The Hekla 2000 tephra deposit: Grain-size characteristics and eruptive parameters

Abstract UV 1 from Nordic Geological Winter Meeting 2012

Kate Smith, Costanza Bonadonna, Thorvaldur Thordarson, Ármann Höskuldsson, Guðrún Larsen, Stefán Árnason, Freysteinn Sigmundsson

The highly active volcano Hekla last erupted in 2000 and current crustal deformation indicates that the next eruption may be soon. Hekla eruptions usually begin with a short subPlinian to Plinian explosive phase. The associated volcanic plume can cause disruption to air-traffic while tephra fall can affect the environment and infrastructure nearby. The total grain size distribution (TGSD) of tephra and the erupted mass and volume are important input parameters for models used for forecasting dispersal and ash concentration in the volcanic plume, as well as hazard and risk evaluation. This paper presents new calculations of eruptive parameters for the Hekla 2000 eruption, new tephra grain size data and TGSDs.
Haraldsson (2000) and Höskuldsson et al. (2007) calculated Hekla 2000 tephra volumes but with considerably different results.

New calculations, based on mass loading data (Haraldsson, 2000) and reassessment of the isomass map, result in an erupted mass value between 7.8 x 109kg and 2.3 x 1010kg, depending on the method used. The associated volume lies between 1.1 and 3.3 x 107m3, corresponding to a dense rock equivalent volume of 2.9 to 8.5 x 106m3 and VEI 3.

The eruption deposited tephra with a main dispersal axis to the north and a minor dispersal axis to the south. Tephra was sampled by Icelandic scientists and members of the public within days to weeks of deposition. Granulometric analysis was performed on 31 samples using hand sieving and laser diffraction. This data was used to construct TGSDs using weighted mean and Voronoi Tessellation methods.

Median diameter decreases with distance from the vent from medium-grained lapilli (15.24mm) in the proximal zone, falling sharply over 30km to coarse-grained ash (1.85mm) and then gradually fining to 102μm on Grímsey (294km distance). Proximal to medial distributions are generally weakly bimodal, whereas
peripheral samples and most samples more than 60 km from the vent are unimodal or very weakly bimodal. Spatial variations in mode and fine ash concentration are interesting. The majority of the samples have 2% or less fine ash (<63μm).  owever, concentration of fine ash is distinctly higher in two domains; 8 to 12 wt% of fine ash is found within 20km west of the vent, and 4 to 8% fine ash is found from 80 to 190km north of the vent. The bimodality and fine ash concentration variations can be attributed to different transport pathways, deposition from different eruptive phases, aggregation of fines, multiple fragmentation mechanisms and reworking; the nature and contribution of these will be discussed. Sorting values improve with distance from the vent, ranging from 2.04 to 0.70 along the dispersal axis and reflecting effective fractionation of the coarser clast  populations by transport and deposition.

All TGSDs determined for Hekla 2000, using these analyses, are bimodal, but different TGSDs emphasise the bimodality and the proportion of fine-grained ash more than others. The TGSD has a significant impact on dispersal model results and it is therefore important to understand the impact of different techniques and sampling patterns for local and international hazard assessment.