Prolonged volcanic activity can induce surface weathering and hydrothermal alteration that is a primary control on edifice instability, posing a complex hazard with its challenges to accurately forecast and mitigate. This study uses a frequently active composite volcano, Mt Ruapehu, New Zealand, to develop a conceptual model of surface weathering and hydrothermal alteration applicable to long-lived composite volcanoes. The rock samples were classified as non-altered, supergene argillic alteration, intermediate argillic alteration, and advanced argillic alteration. The first two classes have a paragenesis that is consistent with surficial infiltration and circulation of the low-temperature (40 degree C) neutral to mildly acidic fluids, inducing chemical weathering and formation of weathering rims on rock surfaces. The intermediate and advanced argillic alterations are formed from hotter (100 degree C) hydrothermal fluids with lower pH, interacting with the andesitic to dacitic host rocks. The distribution of weathering and hydrothermal alteration has been mapped with airborne hyperspectral imaging through image classification, while aeromagnetic data inversion was used to map alteration to several hundred meters depth. The joint use of hyperspectral imaging complements the geophysical methods since it can numerically identify hydrothermal alteration style. This study established a conceptual model of hydrothermal alteration history of Mt Ruapehu, exemplifying a long-lived and nested active and ancient hydrothermal system. This study highlights the need to combine mineralogical information, geophysical techniques and remote sensing to distinguish between current and ancient hydrothermal and supergene alteration systems, to indicate the most likely areas of future debris avalanche initiation.

The rhyolite-producing Laguna del Maule volcanic field (LdMVF) records magma-induced surface inflation rates of ~ 25 cm/year since 2007. During the Holocene, ~60 meters of cumulative surface uplift is recorded by paleoshorelines of the Laguna del Maule, located on the southeast edge of the LdMVF (Chile-Argentina border) near the Barrancas volcanic complex. Rhyolites from the Barrancas complex erupted over ~14 ka including some of the youngest (1.4 ± 0.6 ka) lava flows in the field. New gravity data collected on the Barrancas complex reveals a Bouguer low (-6 mGal, Barrancas anomaly) that is distinct from the pronounced gravity low (-19 mGal; Lake anomaly) associated with present-day deformation and magma intrusion to the north. Three-dimensional inversion of the Barrancas anomaly indicates the presence of a magma body with a maximum density contrast of -250 kg/m3 centered at a depth of ~ 3 km below surface. Comparison of model densities with measured densities from nearby silicic plutons suggest that a magma body, containing < 30 % melt phase and a low volatile content, exists beneath the Barrancas complex. The Barrancas and Lake gravity lows represent magma in different physical states, associated with past and present-day storage beneath the LdMVF. The gravity model mirrors existing geochemical observations which independently indicate that at least two distinct rhyolites were generated and stored as discrete magma bodies within the broader LdMVF. Small temperature changes of these discrete bodies could reverse crystallization and viscous lock-up and propel magma toward a crystal-poor eruptible state.