The rheological evolution of the 2014/2015 eruption atHoluhraun, central Iceland

In the period from August 29, 2014, to February 27, 2015, the largest basaltic eruption of the last 200 years in Iceland occurred within the tectonic fissure swarm between the Bárðarbunga-Veiðivötn and the Askja volcanic systems. The eruption took place in the Holuhraun lava field, 45 km northeast of the Bárðarbunga volcano and 20 km south of the Askja volcano. It emplaced over 1.2 km3 dense rock equivalent (DRE) of lava in an area of very low topographic relief. In light of the minimal topographic forcing, lava flow emplacement can be viewed as having been effectively controlled by the lava’s temperature-dependent rheology. Here we combine field and remote sensing data collected during the week of the 17th to 22nd of November 2014 to constrain the lava’s flow path, its velocity, deformation rate and thermal evolution during emplacement. We combine these with measurements of the pure liquid viscosity and the sub-liquidus rheological evolution of the lava during crystallization. Sub-liquidus experiments were performed at a range of constant cooling and shear rates, to mimic the conditions experienced by the lava during emplacement. These data can also be used to infer flow conditions of partly degassed magma within dike-swarms during transport towards the surface. The data show that the effective viscosity of the lava drastically increases until reaching a specific sub-liquidus temperature, the “rheological cutoff temperature” (T cutoff). This departure to high viscosity is a consequence of the onset of crystallization and is found to be primarily controlled by the imposed cooling rate. Our data indicate that shear rate exerts a second-order effect on this rheological departure and T cutoff. We discuss the experimental dataset in the context of the reconstructions of the natural emplacement conditions and describe the implications for the 2014–2015 lava flow field at Holuhraun as well as lava flow modelling in general.