Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones

Herlan Darmawan,Daniel Müller,Chris Harris,Valentin R Troll,Frances M Deegan,Nadhirah Seraphine,Thomas R Walter,Michael J Heap,Hanik Humaida,Harri Geiger

doi:10.1038/s41598-022-06765-9

Abstract

Catastrophic lava dome collapse is considered an unpredictable volcanic hazard because the physical properties, stress conditions, and internal structure of lava domes are not well understood and can change rapidly through time. To explain the locations of dome instabilities at Merapi volcano, Indonesia, we combined geochemical and mineralogical analyses, rock physical property measurements, drone-based photogrammetry, and geoinformatics. We show that a horseshoe-shaped alteration zone that formed in 2014 was subsequently buried by renewed lava extrusion in 2018. Drone data, as well as geomechanical, mineralogical, and oxygen isotope data suggest that this zone is characterized by high-porosity hydrothermally altered materials that are mechanically weak. We additionally show that the new lava dome is currently collapsing along this now-hidden weak alteration zone, highlighting that a detailed understanding of dome architecture, made possible using the monitoring techniques employed here, is essential for assessing hazards associated with dome and edifice failure at volcanoes worldwide.

Highlights

Catastrophic lava dome collapse is considered an unpredictable volcanic hazard because the physical properties, stress conditions, and internal structure of lava domes are not well understood and can change rapidly through time
Processes observed at Merapi and the frequency of hydrothermal alteration recorded in clasts within its deposits suggest that hydrothermal activity plays a role in short-lived pressure build up at Merapi-type dome-building volcanoes, and that it may be the crucial factor in causing progressive destabilization and subsequent large-scale gravitational dome collapses over somewhat longer time scales
Quantifying the degree of hydrothermal alteration and identifying ore and mineral deposits through alteration mapping has been a main motivation for employing imaging remote sensing techniques at active volcanoes for several d ecades[35,36]

Summary

Introduction

Catastrophic lava dome collapse is considered an unpredictable volcanic hazard because the physical properties, stress conditions, and internal structure of lava domes are not well understood and can change rapidly through time. As direct rock sampling and analysis is frequently challenging at active lava domes, there is a need for a more integrated (crossmethod) approach for improving our understanding of the structural, chemical, and mineralogical processes at work This is especially true with respect to buried (or hidden) zones of structural weaknesses which, if addressed, could help to substantially advance our predictive capability of catastrophic dome failures. While gravitational instabilities operate on a timescale of weeks to months, recent work on altered Merapi dome samples postulated that the precipitation of secondary minerals can temporarily reduce dome permeability due to hydrothermal alteration (e.g. hours to days)[27] This process could seal outgassing channels and promote a temporary build-up of gas pressure under the dome, which could be released in small-scale erratic e xplosions[27]. We show that currently ongoing instability of the Merapi dome occurs along previously altered structures that were buried by renewed dome extrusions and act as mechanical weaknesses within the Merapi dome complex

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Conclusion