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Explosive activity in flood lava eruptions: a case study of the 10th century Eldgjá eruption, Iceland

Explosive activity in flood lava eruptions: a case study of the 10th century Eldgjá eruption, Iceland


Titill: Explosive activity in flood lava eruptions: a case study of the 10th century Eldgjá eruption, Iceland
Aðrir titlar: Explosive activity in flood lava eruptions
Höfundur: Moreland, William   orcid.org/0000-0002-5801-1496
Útgáfa: 2017-07-14
Tungumál: Enska
Háskóli/Stofnun: Háskóli Íslands
University of Iceland
Svið: Verkfræði- og náttúruvísindasvið (HÍ)
School of Engineering and Natural Sciences (UI)
Deild: Jarðvísindadeild (HÍ)
Faculty of Earth Sciences (UI)
ISBN: 9789935930668
Efnisorð: Volcanology; Eruption; Explosive; Eldgjá; Fissure; Basaltic; Tephra; Total grain-size distribution; Vesicle-size analysis; Eldfjallafræði; Sprengigos; Sprungur (jarðfræði); Basalt; Gjóskudreifing; Doktorsritgerðir
URI: https://hdl.handle.net/20.500.11815/324

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Tilvitnun:

William Michael Moreland, 2017, Explosive activity in flood lava eruptions: A case study of the 10th century Eldgjá eruption, Iceland, PhD dissertation, Faculty of Earth Sciences, University of Iceland, 92 pp.

Útdráttur:

 
The 10th century Eldgjá flood lava eruption, southern Iceland, was the most voluminous eruption on Earth in the last 1100 years, erupting up to 21.0 km3 of transitional alkali basaltic magma of rather uniform composition. While 19.7 km3 was erupted as lava in the form of two extensive lava fields covering 780 km2 in total, 1.3 km3 (dense rock equivalent) was erupted as tephra in at least 16 explosive phases. The Eldgjá vents form a ~70-km discontinuous mixed cone-row which stretches from beneath Mýrdalsjökull to the edge of Vatnajökull. Explosive activity took place as discrete events restricted to distinct lengths of the fissure and alternated between subglacial and subaerial until phase 10 after which all activity was subaerial. Each phase contributed a tephra unit to what became a thick composite tephra deposit over 2 m thick 10 km away from source. Eruption column heights are estimated to have reached between 11 and 18 km, well above the 10-km tropopause above Iceland. The combination of subaerial and subglacial vents lead to both magmatic and phreatomagmatic tephra being produced. Individual explosive phases have been classified as Plinian and Phreatoplinian. Vesicle-size analysis reveals that the magma beneath all Eldgjá fissure segments had identical vesiculation histories. However, total grain-size distributions of magmatic and phreatomagmatic tephra exhibit stark differences with the magmatic having one medium lapilli mode and the phreatomagmatic having one broad peak of medium lapilli to medium ash and a narrower peak of very fine ash. Thermal granulation is suspected as having reduced lapilli-sized magmatic foam into finer particles but without adding intensity to the eruption.
 
Eldgjárgosið á 10. öld, sem framleiddi um 21.0 km3 af einsleitri alkalískri basaltkviku, er stærsta eldgos jarðar á síðustu 1100 árum. Rúmtak hrauna er 19.7 km3 og gjóskan eru 1.3 km3 (reiknað sem hraun). Gjóskan myndaðist í að minnsta kosti 16 goshrinum. Blandgígaröð Eldgjárgossins er ósamfelld og nær yfir 70 km vegalengd frá Mýrdalsjökli í suðvestri og að jaðri Vatnajökuls í norðaustri. Sprengivirknin í Eldgjárgosinu voru einstakir atburðir (þ.e. hrina) sem afmörkuðust við ákveðna hluta gígaraðarinnar á hverjum tíma. Hver hrina leiddi til gjóskufalls sem samsvarar sérstakri einingu í gjóskulaginu. Saman mynda þessar einingar gjóskulag sem er 2 m þykkt í 10 km fjarlægð frá upptakagígum. Í fyrstu 10 hrinunum var þessi virkni ýmist í Mýrdalsjökli eða rétt utan jökulsins og skiptust því á sprengigos (freatómagmatísk virkni) í jöklinum og þeytigos (magmatísk virkni) utan hans. Síðustu sex hrinurnar voru þeytigos utan jökuls. Þessi sprengivirkni var Plínísk í eðli sínu og gosmekkirnir frá goshrinunum stigu 11 til 18 km í loft upp og vel upp fyrir veðrahvolfið yfir Íslandi. Mælingar á blöðrumagni og blöðrustærðardreifingu Eldgjárvikurs sýnir að engin munur er á freatómagmatísku og magmatísku gjóskunni, sem bendir til þess að utanaðkomandi vatn komst ekki í snertingu við kvikuna fyrr en eftir sundrun hennar efst í gosrásinni. Verulegur munur er á heildarkornastærðardreifingu magmatísku og freatómagmatísku gjóskunnar, þar sem sú síðarnefnda inniheldur meira af fínni ösku. Leidd er rök að því að þessi fína aska myndaðist við hraðkælingu á vikurkornunum þegar gosstrókurinn reis upp í gegnum bræðsluvatn jökulsins.
 

Athugasemdir:

The research presented here has been carried out at the Faculty of Earth Sciences, University of Iceland. The primary aim of this project has been to investigate and characterise the explosive phases of the 10th century Eldgjá eruption. Using data gathered from this eruption more general conclusions have been reached regarding processes active during fissure eruptions and subglacial eruptions. The main results are presented as papers I-III. The papers are either submitted, or in preparation to be submitted, to peer-reviewed international journals.

Leyfi:

Closed until all three papers are published

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