Furthering the Investigation of Eruption Styles Through Quantitative Shape Analyses of Volcanic Ash Particles

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Furthering the investigation of eruption styles through quantitative shape analyses of volcanic ash particles

Nurfiani, Dini; Bouvet de Maisonneuve, Caroline

2018

Nurfiani, D., & Bouvet de Maisonneuve, C. (2018). Furthering the investigation of eruption styles through quantitative shape analyses of volcanic ash particles. Journal of Volcanology and Geothermal Research, 354102‑114. doi: https://hdl.handle.net/10356/88325 https://doi.org/10.1016/j.jvolgeores.2017.12.001

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Furthering the investigation of eruption styles through quantitative shape analyses of volcanic ash particles

D. Nurﬁani ⁎, C. Bouvet de Maisonneuve

Earth Observatory of Singapore, Nanyang Technological University, 639798, Singapore article info abstract

Article history: Volcanic ash morphology has been quantitatively investigated for various aims such as studying the settling Received 21 June 2017 velocity of ash for modelling purposes and understanding the fragmentation processes at the origin of explosive Received in revised form 30 November 2017 eruptions. In an attempt to investigate the usefulness of ash morphometry for monitoring purposes, we analyzed Accepted 1 December 2017 the shape of volcanic ash particles through a combination of (1) traditional shape descriptors such as solidity, Available online 5 December 2017 convexity, axial ratio and form factor and (2) fractal analysis using the Euclidean Distance transform (EDT) Keywords: method. We compare ash samples from the hydrothermal eruptions of Iwodake (Japan) in 2013, Tangkuban Volcanic ash Perahu (Indonesia) in 2013 and Marapi (Sumatra, Indonesia) in 2015, the dome explosions of Merapi (Java, shape Indonesia) in 2013, the Vulcanian eruptions of Merapi in 2010 and Tavurvur (Rabaul, Papaua New Guinea) in fractal analysis 2014, and the Plinian eruption of Kelud (Indonesia) in 2014. Particle size and shape measurements were acquired magma fragmentation from a Particle Size Analyzer with a microscope camera attached to the instrument. Clear differences between eruption style dense/blocky particles from hydrothermal or dome explosions and vesicular particles produced by the fragmentation of gas-bearing molten magma are well highlighted by conventional shape descriptors and the fractal method. In addition, subtle differences between dense/blocky particles produced by hydrothermal explosions, dome explosions, or quench granulation during phreatomagmatic eruptions can be evidenced with the fractal method. The combination of shape descriptors and fractal analysis is therefore potentially able to distinguish between juvenile and non-juvenile magma, which is of importance for eruption monitoring. © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction The shape of volcanic particles has long been recognized to reflect numerous eruptive parameters such as magma viscosity, volatile The use of ash characterization for eruption monitoring is a growing content, degree of interaction with external water, particle transport field of study. This involves investigating ash componentry, color or and sedimentation (e.g. Heiken, 1972; Wohletz, 1983; Heiken and luminance, geochemistry, size and shape (Taddeucci et al., 2002; Cioni Wohletz, 1985; Dellino and La Volpe, 1996; Zimanowski et al., 1997; et al., 2008; Yamanoi et al., 2008; Miwa et al., 2015; Gaunt et al., 2016; Carey et al., 2000; Dellino and Liotino, 2002; Maria and Carey, 2002; Pompilio et al., 2017). As it provides a direct sampling of the internal Riley et al., 2003; Ersoy et al., 2007; Maria and Carey, 2007; Jordan volcanic system, the continuous study of ash can give timely insights et al., 2014; Rausch et al., 2015; Pompilio et al., 2017). High-viscosity on the processes that are generating the eruptive activity. Through a magma will fragment in a brittle manner due to hindered bubble time series analysis, the evolution of the eruptive activity can be better growth and thus bubble overpressure exceeding the magma's tensile predicted, especially in conjunction with other monitoring signals. strength. Low-viscosity magma fragmentation on the other hand will Periods of eruption intensification or waning and the ending of an occur by fluid-dynamic breakup because of the large inertial forces eruption can potentially be forecasted (e.g. Taddeucci et al., 2002). In related to the very rapidly expanding bubbles. The rheology of the addition, the timely recognition of juvenile magma in early phreatic magma will be affected by the chemistry of the melt, the amount of eruptions is essential for distinguishing between low-level volcanic microlites and the abundance of volatiles, all of which will vary during restlessness and precursory activity to a large-scale magmatic eruption magma ascent and eruption (see Gonnermann, 2015 for a review). In (e.g. at Mount Saint Helens in the USA, Cashman and Hoblitt (2004), addition, interaction with an external body of water may promote Rowe et al. (2008); Mount Unzen in Japan, Watanabe et al. (1999); magma fragmentation due to heat exchange, water vaporization and Kirishima volcano in Japan, Suzuki et al. (2013)). partial quenching of the melt (Gonnermann, 2015).The sizes and shapes of ash particles will thus reflect the magma properties (abundance ⁎ Corresponding author. and sizes of phenocrysts, microlites, and vesicles), fragmentation E-mail address: [email protected] (D. Nurfiani). mechanism, and intensity.

https://doi.org/10.1016/j.jvolgeores.2017.12.001 0377-0273/© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). D. Nurﬁani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 103

Objective and quantitative characterizations of ash particle shapes In this study, we test whether the analysis of ash particle shapes and started with simple non-dimensional parameters such as circularity, sizes can be used to (1) discriminate between different styles of activity compactness, elongation, and rectangularity (Dellino and La Volpe, independently of the source volcano and magma composition, and 1996) that have evolved and been refined through time (see Liu et al., (2) provide quantitative insights on the presence of juvenile material 2015a and references therein). However, more than one parameter is that would presage a larger eruption in the near future. We use a meth- really needed to allow reliable comparisons between particles since odology that is both “cheap” and requires little to no sample preparation most of the traditional shape descriptors are relevant for a specific in order to evaluate a possible implementation in volcano observatories scale only. The fractal method, on the other hand, measures the self- as a routine monitoring system (e.g. Leibrandt and Le Pennec, 2015). similarity of a shape at different scales (Mandelbrot, 1967). It has thus We investigate ash from seven eruptions of Asian volcanoes that span been proposed as another way to objectively and quantitatively charac- a range in eruption styles from low-level phreatic (hydrothermal) terize the morphology of an ash particle outline (Carey et al., 2000)and explosions to dome explosions, Vulcanian and Plinian eruptions. We has proven to be very efficient (e.g. Dellino and Liotino, 2002; Maria and find that both traditional shape descriptors and the fractal method Carey, 2002; Németh, 2010; Rausch et al., 2015). Both the traditional provide robust criteria to distinguish between eruption styles and im- shape descriptors and the fractal method have their advantages and portant insights on the magma fragmentation process can be gleaned disadvantages, but more importantly both methods use the large scale from the particle shape analysis. morphological roughness and the fine scale textural roughness of a particle outline to fully characterize it. 2. Samples and eruptions Most investigations of ash morphology are targeted at unravelling the origin of ash and the related magma fragmentation dynamics We analyzed ash from seven eruptions of SE Asian & Asia-Pacificvol- (Walker and Croasdale, 1971; Heiken, 1972; Wohletz, 1983; Fisher canoes (Fig. 1) that occurred between 2010 and 2015. A short descrip- and Schmincke, 1984; Heiken and Wohletz, 1985, 1991; Zimanowski tion of the eruptions and sampling locations are given below. In order et al., 1997; Carey et al., 2000; Cioni et al., 2008; Lautze et al., 2012), par- to distinguish between (i) phreatic eruptions that are the product of ticularly to distinguish between purely magmatic and phreatomagmatic magma interaction with external, non-hydrothermal water such as fragmentation i.e. interaction of magma with external water such as groundwater, seawater, or surface water, and (ii) phreatic explosions groundwater, seawater, surface water, etc. (Dellino and La Volpe, that originate from perturbations in the volcano's hydrothermal system 1996; Büttner et al., 1999; Büttner et al., 2002; Dellino and Liotino, and are characteristic of low-level volcanic restlessness, we refer to the 2002; Maria and Carey, 2002; Ersoy et al., 2007; Németh, 2010; former as phreatomagmatic eruptions and to the latter as hydrothermal Dellino et al., 2012; Murtagh and White, 2013; Jordan et al., 2014; explosions. There is no assumption on the presence or absence of Pardo et al., 2014; Liu et al., 2015a; Rausch et al., 2015). Some studies magma involvement in either type of activity. A more in-depth discus- are targeted at the processes of ash dispersal and its settling properties sion about the terminology “phreatic” can be found in Barberi et al. (Riley et al., 2003; Durant et al., 2009; Mele et al., 2011; Dellino et al., (1992). 2012; Klawonn et al., 2014), as these have implications for aviation, human health, infrastructures and agriculture. However, few or no stud- 2.1. Hydrothermal explosions: Iwodake (2013), Tangkuban Perahu (2013), ies () have investigated ash produced during small scale phreatic (hy- Marapi (2015) drothermal) explosions that are typical of low-level volcanic restlessness and in some cases may be precursors of impending, larger Iwodake is a rhyolitic lava dome at the eastern end of the island scale magmatic activity. of Satsuma Iwojima (or Tokara-Iwojima), on the northern rim of the

Fig. 1. Location map of the volcanoes that produced the eruptions and ash analyzed. 104 D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 underwater Kikai caldera (Ryukyu Islands south of Kyushu, Japan; 2.2. Dome explosions: Merapi (2013) Fig. 1). Mild-to-moderate explosive eruptions have occurred from this vent during the past few decades. On 4 June 2013, intermittent Merapi is a tall stratovolcano near Yogyakarta (Java) and is one of explosions occurred between ~05:17 and 15:00 local time (2GVP, Indonesia's most active volcanoes. Its eruptive history is characterized 2014). An ash plume drifted west and deposited ash in the village of by numerous repeated events of lava dome growth and subsequent Mishima, ~3 km WSW of the summit. Some of this ash was shared collapse into block-and-ash-flows (Simkin and Siebert, 1994). It has during a post-conference field trip to the Kikai caldera following the become a type locality for such eruptive behavior (Costa et al., 2013). 2013 IAVCEI meeting held in Kagoshima (July 20–24). The eruption Since the major eruption in October–November 2010, a small lava was classified with a Volcanic Explosivity Index (VEI) of 1 (Newhall dome has been occupying the summit crater. No lava dome growth and Self, 1982). has occurred since 2010. In 2013, Merapi produced a few minor to mod- Tangkuban Perahu is a shield-like stratovolcano located near the city erate explosions that have left a fracture / fissure zone in the summit of Bandung on Java, Indonesia. Historically, it only produced small dome. On 22 July between 04:15 and 05:30 local time a dense black hydrothermal eruptions recorded since the 19th century. They have plume rose 1 km and deposited ash on the southern flank (2GVP, originated from several nested craters within an elliptical summit 2013). Ash from this explosion was sampled in two different localities; caldera. Between February 21st and March 6th, 2013, the Ratu crater pro- sample (1) was collected at the summit of the volcano on the same day duced a series of very small explosions with plume heights of b100–500 m of the explosion and sample (2) was collected about 3 km SE of the vent (1GVP, 2013). Ash was collected from the summit area by staff from the one month after the event. On 18 November between 04:53 and 10:00 Centre for Volcanology and Geological Hazard Mitigation (CVGHM). local time another eruption generated an ash plume that rose to an ele- Marapi is one of the most active volcanoes in Sumatra, Indonesia. vation of 12.2 km a.s.l. and deposited ash as far as 60 km E (3GVP, 2014). More than stratovolcano with a series of four craters aligned in an Ash from this explosion was sampled at the volcano summit by CVGHM ENE-WSW direction. Eruptions typically consist of small-to-moderate four days after the explosion. explosive activity. N50 events have been recorded since the end of the 18th century. On 14 November 2015 at 22:33 local time, a phreatic ex- 2.3. Vulcanian eruptions: Merapi (2010), Rabaul - Tavurvur (2014) plosion (VEI 1) generated an ash plume (GVP, 2016). Ash was collected by CVGHM staff ~5 km from the summit on the SW flank, in the direc- Merapi (Java) produced an unusually large (VEI 4) eruption in tion of prevailing winds. October–November 2010. The eruption began on 26 October with an

a b

Fig. 2. Processing steps for the fractal analysis using the simplified Bouligand-Minkowski method and the fractal spectrum technique. The image of the original particle (a) is processed to i obtain a binary image of the outline (b), which is then dilated isotropically to produce a Euclidean Distance Map (c) that is thresholded at distance intervals of εi =2, where i is the iteration number (from 1 to 5). (d) An equivalent of the Richardson plot is obtained by plotting the area (A(εi)) of the dilated boundary as a function of the width (εi) at every iteration. (e) The multi-fractal spectrum is obtained by calculating the fractal dimension (FD = 2 – slope) for each line segment in (d). D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 105 ash plume that rose to 12 km a.s.l. and pyroclastic flows that reached 8 km and 1 and that have been argued to be the most effective (Liu et al., from the summit. This initial explosion was followed by smaller explosive 2015a): Solidity (S = Ap /ACH), Convexity (C = PCH /Pp), Axial Ratio 2 eruptions on 29 and 31 October and 1 November, and then by 4 days of ex- (AxlR = B / A), and the Form factor (FF = 4πAp /Pp). Solidity is a measure tremely rapid dome growth (N25 m3/s; Pallister et al., 2013)thatledupto of the larger scale morphological roughness, convexity is a measure of the the climactic eruption on the night of 4–5 November. The 5 November fine scale textural roughness, and axial ratio is a measure of the form. The eruption produced a plume that rose to 17 km a.s.l. and pyroclastic form factor is sensitive to both form and roughness. flows that extended 16 km down the southern flank. Ash from this This analytical procedure has a number of advantages. First of all, it event was collected in Yogyakarta. requires no sample preparation. Samples are measured in water (and Tavurvur is a small cone on the eastern rim of the Rabaul caldera gently ultrasonicated), which enables maximum dispersion and avoids (Papua New Guinea). It has been very frequently active since 1994, finer particles from attaching to larger ones. This is equivalent to the dominantly producing Strombolian and Vulcanian explosions accompa- washing step of Leibrandt and Le Pennec (2015) but does not require nied by lava flows (Bouvet de Maisonneuve et al., 2015). On 29 August an independent task in our workflow. The image and data acquisition 2014, Tavurvur produced a violent Strombolian eruption with an ash is semi-automated, which enables the full process to be quick. It takes plume that rose to 18 km a.s.l. and drifted west. Ash was deposited up about 1–2 h to complete the full process for about 20 fully reliable to 14 km from the summit in the direction of prevailing winds and particle images for one sample. Image acquisition is the slowest part, severely damaged the vegetation. Ash was collected ~4 km from the as particles need to be individually photographed for the analysis. We summit, near the town of Rabaul, by staff from the Rabaul Volcano focus here on glass shards to identify fragmentation mechanisms, and Observatory (RVO). exclude loose crystals. The alternative is to take a large number of images and then throw the inadequate ones away (similar to the method 2.4. Plinian eruption: Kelud (2014) of Leibrandt and Le Pennec, 2015). Either way is a bit time consuming. Although the use of a single grain size fraction has been advocated by Kelud is a stratovolcano in eastern Java that has dominantly been erupting explosively in its history (Hidayati et al., 2009), except in 1.0 2007 when the eruption ended with the formation of a lava dome. a Seven years later, in 2014, the volcano returned to its explosive character by destroying the lava dome. The event was preceded by an increas- 0.9 ing number of shallow and deep volcanic earthquakes causing CVGHM to raise the alert level to 4 on 13 February at 21:15 local time. Less 0.8 than two hours later at 22:50 a VEI 4 eruption ejected a plume that – 1 rose up to 20 26 km a.s.l. ( GVP, 2014). The ash mostly drifted to the 0.7 west where it was collected in Solo (~170 km WNW) and Dieng (~260 km WNW) by CVGHM staff. 0.6 Convexity 3. Methods 0.5 3.1. Particle size and shape analyzer 0.4 The size and shape of ash particles were measured simultaneously with a CILAS 1190L Particle Size and Shape Analyzer at the Earth Obser- 0.3 vatory of Singapore, Nanyang Technological University. Measurements 0.0 0.2 0.4 0.6 0.8 1.0 were done in liquid mode with pure water as the carrier phase. A Solidity maximum of 1 g of ash was introduced in the sample tank, stirred and ultrasonicated for 60 s before being circulated three times through the system. The particle size distribution was measured from the light 1.0 diffraction of 3 lasers using the Fraunhofer Diffraction technique. b The method accurately resolves particles with sizes ranging between 0.04 μm and 2500 μm. Particle size distributions are reported as frequency histograms with particle sizes on a logarithmic scale. In addition, 0.9 we report the diameter at 10, 50, and 90 percentiles of the distribution. Particles were then circulated under an optical microscope equipped with a CCD camera and (high resolution) images were taken. We im- 0.8 aged 17–25 particles per sample (Table 1 in Supplementary material). These were randomly selected, although free crystals were avoided.

Various magnifications can be chosen for the best particle resolution Axial Ratio 0.7 (4×, 10×, 20×, and 40×). We used only the 10× magnification to obtain a consistent and comparable dataset at a magnification that best suited all our samples. Particles analyzed for their shape range from 27 to 262 μmin 0.6 diameter. The image resolution at 10× magnification is of 0.6 μm/pix and images are 752 × 480 pixels in size. Particle shapes were analyzed from the apparent (2D-)projected shape (equivalent to the APASH of 0.5 Leibrandt and Le Pennec, 2015). After image thresholding, the area of 0.0 0.2 0.4 0.6 0.8 1.0 the particle (Ap), area of the convex hull (ACH), perimeter of the particle Form Factor (Pp), perimeter of the convex hull (PCH), and major and minor axes of the best fit ellipse (A and B respectively) were measured using the Fig. 3. Conventional shape descriptors, (a) convexity versus solidity and (b) axial ratio instrument's shape analysis software. To describe the particles, we focus versus form factor, for synthetic shapes that resemble those of Liu et al. (2015a). Note on four conventional shape descriptors, which values vary between 0 that all the ash particles studied here have values of solidity and convexity N 0.6. 106 D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 several authors: 0ϕ (b1mm;Cioni et al., 2014), 1ϕ (500 μm–1mm; influence the size range of the ash sample studied. To maximize the Cioni et al., 2008), 1.5ϕ (350–500 μm; Miwa et al., 2013), 2ϕ number of ash samples that can be studied and to avoid excluding infor- (250–500 μm; Maria and Carey, 2007; Leibrandt and Le Pennec, 2015; mation from smaller or larger particles, we decided to consider the bulk Miwa et al., 2015), or 4ϕ (63–125 μm; Dellino and La Volpe, 1996; sample. This can be an issue for the comparison of traditional shape Dellino et al., 2012; Murtagh and White, 2013), we did not sieve our descriptors, as they are adimensional and therefore particles of very samples and therefore consider a relatively large range in particle different sizes cannot be investigated together. But it should not be an sizes. The reason behind is that every eruption will produce ash with a issue for the fractal analysis of particle boundaries as the dimensions slightly different size range, and the location of ash collection will also of morphological features are inherent to the fractal dimension.

Table 1 Fractal analysis of synthetic shapes using the Euclidean Distance Map (EDM) method, and the effect of image resolution (235 vs 300 dpi) and particle rotation on the fractal value (FV).

Label Synthetic shape (no rotation) Euclidean Distance Map + dilation (isotropic dilation)

FV – 235 dpi % relative to 0° FV – 300 dpi % relative to 0

0° 30° 45° 90° 30° 45° 90° 0° 30° 45° 90° 30° 45° 90°

SS1 1.077 1.06 1.024 1.077 1.6% 4.9% 0.0% 1.076 1.059 1.022 1.076 1.6% 5.0% 0.0%

SS2 1.077 1.068 1.042 1.077 0.8% 3.2% 0.0% 1.075 1.067 1.039 1.075 0.7% 3.3% 0.0%

SS3 1.082 1.08 1.061 1.082 0.2% 1.9% 0.0% 1.077 1.076 1.056 1.077 0.1% 1.9% 0.0%

SS4 1.132 1.142 1.134 1.132 0.9% 0.2% 0.0% 1.118 1.13 1.123 1.118 1.1% 0.4% 0.0%

SS5 1.148 1.155 1.148 1.148 0.6% 0.0% 0.0% 1.137 1.148 1.142 1.137 1.0% 0.4% 0.0%

SS6 1.062 1.062 1.062 1.062 0.0% 0.0% 0.0% 1.062 1.062 1.062 1.062 0.0% 0.0% 0.0%

SS7 1.071 1.085 1.083 1.072 1.3% 1.1% 0.1% 1.066 1.084 1.08 1.066 1.7% 1.3% 0.0%

SS8 1.076 1.095 1.094 1.076 1.8% 1.7% 0.0% 1.068 1.089 1.09 1.068 2.0% 2.0% 0.0%

SS9 1.121 1.133 1.126 1.121 1.1% 0.4% 0.0% 1.108 1.12 1.116 1.108 1.1% 0.7% 0.0%

SS10 1.151 1.16 1.153 1.151 0.8% 0.2% 0.0% 1.141 1.144 1.143 1.141 0.3% 0.2% 0.0%

SS11 1.08 1.07 1.043 1.08 0.9% 3.4% 0.0% 1.075 1.067 1.039 1.075 0.7% 3.3% 0.0%

SS12 1.109 1.102 1.076 1.11 0.6% 3.0% 0.1% 1.103 1.096 1.069 1.103 0.6% 3.1% 0.0%

SS13 1.15 1.145 1.121 1.15 0.4% 2.5% 0.0% 1.137 1.13 1.107 1.137 0.6% 2.6% 0.0%

SS14 1.068 1.078 1.073 1.068 0.9% 0.5% 0.0% 1.062 1.076 1.071 1.062 1.3% 0.8% 0.0%

SS15 1.084 1.094 1.087 1.084 0.9% 0.3% 0.0% 1.077 1.088 1.082 1.077 1.0% 0.5% 0.0%

SS16 1.131 1.139 1.131 1.131 0.7% 0.0% 0.0% 1.109 1.12 1.112 1.109 1.0% 0.3% 0.0%

SS17 1.185 1.198 1.193 1.186 1.1% 0.7% 0.1% 1.162 1.176 1.171 1.162 1.2% 0.8% 0.0%

SS18 1.215 1.222 1.21 1.215 0.6% 0.4% 0.0% 1.191 1.2 1.189 1.191 0.7% 0.2% 0.0%

SS19 1.226 1.231 1.217 1.226 0.4% 0.7% 0.0% 1.203 1.209 1.195 1.203 0.5% 0.7% 0.0%

SS20 1.124 1.122 1.102 1.124 0.2% 2.0% 0.0% 1.098 1.088 1.072 1.098 0.9% 2.3% 0.0%

SS21 1.121 1.134 1.126 1.121 1.2% 0.4% 0.0% 1.09 1.09 1.099 1.09 0.0% 0.8% 0.0%

SS22 1.229 1.228 1.212 1.229 0.1% 1.4% 0.0% 1.182 1.174 1.165 1.182 0.7% 1.4% 0.0%

SS23 1.254 1.264 1.258 1.254 0.8% 0.3% 0.0% 1.207 1.206 1.205 1.207 0.1% 0.2% 0.0% D. Nurﬁani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 107

3.2. Fractal and multivariate analysis components are computed by multiplying the matrix of score coefficients with the standardized data. The apparent (2D-)projected shape of each ash particle was further investigated with fractal analysis, following a procedure very similar to 3.3. Synthetic shapes that of Carey et al. (2000) and Maria and Carey (2002). Each image (particle) is larger than 100 × 100 pixels in size and most images are We created a set of 23 synthetic shapes that resemble those of Liu significantly larger than that. We used a Euclidean Distance Map et al. (2015a) and that were analyzed in the same way as the volcanic (EDM) method called the simplified Bouligand-Minkowski method ash. The purpose is to provide a framework for the interpretation of ash (sBMM; Luppe, 2015), which is similar to the dilation method of shapes (Fig. 3). It also enables to validate the outcomes of the fractal Maria and Carey (2002) and also less sensitive to particle orientation analysis. The shapes are of two resolution categories: 235 dpi, i.e. ∼250 × than other fractal analysis methods (e.g. the caliper and box-counting 250 pixels and 300 dpi, i.e. ~ 320 × 320 pixels. For each category, the orig- methods). The EDM and dilation methods, although slightly different inal shapes were rotated by 0°, 30°, 45°, and 90° to investigate the effect of (see Bérubé and Jébrak, 1999), have been shown to yield fractal values particle orientation. Rotating the particle affects the fractal value by 1% on that are within 1% mean square error (Luppe, 2015). The sBMM dilates average, and by ≤5% in all cases. This value does not depend on image the particle boundary through Euclidean Distance Transform (EDT), resolution (Table 1). meaning that each pixel stores in a binary image a value that corresponds to its distance from the nearest point of the particle outline. 3.4. SEM images The Distance Map (DM) generated in this process is thresholded from i the particle boundary at distance intervals of εi =2,wherei is the iter- Ash grains were mounted on double-sided carbon tape and gold ation number, to avoid a high concentration of points at one end of the coated for imaging using a JEOL JSM-7800F field emission gun Scanning “Richardson plot” and a bias in the slope (Luppe, 2015). At every itera- Electron Microscope (SEM) at the Asian School of the Environment, tion, the width (εi)andthearea(A(εi)) of the dilated boundary are com- Nanyang Technological University. Secondary electron images were puted (Fig. 2). These values are plotted on a double logarithmic scale, taken at 3 kV and 3 nA in order to minimize charging. These images similarly to a Richardson plot. A purely fractal material would yield a were used to corroborate observations and interpretations based on straight line with a unique fractal dimension (FD) that is proportional the particle shape analysis. to the slope (FD = 2-slope). As volcanic ash is not purely fractal, we compute a spectrum of values by converting the local slope into fractal 4. Results dimensions, following the scheme of Maria and Carey (2002). We used multi-variate analysis to translate the multi-fractal dimen- 4.1. Particle size sion of ash particles into two components using principal component analysis (PCA). All the variables (i.e. fractal dimensions) are standard- The ash samples from Tangkuban Perahu, Marapi, Merapi in 2010 and ized and the correlation matrix is computed. The resultant component in November 2013, Rabaul, and from Kelud collected in Solo (170 km loadings (that correlate the original variables with the components) away) have similar sizes with median diameters of 30.9, 54.9, 61.8, 88.2, were rotated using the Varimax option (Harman, 1976 -seeMaria 44.9, and 33.7 μmrespectively(Fig. 4). The ash sample from Iwodake is and Carey, 2002) to produce a matrix of score coefficients. Principle slightly coarser with a median diameter of 142.6 μm, while the samples

Iwodake Merapi - 2013 10 Jul-1 Jul-2 Nov d10% : 38.5 µm July -2 d10% : 17.4; 18.3; 12.8 µm 8 d50% : 142.6 µm July -1 d50% : 627.7; 536.2; 88.2 µm 6 d90% : 339.3 µm d90% : 1351; 887.1; 241.5 µm 4 November

Frequency (%) Frequency 2 0 Tangkuban Perahu Merapi - 2010 10 d10% : 9.9 µm d10% : 11.3 µm 8 d50% : 30.9 µm d50% : 61.8 µm 6 d90% : 167.7 µm d90% : 207.6 µm Solo; Dieng 4 d10% : 5.8; 13.8 µm

Frequency (%) Frequency 2 d50% : 33.7; 550.7 µm d90% : 184.8; 1165 µm 0 Marapi Rabaul Kelud 10 8 d10% : 9.6 µm d10% : 12.2 µm d50% : 54.9 µm d50% : 44.9 µm Dieng 6 d90% : 237.1 µm d90% : 263.9 µm Solo 4

Frequency (%) Frequency 2 0 0.010.1 1 10 100 1000 0.1 1 10 100 1000 0.1 1 10 100 1000 Particle Diameter (µm) Particle Diameter (µm) Particle Diameter (µm)

Fig. 4. Size distribution histograms of ash samples and particle diameters at 10%, 50%, and 90% of the distribution. The gray box shows the grain size fraction of 63–125 μmrecommended for study by Dellino and La Volpe (1996) and commonly referred to in the literature. This grain size fraction corresponds to a very small fraction of ash in all our samples and would not be ideal for most samples. 108 D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 from Merapi in July and August 2013, and from Kelud collected in Dieng regular in shape with even higher solidity and convexity values around (260 km away) are significantly coarser with median diameters of 0.90–0.99 and 0.94–1 respectively. Their textural roughness measured 627.7, 536.2, 550.7 μm. These three coarser ash samples also show a by convexity is particularly low, meaning that the particles have very bimodal size distribution with two completely distinct peaks around smooth and straight outlines. Ash samples from the Vulcanian and Plinian 20–30 μm and 700–800 μm. The peak at smaller particle diameters eruptions of Merapi in 2010, Rabaul, and Kelud show the highest variabil- corresponds to loose glass shards, while the other corresponds to crystal ity and lowest values of solidity and convexity around 0.66–0.94 and fragments and/or aggregated glass shards and crystals. In this study we 0.76–0.96 respectively. No difference can be observed between the focused on the loose glass shards only (20–30 μm in diameter). The Kelud samples collected in Dieng or Solo. These high levels of morpholog- median grain sizes of our samples are similar to or slightly finer than ical and textural roughness are due to the presence of truncated vesicles the grain size fraction that has been studied in previous works (63 μm walls in the glass shards. to 1 mm; Dellino and La Volpe, 1996; Maria and Carey, 2007; Cioni The same segregation between ash samples from different eruption et al., 2008; Dellino et al., 2012; Miwa et al., 2013; Murtagh and White, styles can be seen on a plot of axial ratio versus form factor (Fig. 5b). The 2013; Cioni et al., 2014; Leibrandt and Le Pennec, 2015; Miwa et al., 2015). ash from the 2013 Merapi dome explosions has rather high axial ratios (0.47–0.87) and form factors (0.62–0.86), while the ash from the hydro- 4.2. Particle shapes thermal explosions has similar axial ratios (0.44–0.85) but slightly lower form factors (0.56–0.80), and the ash from the Vulcanian and Ash samples from the hydrothermal eruptions of Iwodake, Tangkuban Plinian eruptions has the lowest axial ratios (0.29–0.76) and form fac- Perahu, and Marapi show very high values of solidity and convexity tors (0.24–0.67). Particles produced from dome explosions have very around 0.88–0.97 and 0.88–0.98 respectively (Fig. 5a). They are quite regular and relatively equant shapes, while particles from more vio- blocky and display little morphological or textural roughness. Ash lently explosive eruptions have more convolute and elongated shapes, samples from the dome explosions of Merapi in 2013 are even more again due to the presence of occasionally tubular vesicles. The fractal and multivariate analysis of ash shapes is equally efficient to discriminate different ash morphologies and consequently at differ- entiating eruption styles (Fig. 6). Ash from the Merapi dome explosions fi − – 1.0 have low values of the rst principal component ( 1.29 0.42) but a show a range in values for the second principal component (−4.09– 1.49). Ash from the hydrothermal explosions have slightly higher values of the first principal component (−1.12–0.38) and a similar range in 0.9 values for the second principal component (−2.18–2.18). Ash from the Vulcanian and Plinian eruptions have a larger range and higher values of the first principal component (−0.64–5.69) and slightly − – 0.8 higher values for the second principal component ( 1.59 3.84). No difference can be seen between the Kelud ash collected in Dieng or

Convexity Solo. By comparing the morphology of particles and their principal component values, we see that the ﬁrst principal component is a good 0.7 measure of the large-scale morphological roughness while the second Merapi - 2010 Iwodake Merapi - Jul 2013 (1) ﬁ Rabaul principal component is a measure of the ne-scale textural roughness Tangkuban Perahu Merapi - Jul 2013 (2) Kelud (Dieng) of a particle boundary. As for the traditional shape descriptors, there is Marapi (Sumatra) Merapi - Nov 2013 Kelud (Solo) relatively little overlap between the samples from different eruption 0.6 0.60.70.8 0.9 1.0 styles. Solidity 4.3. Insights from SEM images

Ash particles from the hydrothermal explosions of Iwodake and 1.0 b Tangkuban Perahu consist essentially of altered rocks with very rough μ 0.9 surfaces formed by a multitude of very small (~10 m in diameter) interlocking crystals (Fig. 7a–c). These ﬁne crystals are replacement 0.8 minerals or secondary precipitates in cracks and voids that formed in response to the circulation of hydrothermal ﬂuids. Ash particles from 0.7 the hydrothermal explosion of Marapi are very similar to those of Iwodake and Tangkuban Perahu (Fig. 7d), but in addition there is a 0.6 substantial amount of blocky particles with much smoother surfaces

Axial Ratio (Fig. 7h). These latter particles are rather fresh-looking and often host 0.5 one or more large crystals (tens of micrometers in diameter). Most 0.4 ash particles from the dome explosions of Merapi in July and November 2013 are blocky with smooth surfaces (Fig. 7e–g), very similar to the 0.3 smooth Marapi particles. They are generally quite fresh looking and often host one or more crystals - tens of micrometers in diameter - 0.2 (e.g. Fig. 7f). In addition, there are a number of hydrothermally altered 0.20.4 0.6 0.8 1.0 particles similar to those of Iwodake and Tangkuban Perahu and rare Form Factor particles that are highly vesicular and stretched (similar to Fig. 7i). There is no obvious difference in the ash particles from the July and Fig. 5. Conventional shape descriptors, (a) convexity versus solidity and (b) axial ratio versus November 2013 explosions. form factor, for ash particles. Clear distinctions are made between the hydrothermal Ash particles from the Vulcanian and Plinian eruptions of Merapi in explosions of Iwodake, Tangkuban Parahu, and Marapi, the dome explosions of Merapi in fl 2013, and the Vulcanian and Plinian eruptions of Merapi in 2010, Rabaul, Kelud (sampled 2010, Rabaul and Kelud are dominantly highly in uenced by vesicles in Dieng and Solo). and often elongated (Fig. 7i–l), clearly reflecting the fragmentation of D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 109

-1

-2 Principle Component 2 -3

-4 Iwodake Tangkuban Perahu Marapi (Sumatra) -5 -2-1 0 1 2 364 5 Merapi - Jul 2013 (1) Merapi - Jul 2013 (2) Principle Component 1 Merapi - Nov 2013 Merapi - 2010 Rabaul Kelud (Dieng) Kelud (Solo)

Fig. 6. Principle component analysis of the multi-fractal spectrum of ash particle outlines. The first principal component (PC1) is a good measure of the large-scale morphological roughness while the second principal component (PC2) is a measure of the fine-scale textural roughness of a particle boundary. Clear distinctions are made between the hydrothermal explosions of Iwodake, Tangkuban Parahu, and Marapi, the dome explosions of Merapi in 2013, and the Vulcanian and Plinian eruptions of Merapi in 2010, Rabaul, Kelud (sampled in Dieng and Solo). molten magma. A number of free crystals (~100 μm in diameter or (Batiza et al., 1984)(Fig. 8a–b). We observe similar trends, with more) are also present, as well as some hydrothermally altered particles dense/blocky particles produced by quench granulation (Pacificsea- similar to those of Iwodake and Tangkuban Perahu. Aggregates of fine mount), hydrothermal explosions (Iwodake, Tangkuban Perahu, and ash (accreted, b10 μm platy bubble walls) are abundant in the Rabaul Marapi), or dome explosions (Merapi in 2013) having higher values of sample and in the Kelud sample collected in Dieng. There is thus a solidity and convexity than vesicular fragments/shards produced by clear difference in the overall ash particle shapes as a function of erup- magmatic (Tambora, Mount St Helens, Rabaul, Merapi in 2010, and tion style. Kelud) or phreatomagmatic (Surtsey) eruptions. However, our results are not directly comparable because morphometric descriptors were 5. Discussion computed from the apparent projected shape of a particle in this study and from sectioned particles mounted in epoxy in Liuetal. 5.1. Comparisons of ash samples and insights on fragmentation mechanisms (2015a). Projected particle outlines are the two-dimensional shadow images of particles and consist of the projection of irregularities and It is very challenging to compare our results with other studies asymmetry in multiple directions at multiple levels (heights) within a because the values for adimensional morphometric parameters are particle. They are thus inherently smoother and rounder than the out- rarely reported in tables, and when the data is reported it almost lines of sectioned particles, resulting in a shift to higher solidity and con- never includes the original, raw data. It is thus impossible to compute vexity values (e.g. Fig. 3). Conventional shape descriptors can thus not shape parameters different than the ones that had initially been chosen. be used to compare data sets that have not been acquired in the exact We therefore wish to re-emphasize the suggestion of Leibrandt and Le same way.

Pennec (2015) to report basic morphometric data in order to allow We computed the Compactness (Ap/lw), Rectangularity (Pp/(2l + 2 other scientists to recalculate any morphological descriptor. We urge 2w)), Circularity (Pp/2√(πAp)), and Elongation (Dmaxferet/Ap) shape pa- the scientific community to include, in Supplementary tables, raw rameters of Dellino and La Volpe (1996) in order to compare our data shape data such as the area (Ap) and perimeter (Pp) of a particle, the with other studies (Fig. 8c–e). The plot of Circularity x Rectangularity width (w) and length (l) of the bounding box, the major (A) and vs. Compactness x Elongation (Fig. 8c, e) has been proposed to discrimi- minor (B) axes of the best fit ellipse, the area (ACH) and perimeter nate between hydromagmatic (or phreatomagmatic) and magmatic (PCH) of the convex hull, and the maximum feret diameter (Dmaxferet). fragmentation mechanisms (Schipper, 2009; Murtagh and White, This short list of nine measurements would enable the calculation of 2013). Ash from the eruptions of Monte Pilato - Rocche Rosse, Lipari, Solidity, Convexity, Axial Ratio and Form Factor that have been proposed Italy (Dellino and La Volpe, 1996), the Palizzi base surge of La Fossa, by Liu et al. (2015a) to be the most effective, and the calculation of Vulcano, Italy (Büttner et al., 2002), Black Point volcano, Mono Lake, Circularity, Rectangularity, Compactness and Elongation that have been USA (Murtagh and White, 2013), and the AS1a eruption of Vesuvius, introduced by Dellino and La Volpe (1996) and are commonly used Italy (Cioni et al., 2008) straddle the boundary between hydromagmatic since then. Our full data set is reported in Supplementary Table 1. and magmatic fragmentation regimes. These eruptions (except maybe Despite this issue, we compared our measurements of solidity and AS1a) have clear evidence of interaction (at some point in time) with convexity with those reported by Liu et al. (2015a) for ash from the a large body of external, non-hydrothermal water, which supports the well-documented eruptions of Eyjafjallajökull in 2010, Grímsvötn in observed range of ash morphometric parameters. Particles produced 2011, Tambora in 1815, Surtsey in 1963–64, Mount St Helens in 1980, by Molten Fuel Coolant Interaction (MFCI) experiments (Büttner et al., and a submarine eruption from a Pacific seamount at 2000 m depth 2002), representative of phreatomagmatic eruptions, also straddle the 110 D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114

a. Iwodake b. Iwodake c. Tangkuban Perahu d. Marapi

10 µm 10 µm 10 µm 10 µm

e. Merapi (Jul 2013-1) f. Merapi (Jul 2013-2) g. Merapi (Nov 2013) h. Marapi

10 µm 10 µm 100 µm 10 µm

i. Merapi (2010) j. Rabaul k. Kelud (Dieng) l. Kelud (Solo)

10 µm 10 µm 10 µm 100 µm

Fig. 7. SEM images show that ash particles from hydrothermal explosions have rough surfaces due to fine grained secondary mineral precipitates (a–d), while particles from dome explosions are blocky but with smooth surfaces (e–g). Marapi ash particles display both characteristics (d, h). Particles from Vulcanian and Plinian eruptions are vesicular and often elongated (i–l). The arrow in (f) highlights the presence of a crystal. All scale bars are 10 μm or otherwise stated. boundary between hydromagmatic and magmatic fragmentation re- expected. But again, a closer look at the data shows that particles 1 gimes, as expected. However, a closer look at the data shows that parti- and 3 (Dellino and La Volpe, 1996, Fig. 10) plot in the ductile instead cles 6 (Dellino and La Volpe, 1996,Fig.10)and3g(Büttner et al., 2002, of the brittle fragmentation domain, while particle 5 (Dellino and La Fig. 3) plot in the hydromagmatic instead of the magmatic fragmenta- Volpe, 1996, Fig. 10) plots in the brittle instead of the ductile fragmen- tion domain. Conversely, particles 2 (Dellino and La Volpe, 1996, tation domain. Ash from the AS1a eruption of Vesuvius, Italy (Cioni Fig. 10), 3c and 4b (Büttner et al., 2002, Figs. 3 & 4) plot in the magmatic et al., 2008) and from the present study plot solely in the ductile instead of the hydromagmatic fragmentation domain. Ash particles from fragmentation regime. Ductile fragmentation is unlikely for the dome ex- the present study straddle the boundary between fragmentation regimes, plosions of Merapi in 2013 or the hydrothermal explosions of Iwodake, with a tendency for blocky particles to plot on the hydromagmatic side of Tangkuban Perahu and Marapi. This diagram may thus also not be fully the discriminating line. This implies that particles produced by dome reliable for discriminating between brittle and ductile fragmentation explosions are classified as having a phreatomagmatic origin, which is regimes. erroneous. In addition, a substantial amount of vesicular ash grains from Conventional shape descriptors are efficient at discriminating the magmatic eruptions of Merapi in 2010, Rabaul, and Kelud plot in the between dense or blocky particles on the one hand and vesicular hydromagmatic fragmentation domain. This diagram may thus not be fragments or shards on the other hand. However, blocky/dense particles fully reliable for discriminating between magmatic and phreatomagmatic produced by hydrothermal explosions, dome explosions, and MFCI fragmentation regimes. experiments are poorly to not differentiated. Although there is a resem- The plot of Circularity x Elongation vs. Rectangularity x Compactness blance in the fragmentation mechanisms of hydrothermal explosions (Fig. 8d) has been proposed to discriminate between brittle and ductile and phreatomagmatic eruptions due to the involvement of external fragmentation mechanisms (Büttner et al., 2002). Ash from the erup- fluids, one would expect to see a distinction in the morphology of the tions of Monte Pilato - Rocche Rosse, Lipari, Italy (Dellino and La particles due to differences in the material that is fragmenting. In the Volpe, 1996), the Palizzi base surge of La Fossa, Vulcano, Italy (Büttner case of phreatomagmatic eruptions and MFCI experiments, magmatic et al., 2002) Turrialba volcano, Costa Rica (Alvarado et al., 2016) and melt is in contact with hydrous fluids and fragments in a brittle manner particles produced by MFCI experiments (Büttner et al., 2002) straddle in response to rapid quenching (thermal granulation or brittle disinte- the boundary between brittle and ductile fragmentation regimes, as gration of variably vesicular clasts; see Liu et al. (2015b), Büttner et al. D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 111

Fig. 8. Comparisons with ash samples from other studies based on conventional shape descriptors. (a–b) Comparison of solidity and convexity with the data from (Liu et al., 2015a). Projected particle outlines (this study) are inherently smoother and rounder than the outlines of sectioned particles (Liu et al., 2015a), resulting in a shift to higher solidity and convexity values. (c–e) Various combinations of circularity, rectangularity, compactness, and elongation used to discriminate between hydromagmatic and magmatic fragmentation (Murtagh and White, 2013) or between brittle and ductile fragmentation (Büttner et al., 2002). Particles studied by Dellino and La Volpe (1996) and Büttner et al. (2002) and discussed in the text are highlighted with numbers.

(2002), and references therein). In parallel, ash may be produced by the geothermal system (Barberi et al., 1992). This material is generally shattering of host rocks during the violent expansion of steam, but holocrystalline, more or less oxidized, contains a variety of replacement these lithic particles are generally avoided for morphometric studies minerals of varying sizes and shapes, and bears secondary mineral precip- and are not the types of particles analyzed by Dellino and La Volpe itates as coatings and vesicle fillings (e.g. Alvarado et al., 2016; Gaunt (1996) or Büttner et al. (2002). In the case of hydrothermal explosions, et al., 2016). Different external grain morphologies should thus be ob- hydrothermally altered conduit rocks are shattered due to gas accumula- served, with very angular particles produced by quench granulation tion and overpressurization in response to perturbations of the shallow and/or brittle disintegration during phreatomagmatic eruptions and less 112 D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 angular and more rugged or cauliflower-shaped particles produced by shape for Kelud ash versus sliced particles for Katla ash), Katla ash hydrothermal explosions (e.g. Fig. 7a–d). Dome explosions result from shows wider incisions and lower values of PC2 than Kelud ash. Similarly gas overpressurization in a lava plug (Sparks, 1997; Melnik and Sparks, to Katla, Grímsvötn ash shows low PC2 values. However, this is not only 1999), and should thus produce dense, subrounded and blocky particles due to differences in the sample preparation technique but also reflects with shapes controlled by the high crystallinity of the plug material (e.g. differences in particle morphology. Grímsvötn ash is more angular and Fig. 7e–g). Such differences in particle morphologies are actually observed has wide and shallow incisions compared to the rounded ash from under the optical microscope and SEM but are not reflected with the Merapi and Iwodake. Ash from Iwodake and Merapi are also distin- conventional shape descriptors (Figs. 5 & 8). guishable from each other, with the former being more finely incised In order to evaluate whether the fractal method is more efficient at and having a cauliflower-like outline (slightly higher PC1 values) and discriminating different types of blocky particles, we compare our data the latter having very smooth and straight outlines. Grímsvötn ash cor- to that of Maria and Carey (2002) for the hydromagmatic eruption of responds to melt that has been quenched rapidly and fragmented in a Grímsvötn and the magmatic eruption of Katla. Due to a lack of data brittle way. Merapi ash is finely to coarsely crystalline plug material reporting, as for the conventional shape descriptors, it is not currently that has been fragmented in a quasi-solid state. Iwodake ash is hydro- possible to make comparisons with more studies. Fractal values for thermally altered volcanic material that has been shattered. The fractal each dilation step must be reported in order for the PCA to be run on method is therefore quite effective at distinguishing fine scale structures the joint dataset. For consistency, we applied the dilation method to and textures, and is a great tool to discriminate between fragmentation the ash particles from the hydrothermal explosion of Iwodake, the mechanisms. dome explosion of Merapi in November 2013, and the magmatic eruption of Kelud (sampled in Dieng) and compare fractal values for the 5.2. Insights on the source of Marapi ash from particle shape same dilation widths (~8.1, ~12.8, ~20.3, and ~32.2 μm). The dense particles from Iwodake and Merapi slightly overlap but do not overlap with According to conventional shape descriptors and the fractal method, the particles from Grímsvötn in the principal component diagram ash from the 2015 hydrothermal eruption of Marapi bears similarities (Fig. 9). These dense particles have lower first principal component with ash from the dome explosions of Merapi in 2013 (Figs. 5–6). (PC1) values than the vesicular glass shards produced by the magmatic Some particles are particularly rounded and smooth (very high solidity eruptions of Kelud and Katla. PC1 can be seen as a measure of the depth and convexity, very low PC1 values), suggesting that part of the ash of incisions in a particle outline (shallow incisions with PC1 b 0and could have originated from the fragmentation of a lava plug in addition deep incisions at PC1 N 0), while the second principal component to the shattering of hydrothermally altered conduit and crater rocks. (PC2) can be seen as a measure of the width of the incisions within This is also supported by the SEM images of Marapi ash (Fig. 7d, h). the particle outline (wide incisions at PC2 b 0 and narrow incisions at The last magmatic eruptions of Marapi occurred between 1985 and PC2 N 0). Katla ash shows similar to deeper incisions than Kelud ash 1994 with the ejection of incandescent material during small Vulcanian (i.e. similar to greater values of PC1) but, most likely due to the differ- eruptions and a period of lava dome growth (3GVP, 2013). It is thus ence in the sample preparation technique (i.e. apparent projected likely that some dome material is still present as a plug in the shallow

3 Iwodake Merapi 2013 Kelud (Dieng) 2 Grímsvötn Katla

Principal component 2 -1

-2

-3 -2-1 04 1 2 3 5 Principal compenent 1

Fig. 9. Comparisons with ash samples from Maria and Carey (2002) based on fractal analysis using the dilation method and multivariate analysis. The first principal component (PC1) relates to the depth of incisions in a particle outline (shallow incisions with PC1 b 0 and deep incisions at PC1 N 0), while the second principal component (PC2) relates to the width of the incisions within the particle outline (wide incisions at PC2 b 0 and narrow incisions at PC2 N 0). The dense/blocky particles from the hydrothermal explosion of Iwodake, the dome explosion of Merapi in 2013, and the phreatomagmatic eruption of Grímsvötn are successfully distinguished from each other. Differences between the Katla and Kelud ash samples are most likely due to differences in the particle imaging technique, i.e. apparent projected shape for Kelud ash versus sliced particles for Katla ash. D. Nurfiani, C. Bouvet de Maisonneuve / Journal of Volcanology and Geothermal Research 354 (2018) 102–114 113 conduit, although no dome is presently visible in the summit crater. References There have been no magmatic eruptions at Tangkuban Perahu, certainly 1 in the past 200 years and possibly in the past 9500 years (4GVP, 2013). GVP, 2013. Report on Tangkubanparahu (Indonesia). In: Sennert, S.K. (Ed.), Weekly Volcanic Activity Report, 13 March–19 March 2013. Smithsonian Institution and US Similarly, there have been no magmatic eruptions at Iwodake in the Geological Survey. past 1300 years (Saito et al., 2001). Hydrothermal eruptions at these 1GVP, 2014. Report on Kelut (Indonesia). In: Wunderman, R. (Ed.), Bulletin of the Global volcanoes thus essentially consist of the fragmentation of hydrother- Volcanism Network. 39:2. Smithsonian Institution. 2GVP, 2013. Report on Merapi (Indonesia). In: Sennert, S.K. 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