Porosity, Strength, and Alteration – Volcano Stability Assessment Using VNIR-SWIR Reflectance Spectroscopy

Volcanic rocks are characterised by pore and crack structures that can govern hydrothermal fluid transport, volcanic outgassing, and the strength and stability of a rock mass. Volcano slope stability analysis is a critical component of volcanic hazard assessments and monitoring. Here, visible to shortwave infrared (350–2500 nm; VNIR–SWIR) reflected light spectroscopy on laboratory-tested rock samples from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria) and La Soufrière de Guadeloupe (Eastern Caribbean) volcanoes was used to design a novel rapid chemometric-based method to estimate uniaxial compressive strength (UCS) and porosity.  

Our Partial Least Squares Regression models return moderate accuracies for both UCS and porosity, with R2 of 0.427–0.493 and Mean Absolute Percentage Error (MAPE) of 0.212–0.391. When laboratory-measured porosity is included with spectral data, UCS prediction reaches an R2 of 0.82 and MAPE of 0.105. Our models highlight that the observed changes in the UCS are coupled with subtle mineralogical changes due to hydrothermal alteration at wavelengths of 360–438, 532–597, 1405–1455, 2179–2272, and 2460–2490 nm.  

Hydrothermal alteration can reduce the strength of the volcanic rocks by replacing strong volcanic glass and primary minerals in the groundmass with weaker secondary minerals, including sulfates and phyllosilicates, but it can also increase strength because of porosity-filling precipitation and/or silicification.  

Our approach highlights that spectroscopy can provide a first order assessment of rock strength and/or porosity or be used to complement laboratory porosity-based predictive models. VNIR-SWIR spectroscopy therefore provides an accurate non-destructive way of assessing rock strength and alteration mineralogy, even from remote sensing platforms.