edu.tw:88/thesis/view_etd.asp?URN=105622604&fileName=GC105622604.pdf 24 March 2024 It is possible that the first large event was associated with a subglacial eruption within the Bárðarbunga caldera a couple of days before the Gjálp eruption. The seismological study sees a parallel to the 2014–2015 eruptions and to the caldera drop in Bárðarbunga central volcano in that eruption, and postulate a similar magma migration to the eruption site though on a smaller scale. This could mean that the volcano is part of the fissure system of Bárðarbunga, not Grímsvötn. There had been seismic studies that suggested an east west line of seismic activity in the Bárðarbunga volcanic system at the Loki Ridge intersected the eruption location, https://www.researchgate.net/publication/260322845 Jakobsdóttir S.S. 2008 Seismicity in Iceland: 1994–2007. Jökull 58 1 75–100 10.33799/jokull2008.58.07587 but the Loki Ridge was not seismically active during the eruption. Another possibility is that Bárðarbunga magma entered a portion the magmatic system of Grímsvötn and started the eruption by this intrusion. Bárðarbunga is known for such tendencies, as its magma mingled with Torfajökull magma at least three times in the past which resulted in bimodal eruptions, e.g. of the Veiðivötn and at Landmannalaugar by the end of the 15th century. 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 3–4 388–398 10.1016/j.epsl.2008.02.026 2008E&PSL.269..388Z Formation of the tindar volcano The Gjálp eruption took place at a some kilometers long known fissure under 550 – 700 m of glacier ice within Vatnajökull. The eruption in October 1996 could push through this ice in about 30 hours and took place from 30 September to 13 October 1996. The eruption fissure had a length of 6 – 7 km. The location is some kilometers to the north of Grímsvötn caldera. In the beginning, a 2 – 4 km long N–S trending depression was formed above the fissure, with time three ice cauldrons were built at each end and in the middle, but the eruption concentrated later on one of them where a 200 – 300 m wide crater came to light. After some time, an open ice canyon was built above the fissure. It had a length of about 3.5 km and was up to 500 m in width. The meltwater drained first through the ice canyon and then disappeared into subglacial channels and run from there to the subglacial caldera lake of Grímsvötn. The subglacial channels were easily recognized, because continuous melting caused by the hot water from the eruption site initiated the formation of depressions on the ice surface. And so the scientists followed the melting path down to Grímsvötn caldera. Though the eruption was mostly explosive, the ash was not expelled far from the vents, but fell back into the canyon. The quantity of eruption products stayed more or less the same the whole time which was explained by ice flow into the crater. During the two weeks of eruption, volcanic activity thawed no less than 3 km3 of ice, and this continued to a lesser extent for some time after the end of the eruption. The newly formed tindar disappeared again completely under the glacier ice about 1 year later, but an identifiable ice cauldron remained until at least 2007. The tindar was a 6 km long ridge newly deposited to a height of 500 m above the pre-existing bedrock with a volume of 0.7 km3. It is postulated that the original unconsolidated hyaloclastitic volcanic glass and tephra of the ridge could have by now undergone a process called palagonitization due to hydrothermal alteration, to palagonite, a consolidated rock more resistant to erosion, but it is unknown if this has happened. Eruption products The eruptive products consisted of predominantly basaltic andesite which surprised the scientists as these more evolved rocks are neither typical for Bárðarbunga nor for Grímsvötn, both more connected to basaltic volcanism. Some scientists thought therefore that Gjálp could be an independent volcano. The bulk samples obtained shortly after the eruption ranged from basaltic andesite to basalt and were of distinctive Grímsvötn composition.The tephra layer formed in the 1996 eruption of Gjálp: Dispersal and volume. Magister Scientiarum thesis https://skemman.is/handle/1946/42713 Irma Gná Jóngeirsdóttir 2022 Faculty of Earth Science School of Engineering and Natural Sciences, University of Iceland33 Basaltic andesite from a 1887 eruption had been previously attributed to the Grímsvötn volcanic system and had very similar composition. Tephra assigned to the eruption has been analysed by several researchers and has composition that is Grímsvötn basaltic andesite with rarely Grímsvötn basalt. A total of three samples out of the several hundred in the literature had some tephra with Bárðarbunga basalt composition. It is unknown if this was due to contamination from pre-existing tephra layers in the ice that was overlying Gjálp or if the Bárðarbunga basalt was erupted together with the Grímsvötn basaltic andesite.62 Jökulhlaup in 1996 thumb|Memorial of the jökulhlaup on Skeiðarársandur, outlet glacier Svínafellsjökull in the background thumb|Parts of the destroyed bridge over Gígjukvísl river on Skeiðarársandur In the beginning, scientists presumed that the eruption would be followed immediately by a big jökulhlaup (a sort of a meltwater tsunami including large blocks of ice and a high quantity of sediment). But it took some time to fill the subglacial lake of Grímsvötn in such a manner that the ice wall holding it back would break. Not before some weeks had passed after the eruption was terminated, did the expected jökulhlaup happen. This was from 4 to 7 November 1996. The melt water streamed mostly in subglacial channels and in the end under the outlet glacier Skeiðarárjökull. There, to everybody's surprise, the water masses streamed in such a quantity that the whole glacier was lifted up.https://www.researchgate.net/publication/232996449 Tomas Jóhannesson Propagation of a subglacial flood wave during the initiation of a jôkulhlaup Hydrological Sciences-Journal-des Sciences Hydrologiques 47 3 2002 417 10.1080/02626660209492944 2002HydSJ..47..417J 8 August 2020See also: Helgi Björnsson Understanding jökulhlaups: from tale to theory Journal of Glaciology 56 200 2010 1002 10.3189/002214311796406086 2010JGlac..56.1002B 8 August 2020 https://www.cambridge.org/core/services/aop-cambridge-core/content/view/21FD9F138CE4F1F74856701D1BE0F51A/S0022143000213221a.pdf/div-class-title-understanding-jokulhlaups-from-tale-to-theory-div.pdf 28 July 2018 https://web.archive.org/web/20180728033118/https://www.cambridge.org/core/services/aop-cambridge-core/content/view/21FD9F138CE4F1F74856701D1BE0F51A/S0022143000213221a.pdf/div-class-title-understanding-jokulhlaups-from-tale-to-theory-div.pdf bot: unknown In the end, the water sprang up from under the glacier edge and the flood covered most of Skeiðarársandur glacial outwash plain, destroying on its way large parts of the main road Hringvegur including two bridges and some communication installations. Luckily, the road had been closed before the flood so that nobody was injured. The volume of melt water produced by this eruption was around 4 km3.https://notendur.hi.is/mtg/pdf/2008Jokull58_MTGetal_volchaz.pdf M.T. Gudmundsson G. Larsen Á. Höskuldsson Á.G. Gylfason Volcanic hazards in Iceland Jökull 58 2008 251–268 10.33799/jokull2008.58.251 8 August 2020 28 April 2021 https://web.archive.org/web/20210428133008/https://notendur.hi.is/mtg/pdf/2008Jokull58_MTGetal_volchaz.pdf dead Water flowing over the sandur streamed at up to 50 – 60000 m3/s. The first estimates had been somewhat lower. Former eruption in 1938 At more or less the same place another eruption had taken place in the 1930s. It had also caused a jökulhlaup, but at the time, science could not yet analyse the events. That eruption stayed subglacial. See also List of volcanic eruptions in Iceland Subglacial volcano Grímsvötn Subglacial eruption Bárðarbunga Ice cauldron Further reading Helgi Björnsson: Subglacial lakes and jökulhlaups in Iceland. Global and Planetary Change 35 (2002) 255–271 https://web.archive.org/web/20210731062914/http://www.norvol.hi.is/pdf/HB2003GlobPlanCh.pdf 31 July 2021 Notes References Category:1996 in Iceland Category:1990s disasters in Iceland Category:Bárðarbunga Category:East Volcanic Zone of Iceland Category:Grímsvötn Category:Subglacial eruptions Category:Subglacial volcanoes of Iceland Category:VEI-3 eruptions Category:Volcanic eruptions in Iceland Category:Jökulhlaups