Vesicle size analysis of magmatic and phreatomagmatic phases of the 934-40 AD Eldgjá fissure eruption, southern Iceland

Abstract:

Eldgjá is a ~75 km volcanic fissure which erupted in the middle of the 10^th century. It belongs to the Katla volcanic system and is of the mixed-cone row type. The eruption lasted several years and produced two large lava fields (18.3 km³) and a widespread basaltic tephra deposit (4.5 km³ or 1.3 km³ DRE). Proximal tephra deposits are up to 5 m thick and contain as many as 10 sub-units which vary in thickness parallel to the fissure, indicating multiple points of origin. Many of the lowest units were produced by phreatomagmatic activity and dispersion of these units indicate an origin below the Mýrdalsjökull glacier. Two sections were chosen to represent magmatic and phreatomagmatic activity. A 3.7 m section at Skælingar, 0.5 km south-east of Eldgjá, represents the magmatic products. The phreatomagmatic products are represented by a 2.7 m thick section at Stóragil, located 10-15 km south and east of the vent system. The sampled phreatomagmatic part of the section is 1.12 m thick. These sections were logged and then each sub-unit was sampled for density analysis which involves picking 100 clasts in the 8 to 32 mm size range from horizons less than 5 cm thick. Thin-sections were then made from representative clasts of each sub-unit. Nested images of these thin-sections were analysed for vesicle size and shape. The magmatic clasts have unimodal, normal to moderately log-normal density distributions with a mean density of 7900-7500 kg.m^-3 (60-70% vesicularity) and a range of around 8600 kg.m^-3 (40% vesicularity). The log-normal distribution of some samples may be explained by the inclusion of previously degassed magma in the eruption. The section at Stóragil contains both phreatomagmatic and magmatic products. The phreatomagmatic samples have characteristic plateau-like distributions with a mean density of 7200-7500 kg.m^-3 (70-80% vesicularity) and a range of around 8420 kg.m^-3 (45% vesicularity). One layer shows a change from phreatomagmatic to magmatic vesicularity distributions. Initial results for vesicle size analysis indicate simple nucleation patterns, vesicle number densities per volume (N_V) in the order of 4×10³ mm^-3 and vesicle number densities per melt volume (N_VM) in the order of 1 4×10⁴ mm^-3 The complete set of vesicle size analysis results will be presented at the conference.