Investigation of the pre-eruptive processes of the 2014/15 Holuhraun eruption based on extracted volcanic tremor signals

<p>During volcanic unrest, multiple subsurface processes can happen simultaneously and may lead to an eruption. The analysis of seismic records in an unrest period before an eruption reveals information about the pre-eruptive processes and might be able to provide hints for a possible future eruption.</p><p>The 2014–2015 Holuhraun eruption was the largest one in Iceland in 230 years. It was extensively monitored and studied in a variety of multidisciplinary research approaches. Intense seismicity and ground deformation were interpreted as magma propagation from Bárðarbunga volcano 48 km laterally at ∼6 km depth over two weeks before an eruption started at Holuhraun. Different processes including vertical and lateral magma migration, dike propagation, caldera subsidence, and subglacial eruptions happened in this period and some models linking these processes are suggested. In the two-week interval preceding the eruption, there is still no clear connection between the observed tremor and pre-eruptive processes. Both the tremor source location and tremor generation process are not well understood yet. While cauldrons as a sign of subglacial eruptions were identified on the glacier surface from aerial photos, these cauldrons might have been formed earlier and there is hence an uncertainty of a few days. A tremor location might help to constrain these dates. However, the simultaneous occurrence of intense seismicity and tremor hinders the study and location of tremor. Here, we use a recent volcanic tremor extraction algorithm (Zali et al., 2020) and extract pre-eruptive tremor signals in order to better locate them using the Seismic Amplitude Ratio Analysis (SARA) method. Furthermore, the occurrence of the tremor could open new insights into ascending magma and fluid migration as well as the timing and duration of the subglacial eruptions.</p><p>We also observed short-lived tremors before the eruptions on August 29 and 31, which could be considered as eruption precursors. The primary investigation on the extracted tremor signals is promising while further analysis is on-going.</p>

Sub-Ice Sheet Environments in North Victoria Land during the Last Glacial Maximum

<p>Subglacial calcite precipitates from Boggs Valley (71<sup>o</sup>55’S; 161<sup>o</sup>31’E; elevation 1,160 m asl., Northern Victoria Land, Antarctica), provided the first radiometrically-dated petrographic, geochemical and genomic evidence of thermogenic subglacial drainage events linked to subglacial eruptions during the Last Glacial Maximum (LGM). The crusts consist of two fabrics: i) a dirty (particulate-rich) microsparite, which marks catastrophic subglacial discharges of meltwater and a ii) dark columnar calcite that formed in pockets of basal melt. Synchrotron Radiation-based micro X-Ray fluorescence reveal that the dirty microsparite is S-rich, and embeds particulates characterized by high Manganese (Mn), Yttrium (Y) and Iron (Fe) concentrations. From previous work, we also know that the microsparite layers contain organic compounds, including amino acids, from which we extracted DNA fragments of microorganisms that lived in diverse sub-Antarctic environments (Frisia et al., 2017). The elongated columnar calcites are characterized by the presence of Arsenic (As) associated with low concentrations of  Mn. Both elements suggest local anaerobic, chemolitothrophic metabolism. Columnar calcite becomes increasingly rich in S near the “discharge” layers.  </p><p>Our preliminary interpretation is that during the LGM subglacial volcanism was crucial to sustain life in sub-ice sheet refugia by injecting both nutrients and diverse microbes into the basal ecosystem. The otherwise nutrient-poor, anoxic subglacial environment sustained a population of chemolithotrophs, which may have also been “allochthonous”.   </p>

Will subglacial rhyolite eruptions be explosive or intrusive? Some insights from analytical models

Simple analytical models of subglacial eruptions are presented, which simulate evolving subglacial cavities and volcanic edifices during rhyolitic eruptions beneath temperate glaciers. They show that the relative sizes of cavity and edifice may strongly influence the eruption mechanisms. Intrusive eruptions will occur if the edifice fills the cavity, with rising magma quenched within the edifice and slow melting of ice. Explosive magma–water interaction may occur if a water- or steam-filled gap develops above the edifice. Meltwater is assumed to drain away continuously, but any gap above the edifice will be filled by meltwater or steam. Ductile roof closure will occur if the glacier weight exceeds the cavity pressure and is modelled here using Nye’s law. The results show that the effusion rate is an important control on the eruption style, with explosive eruptions favoured by large effusion rates. The models are used to explain contrasting eruption mechanisms during various Quaternary subglacial rhyolite eruptions at Torfajökull, Iceland. Although the models are simplistic, they are first attempts to unravel the complex feedbacks between subglacial eruption mechanisms and glacier response that can lead to a variety of eruptive scenarios and associated hazards.