In geodynamic numerical models of volcanic systems, the volcanic basement hosting the magmatic reservoir is often assumed to exhibit constant elastic parameters with a sharp transition from the host rocks to the magmatic reservoir. We assess this assumption by deriving an empirical relation between elastic parameters and temperature for Icelandic basalts by conducting a set of triaxial compression experiments between 200 °C and 1000 °C. Results show a significant decrease of Young's modulus from ∼38 GPa to less than 4.7 GPa at around 1000 °C. Based on these laboratory data, we develop a 2D axisymmetric finite-element model including temperature-dependent elastic properties of the volcanic basement. As a case study, we use the Snæfellsjökull volcanic system, Western Iceland to evaluate pressure differences in the volcanic edifice and basement due to glacial unloading of the volcano. First, we calculate the temperature field throughout the model and assign elastic properties accordingly. Then we assess unloading-driven pressure differences in the magma chamber at various depths in models with and without temperature-dependent elastic parameters. With constant elastic parameters and a sharp transition between basement and magma chamber we obtain results comparable to other studies. However, pressure changes due to surface unloading become smaller when using more realistic temperature-dependent elastic properties. We ascribe this subdued effect to a transition zone around the magma chamber, which is still solid rock but with relatively low Young's modulus due to high temperatures. We discuss our findings in the light of volcanic processes in proximity to the magma chamber, such as roof collapse, dyke injection, or deep hydrothermal circulation. Our results aim at quantifying the effects of glacial unloading on magma chamber dynamics and volcanic activity.@en

Earthquake-triggered volcanic activity promoted by dynamic and static stresses are considered rare and difficult-to-capture geological processes. Calderas are ideal natural laboratories to investigate earthquake–volcano interactions due to their sensitivity to incoming seismic energy. The Campi Flegrei caldera, Italy, is one of the most monitored volcanic systems worldwide. We compare ground elevation time series at Campi Flegrei with earthquake catalogues showing that uplift events at Campi Flegrei are associated with large regional earthquakes. Such association is supported by (yet non-definitive) binomial tests. Over a 70-year time window we identify 14 uplift events, 12 of them were preceded by an earthquake, and for 8 of them the earthquake-to-uplift timespan ranges from immediate responses to 1.2 yr. Such variability in the response delay may be due to the preparedness of the system with faster responses probably occurring in periods during which the Campi Flegrei system was already in a critical state. To investigate the process that may be responsible for the proposed association we simulate the propagation of elastic waves and show that passing body waves impose high dynamic strains at the roof of the magmatic reservoir of the Campi Flegrei at about 7 km depth. This may promote a short-lived embrittlement of the magma reservoir's carapace otherwise marked by a ductile behaviour. Such failure allows magma and exsolved volatiles to be released from the magmatic reservoir. The fluids, namely exsolved volatiles and/or melts, ascend through a nominally plastic zone above the magmatic reservoir. This mechanism and the associated inherent uncertainties require further investigations but the new concept already implies that geological processes triggered by passing seismic waves may become apparent several months after passage of the seismic waves.@en

We investigate the regional fluid flow dynamics in a highly faulted transform area, the Tjörnes Fracture Zone in northern Iceland which is characterized by steep geothermal gradients, hydrothermal activity, and strong seismicity. We simulate fluid flow within the Tjörnes Fracture Zone using a high-resolution model that was based on the available geological and geophysical data and has the aim to represent the complex geological structures and the thermodynamical processes that drive the regional fluid flow in a physically realistic way. Our results show that convective heat flow and mixing of cold and saline seawater with deep hydrothermal fluids controls the large-scale fluid flow. The distribution of faults has a strong influence on the local hydrodynamics by focusing flow around clusters of faults. This explains the nature of isolated upflow zones of hot hydrothermal fluids which are observed in the Tjörnes Fracture Zone. An important emergent characteristic of the regional fluid flow in the Tjörnes Fracture Zone are two separate flow systems: one in the sedimentary basins, comprising more vigorous convection, and one in the crystalline basement, which is dominated by conduction. These two flow systems yield fundamental insight into the connection between regional hydrothermal fluid flow and seismicity because they form the basis of a toggle switch mechanism that is thought to have caused the hydrogeochemical anomalies recorded at Húsavik before and after the 5.8 M earthquake in September 2002.@en

We present a groundwater flow model that integrates geological observations, field data, effective permeabilities for fractured lava flows, and historical eyewitness records to explain the change from wet to dry explosive activity that occurred in March 1875 at Askja volcano, Iceland. This rhyodacite eruption is the third largest silicic explosive event in Iceland since settlement in this region (approximately 1140 B.P.). It was part of a larger volcano-tectonic episode that took place in the North Volcanic Zone from 1874 to 1876. This eruption is unique in that it features both abrupt and gradual shifts between wet and dry explosive activities. It consisted of an initial and sudden shift from dry subplinian to wet phreatoplinian activity followed by a gradual shift from wet phreatoplinian to increasingly dry pyroclastic density currents to a final dry Plinian phase. The Askja caldera overlaps the Öskjuvatn caldera and it is bounded by steep-sided hyaloclastic mountains on all sides. Its floor is filled by Holocene and historical lava flows which are intensely fractured. The source of water that fuelled the phreatoplinian activity is not obvious, as eyewitness records rule out snow, surface water, and heavy rainfall. Hence we use 3D numerical simulations of the syn-eruptive groundwater flow in the Askja caldera, including an accurate estimation of the effective permeabilities of fractured lava flows, to explain the possible changes in eruptive style. We propose that the water necessary to maintain the phreatoplinian phase was stored within the lava pile and that the rapid groundwater flow through the fractured lavas provided enough water to drive the hour-long phreatoplinian eruption. Our model describes how the decline of the groundwater table caused the progressive dry-out of the wet phase and led to the dry character of the following Plinian activity. This study demonstrates how the availability of groundwater and the geology of the volcanic edifice can strongly affect the eruptive style of a volcanic eruption.@en

We use high-resolution simulations to analyze fluid flow, pore pressure, and fault permeability evolution in the seismically active Tjrnes Fracture Zone (TFZ), a major transform fault zone in the North of Iceland. Our results show that the TFZ is characterized by four distinct areas where pore pressures are above hydrostatic, consistent with geophysical observations. Basement and faults, which are assumed to have low permeabilities, often display pore pressures close to lithostatic. Fault permeabilities are allowed to vary freely as a function of the effective fault normal stress. They hence inflate periodically to release excess pore pressure in a few minutes. This is accompanied by an increase in permeability of over seven orders of magnitude and causes short-lived fluid fluxes of more than 0.01 m s-1. After pore pressures have dissipated, fault permeabilities decay back to their original values in 2 to 3 years as the effective fault normal stress increases. This behavior is consistent with a toggle switch mechanism and could have two important implications for fluid flow in seismically and hydrothermally active oceanic crust. First, the rapid changes in fault permeability and pore pressure provide an explanation for distinct cyclical geochemical changes observed on a similar timescale in thermal waters near the town of Hsavik in the TFZ before and after a magnitude 5.8 Mw earthquake. Second, our results provide another line of evidence in the growing number of observations that crustal permeabilities are constantly evolving and geological processes in hydrothermal systems can be dominated by short-lived and extreme flow events.@en