The 1845 Eruption of Hekla, Iceland: Fragmentation and Vesiculation Processes

The 1845 eruption of Hekla Iceland began around 9 am on the 2 of September, with an intense opening phase. The eruption continued until August 1846. Contemporary accounts mention precursor earthquakes and describe darkness in the Land district to the west of the volcano, which was caused by the ~19km high and east-southeast-drifting eruption plume. The accounts also indicate first sighting of the eruption plume at sites some 80 km downwind from the volcano around 10 am and with visible tephra fall at ~11 am. An hour later the tephra fall began to dissipate and had stopped by 3 pm. Three ships of the southeast coast of Iceland report tephra falling on their deck and sails and records of tephra fall are as also known from the Faeroe and Shetland Islands.

The initial explosive phase of the 1845 eruption lasted for about one hour, featuring mass eruption rate of ~10⁴ m³ s^-1 and produced a well-defined tephra fall sector covering approximately 10000 km² within the 1 mm isopach. The total volume of the tephra from the initial phase is calculated at ~0.04km³ DRE. The average density obtained from three sets of “100-clast” samples ranges from 560-690 kg m^-3. All of these samples exhibit a tight unimodal distribution with a calculated mean vesicularity of 74-79%.. These data also indicate a subtle decrease in maximum and mean vesicularity through the opening phase, from 88% to 82% and 79% to 75% respectively, which coincides with upward decrease in grain size (i.e. intensity) within proximal sections. The initial phase of the 1845 Hekla eruption was short lived and its products are exceptionally well preserved in Icelandic soils. As such it is ideal for studies of vesiculation and fragmentation processes and we will present new data on pumice vesicle size distribution as well as on total deposit grain size distribution.