Recommendations for microgravity campaigns and insight into volcanic subsidence and uplift gained from microgravity data at Askja, Iceland between 1988 - 2022

<p>In August 2021, Askja volcano in Iceland experienced&#160; a sudden onset of rapid uplift that followed decades of continuous subsidence. We review the extensive microgravity record from Askja, revisiting data recorded between 1988 and 2017, and presenting new microgravity data from 2021 and 2022 that were collected after the uplift had started. Based on our findings, we provide a comprehensive set of recommendations that should be followed for optimal microgravity data collection and treatment in volcano monitoring. Without such standards, it becomes increasingly challenging to interpret the microgravity results in terms of volcanic processes. At Askja, from 1988 &#8211; 2016, exponentially decaying surface subsidence was accompanied by a microgravity decrease, potentially signaling the contraction of its magma chamber, eviction of magma to deeper levels, or other density-decreasing processes. Following this, between 2016 and 2021, a gravity increase occurred in the center of the caldera which effectively annuls the microgravity decrease detected between 1988 and 2016. This increase took place either during subsidence or leading up to and during the uplift, and may potentially be explained by mass accumulation below the caldera. After August 2021, gravity changes follow the free-air gradient, despite continuing deformation with a total uplift of up to 40 cm, suggesting subsurface density decreases as a driving process. Such a process could be envisaged as the previously emplaced intrusion before 2021 that is now undergoing magma vesiculation, is causing a change in the hydrothermal system, or represents the replacement of dense basaltic magma with less dense rhyolitic magma. However, uncertainties for these data are large (50&#956;Gal) and small mass intrusions contributing to the uplift may remain undetected. The driving mechanism for the uplift remains enigmatic and future microgravity campaigns will help shed light on its nature.</p>