PhD defence: Johanne Schmith – Københavns Universitet

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PhD defence: Johanne Schmith

Johanne Schmith defends her thesis,

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

Supervisors:
Associate Professor Paul Martin Holm, IGN
Research Professor Ármann Höskuldsson, University of Iceland

Assessment Committee:
Professor Thorvaldur Thordarson, University of Iceland
Senior Lecturer Jennie Gilbert, Lancaster University - UK
Professor Tod Waight (chair), IGN

Summary:
This PhD study revolves around the 1755 and 1625 explosive eruptions from Katla volcano. Katla is one of the most dangerous volcanoes in Iceland and has previously sent ash raining down on Northern Europe, including in 1755 and 1625. A similar eruption in the present day would cause major disruption to air traffic and current unrest suggests that another eruption may be imminent.
There are two aims of this PhD-study. One is obtain eruption parameters for the 1755 and 1625 eruptions to assess the size of the eruptions. The second aim is to understand the high-explosivity basaltic eruptions in Iceland. I answered these two research questions by studying the 1755 and 1625 eruption deposits.
To obtain eruption parameters, I dug 53 holes in soils around Katla to measure thicknesses of the tephra layers. I took ash samples to measure the size of the ash grains. From the thickness measurements I could model the erupted volume and mass in 1755 as 1.20-1.50 km3 and 1.84-2.45×1012 kg and in 1625 as 1.12-1.36 km3 and 1.53-1.94×1012 kg. This shows much more material was erupted than previously estimated. Then I used the weight and volume of some of the field samples as inversion inputs in a tephra dispersal model (Tephra2). The Tephra2 model calculates how tephra settles around the volcano during an eruption using the size of the ash grains and wind conditions. I estimated an average height of the plume of 14.4 km for the 1755 eruption and of 16.6 km for the 1625 eruption. Finally I classified the eruptions as subplinian to plinian (VEI 4-5) showing the eruptions were larger than previously thought.
Furthermore I have used the correlation between ash grain shapes and magma fragmentation to understand the explosivity of Icelandic basaltic eruptions. I measured shapes of ash grains from 6 different basaltic eruptions in the new automated Particle Insight dynamic shape analyzer. I created a reference dataset and used statistics to select the best shape parameters for a new classification diagram. The diagram shows that the explosivity of Katla is a result of both magmatic degassing and water/magma interaction. This is a new fast method to successfully assess magmafragmentation.

The thesis is available for inspection at the PhD administration office, 04.1.413