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Abstract

The crater of Mount St. Helens has undergone significant changes since the end of the 2004–2008 eruption cycle. Heat output from the new lava dome has decreased constantly in different phases of cooling. At the same time, the lava dome morphology changed, including substantial subsidence. Herein we present long-term dome evolution within the St. Helens crater by combining field investigations and remote sensing data. Repeated surveys of subglacial caves provide evidence for a new phase of dome cooling and reduced fumarolic activity. While the caves remained consistent or became larger before 2022, the latest field campaigns in 2023 indicated a strong volume reduction. One example is Mothra Cave, which reduced from 797.4 to 189.8 m in surveyed length in one year (2022 to 2023). These observations are supported by decreasing fumarole temperatures in/near the cave systems located at the dome flanks (trending towards 0 °C) as well as decreasing fumarole temperatures at the dome summit (~380 °C in 2014/2015, ~90 °C in 2021; ~60 °C in 2024). Although ASTER data similarly revealed an overall cooling of the lava dome, field investigations enabled a more precise determination of the temperature distribution. As revealed by LiDAR data, the new 2004–2008 lava dome decreased in elevation more than 35 m from 2009 to 2019. In contrast, almost no elevation change was observed at the old 1980–1986 dome. The elevation loss is strongly related to initial dome growth structures. Drone images further characterized structures on the new dome, demonstrating correlations between fumarolic areas and substrate and morphology. Since lava dome degradation is associated with numerous hazards, information about their long-term development as well as the comparison between various parameters and their response time can contribute to hazard forecasts.