Reconstructing Magma Degassing in the Katla 1918 Eruption through Vesicle Textures and Dissolved Volatile Contents

Abstract:

Iceland’s Katla volcano frequently produces explosive eruptions (VEI 3-5) that generate large quantities of ash and powerful glacial floods (jökulhlaups). Its last eruption (VEI 4, basalt) was in 1918, but another may be imminent, given recent unrest and historic correlation with activity at Eyjafjallajökull, which erupted in 2010.

Investigations of eruption products at nearby Torfajökull [1,2] have shown that volatiles are the primary control on the eruptive behaviour of subglacial rhyolite, rather than ice thickness. Explosive events are characterised by high pre-eruptive H₂O contents (up to ~5 wt. %) and more closed system degassing, demonstrated by H₂O–Cl ratios, microlite contents and vesiculation modelling. We have continued to develop the use of volatile degassing as an sensitive indicator of syn-eruptive pressure conditions [3,4].

We are now applying similar approaches to the basaltic Katla 1918 event, to determine the relative influence of volatiles and meltwater on eruption mechanisms. Sampling has included air-fall tephra from Mýrdalsjökull and jökulhlaup deposits from Múlakvísl. Infrared spectroscopy (FTIR) data reveals that airfall tephra have degassed to atmospheric conditions (0.07 wt % H₂O), whereas jökulhlaup-carried juvenile clasts have elevated H₂O contents (0.18 to 0.32 wt % H₂O), consistent with quenching beneath a load of ice, water or tephra.

Ongoing quantification of vesicle and crystal size distributions, together with experimental vesicle growth rates using hotstage microscopy, will help constrain rates and amounts of magma decompression, degassing and interactions with meltwater. We also aim to investigate chamber-to-surface degassing through analysis of volatile concentrations in melt inclusions.

[1] Owen et al. 2013a Geology 41: 251-254

[2] Owen et al. 2013b J Volcanol Geoth Res 258: 143-162

[3] Tuffen et al. 2010 Earth Sci Rev 99: 1-18

[4] Owen et al. 2012 Bull Volcanol 74: 1355-1378