| Summary: | Iceland is one of the most active terrestrial volcanic regions on Earth with an average of more than 20 eruptions per century. Around 80% of all events are tephra generating explosive eruptions, but less than 10 % of all known tephra layers have been mapped. Recent hazard assessment models show that the two key parameters for hazard assessment modeling are total grain size distribution (TGSD) and eruptive style. These two parameters have been determined for even fewer eruptive events in Iceland. One of the most hazardous volcanoes in Iceland is Katla and no data set of TGSD or other eruptive parameters exist. Katla has not erupted for 99 years, but at least 2 of the 20 eruptions since the settlement of Iceland in 871 have reached Northern Europe as visible tephra fall. These eruptions occurred in 1755 and 1625 and remain enigmatic both in terms of actual size and eruption dynamics. This work presents studies of these two far-reaching eruptions in terms of fragmentation and eruption dynamics as well as the first set of eruption source parameters for any Katla eruption. In order to provide detailed insight into the eruption dynamics a new method for classifying fragmentation mechanisms based on tephra grain morphology was developed and is presented in this work. The deposits are estimated to cover 23400 km2 and 23600 km2 on land in Iceland for the 1755 and 1625 eruptions. Volumes calculated from the power-law integration method are 1.20-1.50 km3 for the 1755 eruption and 1.12-1.36 km3 for the 1625 eruptions. The total erupted mass converted from erupted volume of the 1755 eruption was 1.84-2.45⋅1012 kg with a lower-bound mass eruption rate of 1.25-1.67⋅106 kg/s. In 1625 Katla erupted between 1.53-1.94⋅1012 kg tephra as calculated from the erupted volume with a lower-bound mass eruption rate of 1.61-2.04⋅106 kg/s. The average 1755 plume height was 14.4 km based on mass loading data inversion with an empirical correlation estimate of paroxysmal peaks at 25± 6 km. The average 1625 plume height was 16.6 km based on ...
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