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Volcanology and hazards of phreatomagmatic basaltic eruptions: Eruption source parameters and fragmentation mechanism of large eruptions from Katla volcano, Iceland

Volcanology and hazards of phreatomagmatic basaltic eruptions: Eruption source parameters and fragmentation mechanism of large eruptions from Katla volcano, Iceland


Titill: Volcanology and hazards of phreatomagmatic basaltic eruptions: Eruption source parameters and fragmentation mechanism of large eruptions from Katla volcano, Iceland
Höfundur: Schmith, Johanne
Leiðbeinandi: Paul Martin Holm
Ármann Höskuldsson
Útgáfa: 2017-08-25
Tungumál: Enska
Háskóli/Stofnun: Háskóli Íslands
University of Iceland
Svið: Verkfræði- og náttúruvísindasvið (HÍ)
School of Engineering and Natural Sciences (UI)
Deild: Jarðvísindastofnun (HÍ)
Institute of Earth Sciences (UI)
ISBN: 9789935930057
Efnisorð: Volcanology; Explosive eruptions; Tephra; Source paramenters; Fragmentation; Phreatomagmatic; Katla; Eldfjallafræði; Eldfjöll; Sprengigos; Gjóska; Gjóskulög; Doktorsritgerðir
URI: https://hdl.handle.net/20.500.11815/706

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Tilvitnun:

Schmith, J. (2017). Volcanology and hazards of phreatomagmatic basaltic eruptions: Eruption source parameters and fragmentation mechanism of large eruptions from Katla volcano, Iceland. Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen.

Útdráttur:

 
Iceland is one of the most active terrestrial volcanic regions on Earth with an average of more than 20 eruptions per century. Around 80% of all events are tephra generating explosive eruptions, but less than 10 % of all known tephra layers have been mapped. Recent hazard assessment models show that the two key parameters for hazard assessment modeling are total grain size distribution (TGSD) and eruptive style. These two parameters have been determined for even fewer eruptive events in Iceland. One of the most hazardous volcanoes in Iceland is Katla and no data set of TGSD or other eruptive parameters exist. Katla has not erupted for 99 years, but at least 2 of the 20 eruptions since the settlement of Iceland in 871 have reached Northern Europe as visible tephra fall. These eruptions occurred in 1755 and 1625 and remain enigmatic both in terms of actual size and eruption dynamics. This work presents studies of these two far-reaching eruptions in terms of fragmentation and eruption dynamics as well as the first set of eruption source parameters for any Katla eruption. In order to provide detailed insight into the eruption dynamics a new method for classifying fragmentation mechanisms based on tephra grain morphology was developed and is presented in this work. The deposits are estimated to cover 23400 km2 and 23600 km2 on land in Iceland for the 1755 and 1625 eruptions. Volumes calculated from the power-law integration method are 1.20-1.50 km3 for the 1755 eruption and 1.12-1.36 km3 for the 1625 eruptions. The total erupted mass converted from erupted volume of the 1755 eruption was 1.84-2.45⋅1012 kg with a lower-bound mass eruption rate of 1.25-1.67⋅106 kg/s. In 1625 Katla erupted between 1.53-1.94⋅1012 kg tephra as calculated from the erupted volume with a lower-bound mass eruption rate of 1.61-2.04⋅106 kg/s. The average 1755 plume height was 14.4 km based on mass loading data inversion with an empirical correlation estimate of paroxysmal peaks at 25± 6 km. The average 1625 plume height was 16.6 km based on mass loading data inversion with an empirical correlation estimate of paroxysmal peaks at 25± 6 km. A new quantitative method producing grain shape data of bulk samples of volcanic ash was developed to correlate the bulk average grain shape with magma fragmentation mechanisms. The new shape index: the regularity index (RI) was developed from a manually classified reference morphology dataset using principal component analysis. The systematic change in RI between wet and dry eruptions supports that the RI can be used to assess the relative roles of magmatic versus phreatomagmatic fragmentation. Surtseyan ash has an RI of 0.207-0.191 ± 0.002 (2σ), whereas Hawaiian ash has an RI of 0.134 ± 0.001 (2σ) and these samples represent the extremes of the fragmentation spectrum. Subglacial samples show intermediate RIs of 0.168 ± 0.002 (2σ), 0.175 ± 0.002 (2σ) and lacustrine samples have slightly higher RI of 0.187 ± 0.002 (2σ). The method uses automated image analysis of 2D projections ash grains in the size range 125-63 μm. Loose bulk samples from the deposits of 6 different basaltic eruptions were analyzed and 20,000 shape measurements of 26 shape parameters for each were obtained within ∼45 min using the Particle InsightTM dynamic shape analyzer (PIdsa). The RI modeling showed that subglacial eruptions were controlled by both magmatic and phreatomagmatic fragmentation processes. Further investigations determined the RI of consecutive depositional units of both the 1755 and 1625 eruptions. The detailed models showed that the fragmentation processes in the 1755 eruption was a stable combination of magmatic degassing and magma/water-interaction with only minor perturbations, whereas the 1625 eruption went progressively from predominantly phreatomagmatic to predominantly magmatic and back to phreatomagmatic. This study documents that the amount of and access to melt water can change significantly during subglacial eruptions and that magmatic processes play an important role in the fragmentation process. The RI study was combined with field data on deposit stratigraphy, granulometric modeling, componentry, and written accounts of the eruptions and produced a coherent model of the evolution of the eruptions. The collective data set shows that the 1755 eruption was a continuous uprush eruption much like the recent 2011 Grímsvötn eruption. However, the 1755 Katla eruption had a longer duration of 17 days and a higher mass eruption rate. The 1625 eruption was a dynamic eruption less influenced by magma/melt-water interaction, which agrees well with the higher average plume height as compared to the 1755 eruption. This work has contributed to the general understanding of the fragmentation dynamics of large subglacial eruptions. It has also provided much needed data for improvement of future hazard and risk assessments of one of the most hazardous volcanoes in Iceland. Furthermore the RI method is expected to be widely applicable to tephra morphology studies and to be helpful during the next volcanic ash crises.
 
Island er en af verdens mest aktive vulkanske regioner med gennemsnitligt mere end 20 udbrud pr. århundrede. Omkring 80% af alle eruptioner er eksplosive tephradannende udbrud, men under 10 % af alle kendte tephralag er kortlagt. Nye risikovurderingsmodeller viser at nøgleparametrene til modellering af udbrudsfarer er total kornstørrelsesfordeling (TGSD) og udbrudsstil, hvilket er blevet bestemt for endnu færre udbrud. Katla er en af de farligste vulkaner på Island, og der findes ingen datasæt med TGSD eller andre udbrudsparametre for denne vulkan. Katla ikke har været i udbrud i 99 år, men mindst 2 ud af Katlas 20 historiske udbrud gennem de sidste 1100 år har nået Nordeuropa i form af synligt askefald. Disse udbrud fandt sted i 1755 og 1625 og er fortsat gådefulde både hvad angår faktisk størrelse og udbrudsdynamik. Denne afhandling præsenterer studier af magmafragmentation og udbrudsdynamik for de to vidtrækkende 1755 and 1625 vulkanudbrud samt det første sæt af udbrudsparametre for Katla. For at give en mere detaljeret indsigt i udbrudsdynamikken er der derudover udviklet en ny metode baseret på askekornsmorfologi til klassificering af askefragmentation. Aflejringerne fra udbruddene anslås at dække 23400 km2 og 23600 km2 på land i Island for henholdsvis 1755 og 1625 udbruddet. Volumener beregnet ud fra potensfunktionsmetoden er på 1,20-1,50 km3 og 1,12-1,36 km3 for henholdsvis 1755 og 1625 udbruddet. Den samlede udbrudsmasse af 1755-udbruddet er 1,84-2,45⋅1012 kg omregnet fra udbrudsvolumenet og det nedre estimat for en masseeruptionsrate er på 1,25-1,67⋅106 kg/s. I 1625 erupterede Katla 1,53-1,94⋅1012 kg beregnet ud fra udbrudsvolumenet med en masseeruptionsrate på 1,61-2,04⋅106 kg/s. Den gennemsnitlige askesøjlehøjde for 1755-udbruddet var 14,4 km baseret på inversion af massefordelinsdata. For 1625-udbruddet var denne højde 16,6 km. Den paroxysmale maximumhøjde anslås til 25 ± 6 km for begge udbrud ved beregning ud fra den empirisk korrelation af volumen og askesøjlehøjde for pliniske udbrud. En ny kvantitativ metode til bestemmelse af askemorfologi blev udviklet for at korrelere den gennemsnitlige kornform i askeprøver med magmafragmentationsmekanismer. Det nye formindeks: Regularitetsindekset (RI) blev udviklet ved principal komponent analyse af et datasæt af manuelt klassificerede kornmorfologityper. Den systematiske ændring i RI mellem våde og tørre udbrud viser, at RI kan bruges til at vurdere de relative roller af magmatisk versus phreatomagmatisk fragmentation. Surtseiske udbrud har et RI på 0,207-0,191 ± 0,002 (2σ), mens Hawaiianske udbrud har et RI på 0,134 ± 0,001 (2σ). Disse prøver repræsenterer ekstremerne af fragmentationsspektret. Prøver fra subglaciale udbrud analyseres til middelværdier af RI på 0,168 ± 0,002 (2σ) og 0,175 ± 0,002 (2σ) og prøver fra lakustrine udbrud har et lidt højere RI på 0,187 ± 0,002 (2σ). Metoden bruger automatiseret billedanalyse af 2D-projektioner af askekorn i størrelsesområdet 125-63 μm. Løse askeprøver fra aflejringer af 6 forskellige basaltiske askeudbrud blev analyseret, og 20.000 kornmålinger af 26 formparametre blev opnået indenfor ~45 minutter ved anvendelse af Particle Insight ™ dynamic shape analyzer (PIdsa). RI-modelleringen viste, at subglaciale udbrud blev styret af både magmatiske og phreatomagmatiske fragmentationsprocesser. Yderligere undersøgelser fastslog RI af de aflejrede udbrudsenheder i både 1755- og 1625-udbruddet. De detaljerede modeller viste, at fragmentationsprocesserne i 1755-udbruddet var en forholdsvis stabil kombination af magmatisk afgasning og magma/vand-interaktion. Fragmentationen under 1625-udbruddet gik gradvist fra overvejende phreatomagmatisk fragmentation til overvejende magmatisk og tilbage til phreatomagmatisk. Undersøgelsen dokumenterer, at mængden af og adgangen til smeltevand kan ændre sig signifikant under subglaciale udbrud, samt at magmatiske processer spiller en vigtig rolle i fragmentationsprocessen. RI-studiet blev kombineret med stratigrafiske feltdata, granulometrisk modellering, komponentbestemmelse og skriftlige kilder med beskrivelser af udbruddene. Dette datasæt dannede baggrunden for en sammenhængende model for udbruddenes udvikling. Det samlede datasæt viser, at udbruddet i 1755 var et kontinuerligt opstrømsudbrud ligesom det seneste udbrud i 2011 fra Grímsvötnvulkanen. Dog havde Katla-udbruddet en længere varighed og en højere masseeruptionsrate. 1625-udbruddet var et dynamisk udbrud, som var mindre påvirket af magma/smeltevandsinteraktion, hvilket stemmer overens med den højere gennemsnitlige askesøjlehøjde sammenlignet med 1755-udbruddet. Dette studie har bidraget til den generelle forståelse af fragmentationsdynamikken under store subglaciale udbrud. Det har også leveret nødvendige data til forbedring af fremtidige fare- og risikovurderinger af en af de mest farlige vulkaner i Island. Desuden forventes RI-metoden at være bredt anvendelig til tephra-morfologistudier og være et nyttigt værktøj under de næste vulkanske askekriser.
 

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