Changes in absolute gravity at base stations in ice-covered volcanic areas – the combined effects of isostatic rebound, ice cover and volcanism at Grímsvötn, Iceland, 1971-2025

Glaciers have been retreating globally for more than 100 years. In Iceland, where glaciers cover some of the most active volcanoes, this is causing rapid regional uplift (Glacio-Isostatic Adjustment - GIA). This process has been very prominent over the last three decades, resulting in uplift similar to 4 cm/yr in the volcanic zone covered by Vatnajökull glacier, monitored by continuous GNSS stations. This includes the subglacial central volcano Grímsvötn, in the western part of Vatnajökull, one of the most active volcanoes in Iceland. Gravity surveys are a powerful geophysical tool for investigating surface and subsurface geological processes based on variations in the Earth's gravitational field. Many gravity base stations were established in Iceland in 1968-1971, including in the proximity of the retreating Vatnajökull. In this study, data from several gravity surveys conducted on Vatnajökull over the last 30+ years is used, to detect absolute gravity changes. These surveys include repeated ties of the base station established at Grímsfjall in 1971, a nunatak on the southeastern rim of the Grímsvötn caldera, with the other base stations. As Grímsvötn is a highly dynamic ice-covered volcano, the gravity data series is influenced by several local processes. These are (1) changes in ice cover and ice thickness at the volcano caused by variations in geothermal activity, (2) changes in bedrock topography caused by volcanic eruptions in 1998, 2004 and 2011, (3) variations in water level in the subglacial lake in the Grímsvötn caldera, and (4) potentially variations in groundwater level in the volcanic edifice. In addition, the gravity is affected by (5) inflation and subsidence associated with magma accumulation and the eruptions.  Processes (1), (2), (3) and (5) can be constrained as well as the regional gravity effect caused by uplift due to GIA. The results show large variations with time in the value of g (>0.5 mGal) at Grímsfjall over the last 30 years. While process (2) is too small to register, processes (1) and (3) are very prominent, superimposed on the GIA effect. This contrasts sharply with more regular effects of GIA, seen at the base stations by the edge of the glacier.