Rebecca Ann Lynch flytur fyrirlestur um verkefni sitt til meistaraprófs í jarðfræði. Heiti verkefnisins er Magma vesiculation and fragmentation in the 2011 eruption of Grímsvötn volcano.
Ágrip
Grímsvötn is a basaltic, subglacial volcano and is the most active volcano in Iceland. It is known for phreatomagmatism and historically large fissure eruptions from its associated fissure system. The Grímsvötn central volcano erupted in 2004 from its southern flank, and again in 2011 from the same vent with more explosive intensity, magma volume and with a 20km high eruption plume, suggesting an event of Plinian intensity. The deposits of the 2011 eruption consists of six alternating pumice lapilli and ash-grade units that raise questions about the potential role of magma outgassing in driving fragmentation and about the explosive nature of the 2011 eruption. To assess the degassing and fragmentation characteristics of this eruption, density measurements of 1200 clasts and quantitative image analysis were performed for four selected clasts from the lapilli units. The mean vesicularities are uniform, between 78.8% and 87.8%, indicating a degassed and fully expanded magma column upon magma disintegration. The clasts reached such a high level of vesicularity that the textures of the clasts became convoluted in later stages of the eruption. Bubble-to-bubble interaction resulted in significant bubble coalescence, thin vesicle walls and collapse, as well as the formation of possible tube-like degassing pathways. The highly expanded and effective degassed magma column indicates volatile driven disintegration of the magma upon eruption. However, the alternating lapilli pumice and ash-grade units can be explained by (i) changes in eruption intensity or (ii) two stages of fragmentation, where an initial fragmentation to predominantly lapilli-size pumices is driven by expansion of magmatic gases (i.e. dry eruption) and a second stage quenched fragmentation caused by passive interaction with external water. The latter scenario may be more typical for within-glacier eruptions than thought previously.
Leiðbeinendur eru Þorvaldur Þórðarson, prófessor við Jarðvísindadeild Háskóla Íslands og Bruce F. Houghton, prófessor við Jarðvísindadeild Hawaiíháskóla í Honolulu.
Prófdómari er Dr. John A Stevenson, fræðimaður við Edinborgarháskóla.