Shallow Crustal Density Distribution and its Evolution at the Krafla Volcanic System

Abstract

Active volcanic regions host powerful geothermal systems that are now being intensively exploited as part of global efforts toward sustainable energy and climate-change mitigation. The Krafla Volcanic System (KVS) is one of the most studied volcanic areas in the world, owing to decades of geothermal exploitation and its well-documented volcanic activity. Yet fundamental uncertainties remain regarding the nature, structure, and evolution of coupled magmatic-hydrothermal systems in rift settings, questions that are critical not only for improving geothermal resource potential but also for assessing volcanic hazards and associated risks.

This thesis aims to improve the understanding of the KVS coupled magmatic–hydrothermal system subsurface structure, nature, and evolution through rifting, post-rifting, and the present day, including the impacts of geothermal exploitation. To achieve this, legacy gravity and ground deformation datasets were reprocessed and combined with new gravity and GNSS measurements from three summer field campaigns (2022--2024). From these data, we present: (i) the first high-resolution 3D gravity inversion model of the KVS, (ii) a five-decade compilation of time-lapse gravity and deformation records, and (iii) a finite element model of the Krafla Fires events.

The 3D inversion of the static gravity dataset resolves positive anomalies linked to shallow magmatic intrusions, including caldera ring-shaped dikes and a complex of shallow sill intrusions at $\sim$2\,km depth, where drilling encountered magma. In contrast, negative anomalies coincide with major geothermal areas inside the caldera, consistent with zones of high permeability and fluid content, as well as with the tectonic graben structure of the Húsavík-Flatey Fault.

Time-lapse gravity and deformation data reveal distinct phases in the evolution of the KVS. During the Krafla Fires (rifting episode), a low-gravity anomaly with associated subsidence in the Leirhnjúkur area was best explained by a finite element model with a two-source configuration: a shallow pressurising dike with decreasing density and a deeper depressurising sill source with increasing density. Post-rifting variations are dominated by magmatic cooling and geothermal exploitation. Recent surveys (2022--2024) indicate that present-day gravity changes reflect the background noise system, which is likely related to cavity dynamics, fracture interactions, and fluctuations in the water table.

This thesis provides an integrated gravity-deformation framework for the Krafla Volcanic System by combining high-resolution 3D gravity inversion, long-term geodetic monitoring, and finite-element modelling. The new characterisation of the structure and nature of the magmatic and hydrothermal features beneath the caldera, including its evolution, improves our scientific understanding of rift-zone volcanism and has practical implications for future geothermal exploitation activities and volcanic hazard assessments. In particular, the ability to distinguish between magmatic, hydrothermal, and exploitation-related signals provides a stronger basis for sustainable reservoir management and enhances the reliability of early warning systems for volcanic unrest.

Date of Award	20 Jan 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	James M Wookey (Supervisor) & Alison C Rust (Supervisor)