Midway evaluation in Geology - Melin Barbara Payet--Clerc

Title of thesis: Dynamics of Plinian basaltic eruptions from field reconstruction and physical modelling of historical plumes.

Student: Melin Barbara Payet--Clerc

Doctoral committee:
Dr. Ármann Höskuldsson, Research professor at the Institute of Earth Sciences, University of Iceland
Dr. Þorvaldur Þórðarson Professor at the Faculty of Earth Sciences, University of Iceland
Dr. Guillaume Carazzo, Professor at the Institut de Physique du Globe de Paris, University of Paris
Dr. Anthony Finizola, Professor at the Universite Paris Cite.

Abstract

In the region between Torfajökull and Bárðarbunga is a 100 km-long fissure swarm named after the crater-filling lakes, Veiðivötn. The Veiðivötn fissure swarm sits within the Eastern Volcanic Zone of Iceland and is the surface expression of a segment of the plate boundary between North America and EurAsia. Eruptions in the area are commonly large and typically form widely dispersed tephra layers. The 1477 CE Veiðivötn eruption is the youngest event on this fissure swarm and produced 10km3 of freshly fallen basaltic tephra, covering on land about 53,000km2 (Larsen 2005). The tephra layer is >1 cm-thick at 250 km away from the source vents, which strongly suggests that it was produced by an event of Plinian intensity, which consequently makes it one of the more powerful known explosive eruptions in Iceland (e.g., Larsen 1984). The eruption took place on a ~70 km long discontinuous vent system that runs into the Torfajökull volcanic system. Torfajökull erupted simultaneously and in doing so produced the lava flow fields Laugahraun, Námshraun and Norðurnámshraun which, however, are not considered further in this study. The scientific understanding of the dynamics of highly explosive basaltic eruptions is limited. Until recently, the intensity of such basaltic activity has been linked to explosive magma-water interactions. Water is flashed into steam at the magma-water interface producing an expanding oscillating steam film that sequentially disintegrates/fragments the magma and consecutively leads to an explosive expansion of the particle-steam mixture. However, new research on phreatomagmatic eruptions in Iceland (i.e., Lynch 2015, Decker 2016, Moreland 2017, Moreland et al 2019) indicates that main driving force for the explosivity in these events is exsolution of magmatic gasses into gas bubbles along with fast ascent, growth and accumulation of the bubbles towards the top of the rapidly rising magma column in the conduit. The interaction with external water is confined to quenched granulation, a process that enhances ash generation but does not add any power to the explosive activity. This study explores this explosivity of phreatoplinian basaltic eruption where the magma-to water interaction is confined to surface or near-surface water, using the case study of the 1477 CE Veiðivötn eruption to improve the understanding of the eruption plume dynamics and the mechanism of tephra dispersal in such events. We use field data and a one-dimensional physical model of an eruptive plume to address these research questions, i.e., the Paris Plume Model (PPM). Measured stratigraphic section were used to obtain additional tephra thickness data and maximum clast sizes as a function of distance from source. The latter is used for estimating the eruption column heights during the 1477 eruption. More than 80 tephra samples were collected for grain size analysis to be used in reconstructing the total grain size distribution (TGSD) of the 1477CE tephra deposit and for grain shape measurements using a Camsizer-X2 analyser. The TGSD was obtained using the TOTGS Matlab® script from Biass and Bonadonna (2014). Bulk tephra density is obtained using the tap density method for the ash-grade fraction, and for the lapilli-grade we applied the Archimedes principle as outlined in Houghton and Wilson (1989). The TGSD is used for calculating an exponent (D) of a power law distribution associated with the number of particles of radius superior to a given radius “r” at the vent, which is then implemented in the model, together with the ash-grade density and relevant geochemical compositional data to constrain the critical MER and volatile content required to sustain the estimated maximum plume height, and critical values at which the plume collapses. The field data underpins the data set used to derive the required input parameters for the eruption plume model which is run at a range of given Mass Eruptive Rates (MER) and volatile content under a range of wind