Experimental insight into the role of hydrothermal alteration on the mechanical and microstructural properties of the volcanic rocks: The case of Tutupaca, Peru
Authors: Niclaes Jens , Detienne Marie , Heap Michael , Delmelle Pierre , Rattez Hadrien 
Published: February 2023

The growing structure of active volcanoes due to material addition can lead to oversteepening and overloading (McGuire, 2003). This situation can be worsened by seismic activity as most volcanoes are in seismically active areas. Moreover, the materials forming volcanic edifices are subjected to extreme conditions in terms of temperature, pore pressure (consequence of several combined factors) and chemically aggressive fluids (very low pH for example) which can all destabilize volcanoes&#8217; flanks. Among these factors, hydrothermal activity is of particular interest as it enhances rock dissolution (and thus, increases rock porosity), promotes high pore pressures and leads to the creation of mechanically weaker materials (like clay-rich rocks) and promote the instability (Rattez and Veveakis, 2020). However, the effects that these processes have on volcano stability have been barely quantified (Heap and Violay, 2021).To better understand the influence of different types of hydrothermal alteration on the hydraulic and mechanical properties of volcanic rocks, permeameter and triaxial experiments have been performed on samples retrieved by Detienne et al. (2016) from the Tutupaca volcano (17&#176; 01&#8242; S, 70&#176; 21&#8242; W). This volcano is a dacitic dome complex located at the southern end of the Peruvian arc. The study focuses on a remarkably well-preserved debris avalanche deposit emplaced to the northeast of the volcano. The debris avalanche is sourced to Eastern Tutupaca; it left a horseshoe-shaped crater open to the northeast and was accompanied by a pyroclastic flow (volume: 6.5-7.5 x 107 m3) (Samaniego et al., 2015). The mineralogy and the microstructure of the samples have been investigated using X-ray diffraction and micro-computed tomography respectively. Preliminary results exhibit a high variability of mineralogy, microstructures, and mechanical properties. It appears that the alteration degree may have more influence on the mechanical behavior of volcanic rocks than the porosity. This dataset could be further used in numerical models of flank collapses to better constrain the role of hydrothermal alteration on the nucleation of those events.&#160;ReferencesDetienne M. (2016)&#160;&#8220;Unravelling the role of hydrothermal alteration in volcanic flank and sector collapses using combined mineralogical, experimental, and numerical modelling studies&#8221;. PhD thesis, UCLouvain.Heap M. and Violay M. (2021) &#8220;The mechanical behaviour and failure modes of volcanic rocks: a review&#8221;, Bulletin of Volcanology, 83:33.&#160;Lipman PW, Mullineaux DR (eds) (1981) &#8220;The 1980 eruptions of Mount St Helens, Washington.&#8221; U.S. Geological Survey, Professional Paper 1250, 844 pp.McGuire WJ (2003) Volcano instability and lateral collapse. Revista 1:33&#8211;45.Rattez H, Veveakis M (2020). &#8220;Weak phases production and heat generation control fault friction during seismic slip&#8221;, Nature Communications, doi: 10.31223/osf.io/xupr8 Samaniego, P., Valderrama, P., Mari&#241;o, J., De Vries, B. V. W., Roche, O., Manrique, N., Ch&#233;deville, C., Liorzou, C., Fidel, L. & Malnati, J. (2015). &#8220;The historical (218&#177;14 aBP) explosive eruption of Tutupaca volcano (Southern Peru)&#160;&#187;. Bulletin of Volcanology, 77, 1-18.&#160;