On the Dynamics of Rhyolite Dome Emplacement: Densities and Deformation Fields

Silicic rocks in Iceland are generally associated with central volcanoes and are often emplaced on or around caldera rims. Rhyolite magma can rise, due to buoyancy forces and either form a cryptodome in the shallow crust or rise to the surface, where it erupts. Due to its high viscosity and resistance to flow it often accumulates and forms a lava dome over the vent. Some of these domes were formed subglacially while others were erupted under ice-free conditions. In this thesis some aspects pertaining to the dynamics of dome emplacement are studied, s.a. size and density of domes. A model to describe the deformation field above a rising cryptodome is proposed. A gravity survey was carried out in the area of Krafla in 2007 and 2008 to determine the mean bulk density values of rhyolite domes. Data on density and volumes is essential for meaningful modelling of the emplacement of cryptodomes and lava domes. Such data are scarce. Profiles were measured over three formations, ranging in size from Hlíðarfjall (310 m high and 2 km long), formed under ice 90 000 years BP, to Hrafntinnuhryggur (80 m high and 2.5 km long) formed 24 000 years BP under a glacier to Hraunbunga (125 m high and 1.8 km long) formed 10 000 years BP. The Holocene formation, Hraunbunga was measured as a reference to the subglacially formed ridges Hliðarfjall og Hrafntinnuhryggur on the Krafla caldera's rim. Mean bulk density for each formation was obtained by the Nettleton method. The results are that all the domes have low densities, reflecting both low grain-density and high porosity. The domes's density values are significantly smaller than those of the surroundings, creating a density contrast possibly sufficient to drive the ascent of rhyolite magma. Furthermore, results from gravity data demonstrate that these formations are neither buried by younger volcanic eruptives nor are any roots detected. The domes studied were therefore emplaced as vent-forming domes. No dome eruption has been observed with modern monitoring equipment in Iceland. The persistent seismic cluster at Goðabunga on Katla's caldera rim, located in the South Iceland Flank Zone (SIFZ), has been active for the last forty years. It is has been suggested that it represents a rising cryptodome. No deformation has been observed above this seismic cluster, possibly because of large mesh size of the geodetic network. The Mogi model, due to its simplicity, is widely used to interpret a volcano's inflation or deflation. It has been successfully used to model the deformation fields of the volcanoes Krafla and Katla. An inflating Mogi source assumes addition of magma to the system, resulting in a broad surface deformation anomaly detectable with a dense network. We present here the Mogi-dipole model for a rising batch of magma. It is derived from the Mogi model for a fixed volume of magma. We argue that the Mogi-dipole model's local narrow surface deformation field explains why no deformations have yet been detected at Goðabunga. This model also seems to correspond well with recently published deformation data on the 2004 - 2006 dome eruption at Mount St. Helens, in the Cascade range north-western USA, as well as to the deformation prior to its 1980 eruption. Where significant deformations were only observed at stations located on the domes them selves, and not in the far field. Keywords: rhyolite domes, surface deformation, gravity measurements, Mogi-dipole mode