Understanding volcanic eruption triggers is critical towards anticipating future activity. While internal magma dynamics typically receive more attention, the influence of external processes remains less understood. In this context, we explore the relationship between seasonal snow cycles and eruptive activity at Ruapehu, New Zealand. This is motivated by apparent seasonality in the eruptive record, where a higher than expected proportion of eruptions (post-1960) occur in spring (including the two previous eruptions of 2006 and 2007). Employing recent advancements in passive seismic interferometry, we compute sub-surface seismic velocity changes between 2005–2009 using the cross-wavelet transform approach. Opposite trends in velocities are identified on and off the volcano, with stations closest to the summit recording a winter high closely correlated with the presence of snow. Inverting for depth suggests these changes occur within the upper 200–300 m. Reduced water infiltration (as precipitation falls as snow) is considered the likely control of seasonal velocities, while modeling also points to a contribution from snow-loading. We hypothesise that this latter process may play a crucial role towards explaining seasonality in the eruptive record. Specifically, loading/unloading may influence the volcanic system through increased degassing, thereby increasing the likelihood of small, gas-driven, eruptions. Our findings shed light on the complex interactions between volcanoes and external environmental processes, highlighting the need for more focused research in this area. Pursuing this line of inquiry has significant implications towards improved risk and hazard assessments at not just Ruapehu, but also other volcanoes globally that experience seasonal snow cover.
