Probing permeability and microstructure: Unravelling the role of a low-permeability dome on the explosivity of Merapi (Indonesia)

Abstract

International audienceLow permeability dome rocks may contribute to conduit overpressure development in volcanic systems, indirectly abetting explosive activity. The permeability of dome-forming rocks is primarily controlled by the volume, type (vesicles and/or microcracks), and connectivity of the void space present. Here we investigate the permeability-porosity relationship of dome-forming rocks and pumice clasts from Merapi’s 1888 to 2013 eruptions and assess their possible role in eruptive processes, with particular emphasis on the 2010 paroxysmal eruption. Rocks are divided into three simple field classifications common to all eruptions: Type 1 samples have low bulk density and are pumiceous in texture; Type 2 samples, ubiquitous to the 2010 eruption, are dark grey to black in hand sample and vary greatly in vesicularity; and Type 3 samples are weakly vesicular, light grey in hand sample, and are the only samples that contain cristobalite. Type 2 and Type 3 rocks are present in all eruptions and their permeability and porosity data define similar power law relationships, whereas data for Type 1 samples are clearly discontinuous from these trends. A compilation of permeability and porosity data for andesites and basaltic andesites with published values highlights two microstructural transitions that exert control on permeability, confirmed by modified Bayesian Information Criterion (BIC) analysis. Permeability is microcrack- and diktytaxitic-controlled at connected porosities, φc, < 10.5 vol.%; vesicle- and microcrack-controlled at 10.5 < φc < 31 vol.%; and likely vesicle-controlled for φc > 31 vol.%. Type 3 basaltic andesites, the least permeable of the measured samples and therefore the most likely to have originated in the uppermost low-permeability dome, are identified as relicts of terminal domes (the last dome extruded prior to quiescence). Cristobalite commonly found in the voids of Type 3 blocks may not contribute significantly to the reduction of the permeability of these samples, mainly because it is associated with an extensive microporous, diktytaxitic texture. Indeed, the low permeability of these rocks is more likely associated with their lower fracture density. We propose that diktytaxitic textures may arise from late-stage gas filter pressing of a silica-rich melt phase, which leaves behind a microlite-supported groundmass and cristobalite in neighbouring vesicles. Due to the ubiquity of the Type 3 rocks in all Merapi eruptions, we do not invoke the emplacement of a low-permeability cap as having favoured a particularly high pressurization and subsequent high explosivity of the 2010 eruption. The debate as to the reasons for the highly explosive 2010 eruption rages on

Flank instability assessment at Kick-’em-Jenny submarine volcano (Grenada, Lesser Antilles): a multidisciplinary approach using experiments and modeling

“…Alteration has been previously shown to weaken volcanic rocks (Pola et al 2014;Wyering et al 2014;Heap et al 2015b). Permeability measurements show that permeability increases as porosity increases, in accordance with other measurements on basaltic andesites and andesites (Farquharson et al 2015;Kushnir et al 2016;Heap and Kennedy 2016).…”

Inelastic compaction and permeability evolution in volcanic rock

“…These authors calculate an average porosity of 0.21 for this unit: a high value compared to the average edifice porosity of around 0.15 determined by Setiawan (2002) and the range of 0.05 to 0.10 estimated by Commer et al (2005) for the region directly below the Merapi summit. These values are generally consistent with measured laboratory values of porosity for Merapi samples (Le Pennec et al, 2001;Kushnir et al, 2016). Similar contrasts in density have been inferred from gravimetric studies of several other volcanic regions such as Mauna Loa, Hawai'i (Zucca et al, 1982), Campi Flegrei, Italy (Cubellis et al, 1995), PuyehueCordón Caulle, Chile (Sepúlveda et al, 2005), and in the Central Volcanic Complex of Tenerife, Spain (Gottsmann et al, 2008).…”

“…7b ≤ k ≤ 10 −13 m 2 ) might partially explain why the modal porosity of large datasets of edifice-forming material tends to fall between 0.10 and 0.20 (e.g. Mueller et al, 2011;Bernard et al, 2015;Farquharson et al, 2015;Lavallée et al, 2017).…”

Preexplosive conduit conditions during the 2010 eruption of Merapi volcano (Java, Indonesia)

“…The country rock was probably damaged, allowing the substantial outgassing that our data and the 5 × 10 5 t/d sulfur emissions recorded between 4 and 5 November [ Surono et al ., ] suggest. Our data also support the interpretation that the dome that grew prior to the main explosion was probably not large enough to maintain a fully closed system and associated large overpressure [ Komorowski et al ., ; Kushnir et al ., ].…”

“…The involvement of large amounts of carbonates and CO 2 was at first evoked as a possible cause of the 2010 explosivity [ Deegan et al ., ; Borisova et al ., ], but recent petrologic evidences concur that the role of carbonates was not abnormal [ Costa et al ., ; Erdmann et al ., ]. The cryptodome created degassing conditions that seem similar to those of the previous domes, which suggests that the upper part of the conduit did not sustain unusually high overpressure [ Kushnir et al ., ]. Remarkably high ascent rates are additional observations that set the 2010 eruption apart [ Costa et al ., ; Preece et al ., ; Pallister et al ., ].…”