Steam-driven explosions at Gengissig lake (Kverkfjöll volcano, Iceland): constraints from field and experimental data

Abstract:

On August 15, 2013, a small jökulhlaup occurred when the Gingissig ice-dammed lake was drained at Kverkfjöll, a central volcano with an active geothermal area located at the northern edge of Vatnajökull. Coincidentally after the lake level drop of about 30 m, steam-driven explosions took place at the bottom of the drained lake. The explosions involved the surficial part of a hydrothermally altered glacio-lacustrine deposit mainly composed of pyroclasts, lava fragments and volcanic bombs, interbedded with clay-rich layers. Small fans of ejecta were formed, reaching a distance of 1 km north of the lake covering an area of about 0.3 km², with a maximum thickness of 40 cm at the crater walls.

The explosions were triggered by the abrupt decrease in confining pressure as a consequence to the sudden drainage of the lake which caused rapid boiling in the surficial geothermal reservoir. The thermal and craterization energy, calculated for the explosion areas, is in the order of 10¹³ and 10¹¹J respectively. Comparison of the calculated energies with the ones estimated by the volume of the ejecta and the crater sizes, indicates a good agreement between models and field data.

Morphological analyses (SEM) suggest that fragmentation due to steam expansion within the vesicles affected a low percentage of the ejected material. Impact and abrasion features are significantly more frequent suggesting relatively low energy consumption by fragmentation, in contrast to a high energy transferred to the lifting and the launching of loose particles.

Decompression experiments were performed in the lab mimicking the conditions due to the lake drainage. Then ejection velocities of the gas–particle mixtures were measured trough image analysis of high-speed videos and used to estimate the kinetic energy release.

The aim of this study was to combine field and lab data with theoretical modeling to give a robust constraint on the energies released by low-magnitude steam-explosion event.