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Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
Askja, volcanology, volatiles, Iceland, petrology
The bulk of the eruption of Askja in north central Iceland on March 28-29 1875 consisted of a plinian eruption that lasted 6-7 hours, produced 0.2 km3 of ash and rhyolitic pumice, and created a surge and partially welded ash/pumice fall deposit that crops out on the shore of the modern caldera lake (Sparks et al. 1981). We evaluate the volatile budget of the magma during the eruption and focus on water concentration in glass fragments and shards, glass adjacent to crystals, and melt inclusions (MIs). Sparks et al. (1981) estimated the gas exit velocity at the vent was 380 m/s during the plinian phase, and the water concentration at 2.8 wt%. Measurements of water concentration in basaltic and rhyolitic glass shards from layers C through E range from 0.15 to 0.5 wt%, with variations within layers, a drop in layer D, and increase in layer E. Plagioclase and pyroxene crystals from layers C through E contain rhyolitic MIs with water concentrations ranging from 0.1 to 1.8 wt%, some higher than the matrix glass. Magma underwent degassing on its way to the surface. Rhyolitic glass adjacent to crystals hosting MIs has the highest water concentration, from 0.4 to 2.18 wt%. This, and the initial phreatoplinian eruptive style, both suggest interaction of magma with meteoric water during the eruption. Intimate mixtures of basaltic glass compositions within samples and basaltic glass surrounded by rhyolitic glass support the conclusion of Sigurdsson and Sparks (1981) that magmas mingled prior to and during the eruption.
Sheila J Seaman