[Folda] Ph.D. defence in Geophysics - Hannah Iona Reynolds,?==?utf-8?q? 18. Dec - 13:00 to 16:00 | Location: Askja N-132

[Siqi Li]

Ph.D. student: Hannah Iona Reynolds

Name of thesis: Subglacial volcanic and geothermal activity. Measurement and modelling of heat flow.

Abstract: Thermal anomalies are observed at many volcanoes, resulting from geothermal and magmatic activity, and are usually difficult to quantify since the measurement of heat fluxes from the ground to the atmosphere is subject to large uncertainties. However, many of Iceland’s volcanoes are ice-covered, and this study exploits the special situation that the overlying ice acts as a calorimeter, where estimates of heat release can be made based on the volume of ice melted over a specified time period. This provides opportunities to estimate thermal signals to considerable accuracy. The general aim of this thesis is to advance the understanding and interpretation of thermal signals at ice-covered volcanoes in Iceland, and how they are linked to intrusive and volcanic activity. Three study areas are focused on: Grímsvötn, Bárdarbunga and Dyngjujökull.

Ice calorimetry is used to derive the heat output from Grímsvötn for 1998–2016, using annual mapping of ice surface, providing the most detailed study of the thermal output of Grímsvötn to date. The average heat released over the 18 year period is estimated to be 1.8 GW, whereof about 0.6 GW is composed of peaks above the base flux from melting during eruptions and associated geothermal anomalies. Peaks in geothermal activity were associated with all three eruptions which took place during the study period.

In August 2014, a period of seismic unrest began at the subglacial caldera of Bárdarbunga, and was followed by the propagation of a lateral dyke to the north, beneath the Dyngjujökull glacier, and resulted in a subaerial fissure eruption at Holuhraun. Ice surface depressions were observed above the path of the dyke beneath Dyngjujökull, and around the caldera rim at Bárdarbunga. Rates of heat transfer and duration of thermal signals show that subglacial eruptions rapidly generated the depressions at Dyngjujökull. The depressions around the Bárdarbunga caldera rim formed during and following an episode of caldera collapse, due to associated changes in permeability and possibly intrusive activity. In the two-year period following the onset of increased seismicity in the area, the total power from Bárdarbunga caldera averaged ~270 MW.

The surface thermal signals produced by shallow intrusions were investigated with numerical modelling, using HYDROTHERM. Permeability was found to provide strong control on the surface heat flux, as was the initial temperature of the porous matrix. Lower initial temperatures result in buffering of the heat transfer to the surface, creating a muted thermal signal. High-permeability pathways were found to significantly increase the magnitude of the surface heat flux, and reduce the onset time for the signal.