# Decrypting the sediment record of very late stage volcanic activities in Tatun volcano group, Northern Taiwan: evidence from ashy deposits

Guo-Teng Hong, Damià Benet, Kuo-Lung Wang, Fidel Costa, Liang-Chi Wang

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Guo-Teng Hong, Damià Benet, Kuo-Lung Wang, Fidel Costa, Liang-Chi Wang. Decrypting the sediment record of very late stage volcanic activities in Tatun volcano group, Northern Taiwan: evidence from ashy deposits. Terrestrial, Atmospheric and Oceanic Sciences, 2025, 36, 10.1007/s44195-025-00104-x. insu-05080882

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# Decrypting the sediment record of very late stage volcanic activities in Tatun volcano group, Northern Taiwan: evidence from ashy deposits

Guo-Teng Hong<fnref n="1" />,<fnref n="2" />*, Damià Benet<fnref n="3" />, Kuo-Lung Wang<fnref n="1" />,<fnref n="4" />, Fidel Costa<fnref n="3" /> and Liang-Chi Wang<fnref n="5" />

**Abstract**

The Tatun Volcano Group (TVG) is an active volcano on Taiwan and poses a potential threat to public safety in northern Taiwan. Currently, the youngest eruption records among the TVG occurred around Mt. Chihsing in the late Pleistocene. One of these significant records is from the ashy layers exposed in a lacustrine sediment profile near Mt. Shamao. This study analyzes the mineralogy, petrography, ash componentry, and particle morphology of the ashy layers to characterize the volcanic activity at the TVG, including eruption behavior and style. A previous study inferred that phreatic eruptions occurred near Mt. Chihsing between 17 – 13 ka using radiocarbon dating on finely dispersed organic materials in the neighboring horizons of the ashy layers (Belousov et al. 2010). Based on the evidence of sediment components, petrographic texture, particle morphology, hydrothermal mineral phases, and co-depositional diatoms, this study argues that the ash in question is not an in-situ tephra deposit and proposes two possibilities for its origin: (1) resedimented volcaniclastic deposits from hydrothermal products in a volcanic epithermal environment, not implying any eruption activity; or (2) volcanogenic sedimentary deposits formed by either syn-eruptive or post-eruptive floods. Based on the inconsistent acidity ranges revealed by the mineral assemblages in the ashy samples, the post-eruptive origin is favored, i.e. volcanogenic sedimentary deposits formed by reworked tephra from multiple events. The eruption ages of their tephra sources are unmeasurably older than the radiocarbon age obtained from the current deposits, which are not indicative of volcanic eruption ages.

**Keywords** Tatun volcano group, Ash componentry, Particle morphology, Hydrothermal mineralogy, Eruption activity, Post-eruptive volcanogenic sediments

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© The Author(s) 2025. **Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

<footnote>*Correspondence:
Guo-Teng Hong
gthong@gsmma.gov.tw</footnote>

<footnote n="1">¹Department of Geosciences, National Taiwan University, Taipei, Taiwan</footnote>

<footnote n="2">²Geological Survey and Mining Management Agency, MOEA, Taipei, Taiwan</footnote>

<footnote n="3">³Institut de Physique du Globe de Paris, Université Paris Cité, Paris, France</footnote>

<footnote n="4">⁴Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan</footnote>

<footnote n="5">⁵Department of Earth and Environmental Sciences, National Chung Cheng University, Taipei, Taiwan</footnote>
# 1 Introduction

The Tatun volcano group (TVG) is an active volcano on Taiwan, located close to Taipei, a bustling metropolis with a population of over 2 million people. The TVG poses a significant threat to public safety due to its young eruption history and potential for reactivation. Huang et al. (2021) revealed a p-wave low-velocity zone located between 8 and 20 km beneath the TVG, suspected to be the magma reservoir. As such, the authorities consider the TVG's volcanic activity a realistic concern. Throughout the past 10,000 years, two eruptive events have been reported at the TVG, both near Mt. Chihsing. The two events are 6,000-year-old volcanic avalanche deposit (radiocarbon dating; Belousov et al. 2010) and the lava age less than 1,370 years old (magnetite crystallization age by U-Th-Ra disequilibrium dating; Zellmer et al. 2015).

According to Szakacs (1994), the time between eruptions for active andesitic volcanoes can span thousands to tens of thousands of years. This means that the TVG may have highly potential to erupt again in such a time scale. To accurately assess the potential hazards of the TVG eruption, its eruptive styles (explosivity) and types of hazards generated in the past must be comprehended first. Geological field studies have shown that the TVG typically produced effusive lavas and lahars, with only a few volcanic debris avalanches (Song et al. 2000, 2011; Chen et al. 2003, 2007). This suggests that the volcano tended to erupt in a less explosive and more effusive style. However, the active tectonic setting and extreme climate condition in Taiwan can cause unconsolidated tephra to erode quickly (You et al. 1988; Dadson et al. 2003), potentially obscuring evidence of past explosive eruptions and leading to overlook their potential hazards. It is therefore important to consider the possibility of explosive eruptions, even if there is currently no substantial field evidence supporting large explosive events exceeding VEI 4 (Belousov et al. 2010).

In modern volcanology studies, ash componentry and morphology analysis provide valuable insights into processes driving the activity in the volcanic system. Ash componentry consists of categorizing particles into different components or types such as lithic, free crystal, juvenile, altered material, and hydrothermal material (Schmid 1981; Cas and Wright 1988; Miwa et al. 2009; Gaunt et al. 2016; Benet et al. 2024a, b). Identifying certain particle types and subtypes (e.g. mineral phases) has allowed to infer different stages of activity and their mechanisms (Suzuki et al. 2013; Pardo et al. 2014; Maeno et al. 2016; Benet et al. 2021). For example, abundant altered material in tephra suggests an active hydrothermal system, whereas abundant lithic component in tephra indicates explosions fragmenting older rocks from a volcano (Morgan et al. 2009; Pardo et al. 2014;

Gallagher et al. 2020; Miyagi et al. 2020; Benet et al. 2021). On the other hand, ash morphology analysis consists of characterizing the shape of the particles based on a series of shape parameters that are sensitive to macro- and fine-scale irregularities of the particle silhouette (Liu et al. 2015; Dürig et al. 2018; Nurfiani and Bouvet de Maisonneuve 2018). This allows to characterize the vesicularity and to compare the shape values across particles from different events or volcanoes (Liu et al. 2015; Nurfiani and Bouvet de Maisonneuve 2018). By analyzing the temporal variations of ash componentry and morphology during a consecutive eruption period, changes in eruptive styles in various types of eruptions can be tracked, e.g. dome-forming eruptions (Benet et al. 2021; Suzuki et al. 2013), Strombolian eruptions (Taddeucci 2002, 2004; D'Oriano 2022), Vulcanian eruptions (Miwa et al. 2013), and phreatic explosions (Ohba et al. 2007; Minami et al. 2016). Therefore, ash componentry and morphology analysis provide key insights into volcanic eruption processes with implications for hazard assessment and risk reduction.

The componentry, shape, and mineral assemblage of ashy samples from three layers at the lacustrine sediment profile near Mt. Shamao in the TVG are reported in this study. We aim to characterize the products associated with the TVG eruption and the potential hydrothermal activity. These ashy samples could be analogous to cessation-precursory products of any active volcanoes in recession from elsewhere. Thus, this study is supposed to aid the recognition of eruptive volcanoes going dormant.

# 2 Geological setting

Chen et al. (2007) reported that the TVG consists of over 50 volcanoes scattering across the northern Taiwan (Fig. 1). These volcanoes are primarily composed of andesitic-to-basaltic andesitic lava domes, lava flows, volcanic avalanches, and vast flooded lahars (Song et al. 2011; Hong 2014, 2015; Chen et al. 2003, 2007). As shown in Fig. 2, the magmatic activity of the TVG started at approximately 2.56-3 Ma and continued until 0.29-0.08 Ma, as determined by K-Ar datings (Juang and Bellon 1984; Juang and Chen 1989; Juang 1993; Tsao 1994), Ar-Ar datings (Lee 1996; Chang et al. 2024), and fission track datings (Wang and Chen 1990). The TVG has undergone at least two eruptive surges (0.8 Ma and <0.35 Ma) as identified by zircon U-Pd ages (Chu et al. 2018). However, the reported ages of the TVG extrusive rocks are mostly older than 0.1 Ma, due to the limits of isotopic dating methods and the scarcity of young outcrops.

Although the TVG was once considered dormant due to the lack of young extrusive ages, recent studies have reported young eruption events. A suspicious ash record with a charcoal age of nearly 20 ka in the drilling core of
<figure><img src="image_4.png" /><figcaption>**Fig. 1** The Tatun volcano group in northern Taiwan is divided into six sub-groups based on their spatial distribution and lithologies, namely Chutze, Tatun, Chihsing, Huangtsui, Tinghuohsiu, and Nantze subgroups (Chen 1990). The volcanic cones (purple circles) and craters (black lines and red circles), displaying clear volcano-tectonic features, have been identified through high-resolution DTM (Chen et al. 2007). The study site is marked by a yellow star at the southern foot of Mt. Shamao</figcaption></figure>

the Taipei basin, southwest of the TVG, was disclosed (Chen and Lin 2000, 2002; Chen et al. 2010). Additionally, the 6 ka debris avalanche deposit at Mt. Chihsing (Belousov et al. 2010) and the younger-than-1.37 ka lava flow at Mt. Shamao (Zellmer et al. 2015) have further supported the idea that the TVG was still magmatically active during the Holocene. Furthermore, multi-disciplinary observations have provided firm evidence of an active magma reservoir beneath the TVG (Yang et al. 1999; Song et al. 2000; Lin 2016; Huang et al. 2021). These findings suggest that the TVG is potentially hazardous and the reactivation potentials should not be ignored from a hazard risk perspective, especially considering that the repose time of andesitic volcanoes could last tens of thousands of years (Szakacs 1994).

**3 Outcrop and sample description**

This study focuses on a well-preserved lacustrine sediment profile in the southeastern region of Mt. Shamao, a parasitic lava dome of Mt. Chihsing. The outcrop comprises gray parallel-laminated to uniformly massive mud, with three yellow sandy layers embedded within it. At the

top of the outcrop is a massive laharic deposit (Fig. 3). The three sandy layers, namely A1, A2, and A3 from bottom to top, have thicknesses of 4, 4, and 12 centimeters, respectively. According to the published radiocarbon dating by Belousov et al. (2010), the A1 layer is slightly older than 15,000–17,000 years, while the A3 layer is barely older than 13,000–14,000 years. All three layers were deposited toward the end of the Pleistocene period.

Belousov et al. (2010) proposed that the sandy layers (lithic ash) and finely laminated paleolake sediment were deposited in low-energy, oxygen-poor conditions. They examined the size and shape of the ash particles then inferred that they were likely resulted from small-to-moderate-scale Vulcanian explosions. The sandy layers A1 and A2 have a well-sorted, parallel-laminated texture and show a sharp, conformable contact with the lacustrine sediment. The A3 layer, thicker than the others, changes from massive coarse sand to parallel-laminated mud as it grades upward. These sandy layers are also referred to as “ashy layers” or “event layers” in subsequent sections. The lithologic column is illustrated in Fig. 3.
<figure><img src="image_5.png" /><figcaption>**Fig. 2** Comprehensive geochronological results of the TVG. Most radioactive dating results (diamonds and circles) confined the magmatic activities to the early to late Pleistocene (2.80–0.10 Ma). Only two samples in the Chihsing subgroup area are younger than 10 ka. One is a volcanic avalanche deposit 6,000 BC dated by radiocarbon method (blue triangle) (Belousov et al. 2010), and the other is a lava flow bearing magnetite crystallization age 1,370 years dated by U-Th-Ra disequilibrium method (yellow triangle) (Zellmer et al. 2015)</figcaption></figure>

The lacustrine mud host strata are divided by the three ashy layers into four mud units, namely M1, M2, M3, and M4, from bottom to top (Fig. 3). Except for the bottom of M1 and the top of M4 are massive, the rest parts of mud units generally exhibit planar lamination. The intercalated sandy layers have consistent thickness and delicate sedimentary structures created by water flow, such as planar lamination in A1 and A2 and flame structure at the base of A3. The A3 layer contains poorly sorted particles compared to A1 and A2, indicating that differential settling occurred in a low-energy deep water body, resulting in the normal grading texture of the A3 layer. This suggests that the sandy layers were deposited by a high-density current from a distal event, rather than by regular fluvial processes as the case for the muddy units in the profile.

## 4 Methods

We collected the ashy samples from the lacustrine sediment profile located in the southeastern foot of Mt. Shamao as shown in Fig. 1, specifically from the A1, A2, and A3 layers illustrated in Fig. 3. The particle sizes of the A1 and A2 samples range between 40 and 300 micrometers (µm) in diameter, and those of the A3 sample range between 20 and 500 µm. The lacustrine mud units in between, namely M1, M2, M3, and M4 as shown in Fig. 3, were also investigated for mineral comparison.

According to different analyses applied to the ashy samples, corresponding preparation steps to those analyses were adopted. At first, raw samples were ultrasonically cleaned for 1 min three times to remove the clay-sized fraction and dried at 60 °C in a stove overnight. Then, clean silt-to-sand-sized particles were obtained and sieved into specific grain-size fractions for image
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Field occurrence (on the left) and lithologic column (on the right) of the paleolake deposit outcropped at the southeastern foot of Mt. Shamao. The measuring tape at the left of the outcrop is 1 m long for scale. Organic material collected at mud unit 2, mud unit 4, and laharic deposit yielded calibrated $^{14}$C ages: 15–17 ka, 13–14 ka, and 13–14 ka, respectively (cited from Belousov et al. 2010)</figcaption></figure>

acquisition in routine particle morphology analysis (Benet et al. 2021, 2024a). Cleaned samples without sieving were also applied in the image acquisition for better representativeness, compared to the sieved ones by simply spraying the sample particles evenly on glass slides to mount them with carbon tape. On the other hand, cleaned samples without sieving were also used in thin sectioning for particle petrography and mineral chemistry by scanning electron microscope (SEM). To identify mineral compositions, we grounded raw samples into fine powders to analyze their mineral components by X-ray powder diffraction (XRD). The clay fraction alone was separated from the raw samples to characterize clay minerals. The preparation steps of each analysis are described in the following sections.

For searching diatom-rich fragments in these ashy samples, we analyzed diatoms at 5 cm intervals from top to bottom along the whole studied profile, giving a total of 30 samples. We treated 0.1 g sediment samples with 5 mL 10% HCl and 5 mL 10% H₂O₂, then washed them in deionized water and mounted them with Mountmedia (Wako Pure Chemical Industries, Osaka, Japan). The diatom valves were identified under a LEICA DM750 light microscope at 1000x magnification. The identification of diatom species and their ecological interpretation was conducted using information relating to freshwater diatom flora in assembled references (Kobayashi et al. 2006; Tan et al. 2013; Wang et al. 2009, 2010; Watanabe et al. 2005; Wu et al. 2011).

## 4.1 Componentry and mineral composition

Each sample, about 5 g, was treated for componentry analysis. The aliquots were sieved using multiple meshes of different pore sizes in the scale of Phi (0φ, 1φ, 2φ, and 3φ). The coarsest available grain-size fraction (0φ–1φ) was used for componentry analysis.

We ground the raw samples into fine powder for XRD analysis to identify mineral species. For clay minerals differentiation, the clay fractions of the samples were separated using centrifugation with the addition of a dispersant (sodium hexametaphosphate) (Poppe et al. 2001). Then, we prepared preferred-oriented clay slices and applied additional ethylene glycol treatment to all clay separates to determine the clay mineral species in the X-ray diffraction patterns (Moore and Reynolds 1997; Brindley and Brown 1980). Raw samples and their clay separates were analyzed by a Rigaku TTRAX-III XRD with an accelerating voltage of 35 kV and a tube current of 25 mA, at the National Museum of Natural Sciences (NMNS), Taiwan. The scanning condition was set to 0.5°/min speed and 0.01° sampling interval. We obtained back-scattered electron micrographs and mineral compositional information of raw particle-mounted thin sections using a Hitachi TM3030Plus Tabletop Microscope (SEM) equipped with an Oxford SwiftED 3000 Energy Dispersive X-ray Spectrometer (EDX) at 15 kV and 1 mA, at the Geological Survey and Mining Management Agency (GSMMA), Taiwan.

We observed raw sample particles mounted on carbon tape under a Leica DMRX optical binocular microscope
equipped with a Leica LAS imaging system at GSMMA, Taiwan. Particles were examined under the binocular to describe their properties, including color, luster, degree of alteration, degree of crystallinity, and edge angularity. We opted for classifying ash particles based on these observations, rather than using an automatic classifier because its training set did not include samples associated with cessation (Benet et al. 2024b). The four main types we considered are free-crystal, altered material, juvenile, and lithic components. Free crystals typically exhibit unique optical properties such as cleavage and twinning and are often recognized by their well-faceted shape. Altered material generally contains various amounts of yellowish to reddish coatings, in gray to pale colors, with rounded edges and sometimes the presence of secondary minerals and signs of weathering. Lithic particles are usually dark gray to black, dense, and massive, and their edges' angularity varies from subangular to well-rounded. They often have high crystallinity and show signs of chemical alteration in the SEM like etch pitting. Juvenile particles are typically glossy, sharp-edged, and smooth-surfaced and do not exhibit signs of alteration in the SEM.

## 4.2 Particle morphology analysis

The grain size chosen for analysis usually depended on the total grain size distribution of the samples, which was determined by the tephra characteristics of each eruption and the location where the ash was collected. For instance, some studies analyzed 0 φ (Cioni et al. 2014), while others analyzed 1 φ (Cioni et al. 2008), 1.5 φ (Miwa et al. 2013), 2 φ (Maria and Carey 2007), or 4 φ (Dellino and La Volpe 1996). Nurfiani and Bouvet de Maisonneuve (2018) used bulk samples instead of sieved samples, to utilize more of the sample and avoid excluding information from smaller or larger particles. They found conventional shape descriptors useful in differentiating between eruption styles. In this study, we mostly used bulk samples to discuss particle morphology and petrography because our samples were well-sorted and relatively small in quantity.

We calculated shape parameters (SPs) such as convexity and solidity by analyzing the shapes of particle-projected areas in back-scattered electron images (BSE images) captured by SEM (Liu et al. 2015; Pardo et al. 2020) or in optical images of camera-coupled binocular microscope (Leibrandt and Le Pennec 2015; Miwa et al. 2015; Nurfiani and Bouvet de Maisonneuve 2018). Two kinds of PC-processed particle images were utilized: (1) BSE images of particles with manually subtracted backgrounds; (2) binocular microscope images of particles with automatically subtracted backgrounds using an in-house Python program. The raw scans from binocular images were processed by image fusion, segmentation, and color normalization following the procedure by Benet et al.

(2024a). The silhouette of each particle image was used to compute an array of morphological properties (Online Resource 1: Table S1) necessary for the calculation of SPs (Online Resource 1: Table S2). On the other hand, we followed Liu et al. (2015) to calculate SPs that describe particle shape, texture, and morphological roughness (e.g. solidity, convexity, circularity, rectangularity, compactness, and elongation) to distinguish between different eruption styles (Liu et al. 2015; Nurfiani and Bouvet de Maisonneuve 2018). We analyzed particle shape from electron micrographs to compare our samples from TVG with reference ashes from several known volcanic unrests (which were kindly provided by Dr. D. Nurfiani, Research Center for Geological Disaster, National Research and Innovation Agency, Indonesia) and ensure graphical consistency in data illustration. The raw SPs are available in the Online Resource 2.

## 4.3 Petrography and mineral chemistry

For petrographic analysis, the particles were embedded in epoxy resin, fixed to glass slides, and then ground and polished to a thickness of 30 µm (with the procedure suggested by Humphries 1992). To determine the mineral compositions, we utilized a scanning electron microscope (SEM). We obtained back-scattered electron micrographs and mineral compositional information of particle-mounted thin sections using a Hitachi TM3030Plus Tabletop Microscope (SEM) equipped with an Oxford SwiftED 3000 Energy Dispersive X-ray Spectrometer (EDX) at 15 kV and 1 mA, at the Geological Survey and Mining Management Agency (GSMMA), Taiwan.

To examine the particles' textural characteristics, we observed sample particles mounted on carbon tape under a Leica DMRX optical binocular microscope equipped with a Leica LAS imaging system at GSMMA. Then SEM-EDX was utilized to determine mineral constitutions and internal textures.

# 5 Results

## 5.1 Grain size characteristics

Figure 4a and b show that samples A1 and A2 have similar median diameters, both around 100 µm and normal size distributions with a peak population of 100–110 µm. Sample A3, on the other hand, has a coarser median diameter of about 130 µm and a bimodal size distribution with two peaks at 70 µm and 230 µm (Fig. 4c). The peak population at a particular diameter size in natural ash samples can correspond to specific ash components, such as glass, aggregates, or crystals, depending on the melt depressurization, source rock crystallinity, and tephra fragmentation process (Nurfiani and Bouvet de Maisonneuve 2018; Benet et al. 2021). However, our study revealed that all three samples have evenly mixture of
<figure><img src="image_7.png" /><figcaption>**Fig. 4** Grain-size distribution curves of the samples A1, A2, and A3. The amounts of counted particles are expressed as percentage (No. of counts, in %). (a) The A1 sample particles drawn in a grain-size histogram show a normal size distribution with the peak population at 100–110 µm. 197 particles were counted. (b) Sample A2 has a similar grain-size distribution pattern and peak population as A1 does. 68 particles were counted. (c) The A3 sample particles have a coarser median diameter than those of A1 and A2. In the histogram, A3 displays a bimodal size distribution curve with two peak sizes at 70 µm and 230 µm, respectively. 166 particles were counted</figcaption></figure>

altered material, free crystals, and lithic particles within the peak-size components. Furthermore, when we analyzed the size distribution of each component separately, we observed similar distribution patterns and peak sizes across all components in A1 and A2 (Fig. 5). A similar constraint in grain size was likely imposed by the transporting force (sorting) rather than the fragmentation of the source material. Also, we noticed that altered material in A3 grouped in two size ranges which may reflect the original clast sizes of their raw material/crystals (Fig. 5d).

## 5.2 Ash componentry analysis

The componentry analysis show similar proportions of altered material, lithic and free-crystal components, and lack of juvenile particles (Fig. 5a). Altered material is the most abundant component in all three samples, comprising 50% of A1, 47% of A2, and 64% of A3, suggesting a ubiquitous hydrothermal fluid interaction covered most of the source region of the samples. Free crystals were less abundant than altered material, comprising 42% of A1, 43% of A2, and 23% of A3. Lithic particles, which are fresh to slightly altered rock fragments, were the least abundant component, comprising only 8–14% of the samples, and showed universal oxidation (Fig. 9).
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 5** The size distribution of the four principal components – lithic (L), juvenile (J), free crystal (FC), and altered material (AM) in the samples A1, A2, and A3. (a) Altered material and free crystal constitute most of all samples, but no juvenile particles are included. (b) All components in A1 demonstrate a normal distribution pattern with its peak size near 150 µm. 96 particles were counted. (c) All components in A2 have the same distribution patterns and peak sizes as those in A1. 90 particles were counted. (d) A3 exhibits two peak sizes for altered material due to its larger grain size range and poorer sorting. 22 particles were counted</figcaption></figure>

### 5.2.1 Free crystal component

The same minerals, including feldspar, quartz, pyroxene, amphibole, and Ti-magnetite constitute samples A1, A2, and A3. Feldspar and quartz of sub-rounded to sub-angular shapes are the most abundant crystals. The feldspar grains (plagioclase and alkali feldspar) showing compositional inhomogeneity of varying degrees (Fig. 6a) may suggest the remnants of feldspar phenocrysts with compositional zoning, a common phenomenon in the TVG andesitic magma crystallization (Chen 1975). Ti-magnetite, hypersthene, and augite which represent the crystallization phases of the TVG magma (Chen 1975; Shellnut et al. 2014) are the minor components of free crystals in the samples. They also occur as euhedral to subhedral inclusions within plagioclase crystals (Fig. 6b).

Monocrystalline quartz grains in all samples have a solid crystal structure in subangular to subrounded shapes (Fig. 6c). It is found that orthoclase is attached to the edges of quartz grains. Such a case is also

characterized in quartzose sandstone from the sedimentary strata beneath the TVG. According to Yen et al. (1984), Huang (1988, 1998, 2005), and Song et al. (2011), the subsurface formations beneath the Chihsing volcanic subgroup outcropped in the south of Shanchiao Fault include the Mushan Formation and Wutsishan Formation. The former contains abundant fine-to-medium-sand-sized, well-sorted quartz grains. The latter is characterized by coarse-sand-to-granule-sized quartz grains. Based on the grain sizes of the sandy portion in the studied samples, the Wutsishan Formation is possibly a main source of the monocrystalline quartz clasts. The Mushan Formation is hardly ruled out from the potential sources as well. Cryptocrystalline silica is usually accompanied by altered feldspar and/or iron-bearing clay (Fig. 6d). The XRD analysis showed that the cryptocrystalline silica consists of quartz and α-cristobalite (as described in Sect. 5.4). This kind of silica often contained minor amounts of Al (<2 wt%) and trace amounts of Na
<figure><img src="image_12.png" /><figcaption>**Fig. 6** Back-scattered electron micrographs of free crystals and lithic particles in the A1, A2, and A3 samples. (a) The feldspar crystal including both plagioclase (Pl) and alkali feldspar (Fsp) illustrates its compositional inhomogeneity. Orthopyroxene (Opx) and clinopyroxene (Cpx) are common inclusions found in feldspar. An example from A1. (b) Ti-magnetite (Mgt), hypersthene (Opx), and augite (Cpx) crystals occurred as euhedral to subhedral inclusions enclosed by plagioclase (Pl). Part of the plagioclase was altered to silica (Si). An example from A1. (c) A rare lithic grain shows that quartz grains (Q) in subangular to subrounded shape were cemented along with hornblende (Hbl), plagioclase (Pl), and altered material (AM). An example from A2. (d) The volcanic lithic in A1 lost its primary textures via hydrothermal leaching. The plagioclase phenocryst (Pl) was strongly eroded by acidic fluid to form vermicular voids (v). Amorphous alumino-silicate and silica (Si) were left as the replacement of groundmass. An example from A1. (e) Hornblende (Hbl) exhibited tabular, columnar, or prismatic forms, and often developed good stepped cleavages and wedge-like pits in cutting edges. An example from A2. (f) Orthopyroxene (Opx) and clinopyroxene (Cpx) in sharp contact, with interconnected prismatic voids possibly related to strong hydrothermal dissolution. An example from A1. (g) The volcanic lithic particle is composed of subhedral plagioclase phenocrysts (Fsp) and groundmass including feldspar microlite, magnetite (Mgt) and altered interstitial clay (Prl). An example from A1. (h) The sedimentary lithic particle contains equi-granular quartz and edge-attached orthoclase. An example from A3</figcaption></figure>
<table><caption>**Table 1** The mineral compositions of samples A1, A2, and A3 identified by XRD and SEM-EDX</caption><thead><tr><th colspan="2">Magmatic (volcanic wall rocks)</th><th colspan="2">Non-magmatic (host rocks, hydrothermal altered materials)</th></tr><tr><th colspan="2">Primary Minerals</th><th colspan="2">Sulfate</th></tr><tr><th></th><th></th><th>Clay</th><th>Silicic</th></tr></thead><tbody><tr><td>Pl</td><td>Fsp</td><td>Ha</td><td>Qtz</td></tr><tr><td>A1</td><td>v</td><td>v</td><td>v</td></tr><tr><td>A2</td><td>v</td><td>v</td><td>v</td></tr><tr><td>A3</td><td>v</td><td>v</td><td>v</td></tr></tbody></table>

vv: major component; v: minor component; tr: trace amount; #: characterized only by SEM and not certain; +: characterized only by XRD and not certain

Pl: plagioclase; Fsp: feldspar; Ol: olivine; Opx: orthopyroxene; Cpx: clinopyroxene; Amp: amphibole; Mgt: magnetite; Ha: halloysite; Prl: pyrophyllite; Smc: smectite; Chl: chlorite; Qtz: quartz; Crt: cristobalite; Trd: tridymite; Gp: gypsum; Anh: anhydrite; Alu: alunite; Brt: barite; Cc: calcite

and microlite) (Fig. 8a). Although the felsic particles were altered, mafic phenocrysts such as clinopyroxene, orthopyroxene, and magnetite remained relatively unaffected (Fig. 8b). Some primary minerals in the volcanogenic particles (e.g. plagioclase and microlite) were decomposed into kaolin clay, silica, and/or pyrophyllite to lose their primary texture or outline.

Alunite and sodium-alunite, common sulfate minerals formed by hydrothermal alteration of host rock, occurred as a minor component in the altered material, either grew alone or accompanied by altered feldspar and kaolin clay (Fig. 8c). Crystal clusters composed of micron-sized plagioclase, orthopyroxene, and titanomagnetite in volcanic-altered particles from A1 (Fig. 8d) are recognized as decomposed hornblendes, which is typical of extrusive rocks (Browne and Gardner 2006; Rutherford and Hill 1993).

The altered materials exhibit a rusty-colored and porous appearance (Fig. 9). Some particles of the altered materials and free crystals in A1 and A3 display exceptional roundness (Fig. 9a, c). Free crystals usually share similar alteration features as the altered materials. Typically, feldspar suffered a higher degree of alteration, whereas mafic minerals were less affected.

## 5.3 Ash particle shape analysis

### 5.3.1 Method test-data collection for shape analysis

To test sample representativeness, we compared the shape parameter distributions of two datasets randomly extracted from the same sample. Our findings (Online Resource 1: Fig. S1a) showed that more than 20 particles from each sample are capable of fully representing the sample properties (as suggested by Dr. D. Nurfiani; personal comm.). We also tested the perceived shapes of particles with different image resolutions. Liu et al. (2015) suggested a minimum threshold of 750 pixels per particle (pxl/p) to robustly assess the axial ratio and solidity of 100–120 µm grains. In this study, we used particle images with a higher pixel density, 5,000–10,000 pxl/p, to enhance the sensitivities of shape parameters. For the assessment of shape parameter sensitivity to image resolution, refer to Online Resource 1 (Fig. S1b). We also examined the responses of conventional shape descriptors from different size ranges of our samples and confirmed that there was no systematic bias caused by grain size effect. Our results support the strategy of using bulk samples adopted by Nurfiani and Bouvet de Maisonneuve (2018), referring to Online Resource 1 (Fig. S1c).

### 5.3.2 Particle shape analysis: the relationship between particle shape and eruption style

Volcanic ash from different eruption types and environments usually displays a unique range of shapes and textures that can be described by efficient morphological
descriptors, such as the shape parameters mentioned earlier (Nurfiani and Bouvet de Maisonneuve 2018; Benet et al. 2024a). We used shape parameter discrimination diagrams (Liu et al. 2015; Nurfiani and Bouvet de Maisonneuve 2018) to demonstrate the diagnostic link between ash shape and eruptive style, which was established by reference ashes of known eruptions (Fig. 10a). Our samples were plotted on these diagrams with hints of formation mechanisms, based on their morphological resemblance (Fig. 10b).

Our study on ash particles of different eruption styles showed a scattered pattern in a solidity-to-convexity diagram for samples A1-A3. This pattern exceeded the range of phreatic eruption style and covered the entire range of magmatic/phreatomagmatic eruption style (constrained by the reference ashes from Nurfiani and Bouvet de Maisonneuve 2018). However, when we only consider particles smaller than 125 µm, our samples showed a closer overlap with the range of phreatic eruption style, rather than magmatic/phreatomagmatic eruption style (Fig. 10b). In other words, in the given size range, the solidity and convexity features of our samples are more similar to those by the phreatic eruptions than those by the magmatic/phreatomagmatic eruptions.

## 5.4 Particle petrography and mineral chemistry

The mineral compositions of samples A1, A2, and A3 are highly similar. They all contain volcanic primary minerals (plagioclase, orthopyroxene, clinopyroxene, hornblende, and Ti-magnetite), sedimentary-origin minerals (quartz and orthoclase), hydrothermal minerals (halloysite, quartz, cristobalite, and alunite), and others (smectite and pyrophyllite) as identified by XRD and SEM-EDX (Table 1). Pyrophyllite was found in both A1 and A2 by SEM-EDX, while trace olivine, tridymite, barite, and zeolite identified by XRD in A2, are in question because SEM-EDX could not confirm their existence. [For XRD results of samples A1, A2, and A3, refer to Online Resource 1: Fig. S2].

We also analyzed the host lacustrine mud units, namely M1, M2, M3, and M4 (Fig. 3), in comparison to the embedded ashy layers within them. Our XRD analysis has revealed that these mud units are primarily composed of quarts, smectite, illite, halloysite, cristobalite, and feldspar. Interestingly, the mineral compositions of the mud units are very similar to those in the ashy layers. The only notable difference is the presence of illite exclusively in the mud units. [For XRD results of samples M1, M2, M3, and M4, refer to Online Resource 1: Fig. S3.]

Most lithic particles in our samples have undergone extensive mineral leaching and replacement by hydrothermal fluids, resulting in the loss of their primary textures (Fig. 6d). These particles are referred to as altered material. A small proportion of the particles retain their

primary textures, but their groundmass was altered to clays and silica (Fig. 8b and d) or leached as voids (Fig. 8b and e).

Mafic crystals, such as pyroxene, hornblende, and magnetite, occurring in lithic particles or enclosed in large feldspar crystals are typically resistant to alteration and retain their crystal shapes and compositions (Fig. 8a, b and f). Plagioclase occurring as free crystals or phenocrysts/microlites in lithic particles generally underwent alteration of varied degrees, losing major cations, specifically Ca, K, and Al. In A1 and A3, plagioclase crystals strongly eroded, forming vermicular voids and convex surfaces (Fig. 6d).

It appears that the volcanic lithic particles (especially their matrix and plagioclase microlites) have been extensively replaced by amorphous silica, kaolin mineral, and/or pyrophyllite, forming an irregular mixture without any inherited texture (Fig. 8b). The compact aggregates of silica and kaolin minerals were precipitated by epithermal fluid (Fig. 8g). In contrast, some clay-sized clasts of silica, kaolin mineral, pyrophyllite (or chlorite), and alkali feldspar were randomly oriented and well-sorted (Fig. 8h).

It is worth noting that our samples do not contain fresh lithics, which are common in most volcanic eruption products. All lithic particles in our samples have undergone hydrothermal alteration to varying degrees, giving them a pale yellow and brown color. The presence of the diatom aggregates in the ashy samples (Fig. 7) suggests that the altered lithic particles were sourced from a previously altered region where the diatoms were growing and muddy sediment was supplied. For further discussion on the origin of the ash particles, refer to Sect. 6.3.

Plagioclase crystals in our samples have a wide range of compositions. In particular, the An mol% range from 50 to 94 in A1, 36–85 in A2, and 50–85 in A3. Generally, plagioclase compositions in the TVG have lower An (35–50) (Lo 1982). The plagioclase phenocrysts of the Kuanyinshan andesite, located near the TVG, have An mol% 34–83 (Chen 1982), which are similar in composition to our samples. Alkali feldspar clasts in free crystal form are compositionally consistent. Specifically, Or mol% ranges from 50 to 85 in A1, 60–90 in A2, and 60–80 in A3. These clasts are possibly sourced from the quartzose sandstone (abundant monocrystal or polycrystalline quartz bearing trace orthoclase) of the Mushan or Wutsishan formation, which underlies the TVG (Yen et al. 1984; Huang 2005; Song et al. 2011; Lin 2013).

# 6 Discussion

## 6.1 Origins of the samples

Based on the ash componentry analysis in Sect. 5.2, it is determined that the studied samples are attributed to both volcanic and sedimentary sources. The sedimentary particles include granular quartz (in mono- or
<figure><img src="image_13.png" /><figcaption>**Fig. 8** Back-scattered electron micrographs of altered material in samples A1, A2, and A3. (a) Primary minerals, e.g. feldspar (Fsp) and groundmass in the volcanic lithic particles from A3, were pervasively decomposed to kaolin clay (Kao), silica (Si), and/or pyrophyllite with the destruction of primary texture during hydrothermal alteration. Only mafic minerals (Opx) persisted. (b) The lithic particle from A2 was largely changed in composition by alteration, but the mafic phenocrysts such as clinopyroxene (Cpx), orthopyroxene (Opx) and magnetite (Mgt) were unaffected. (c) Alunite (Alu) and sodium-alunite (Na-Alu) occurred as hydrothermal precipitation forming a part of altered material in A2. (d) Micro-crystal clusters (DC) composed of micron-sized plagioclase (dark gray), orthopyroxene (light gray), and titanomagnetite (bright white) in the highly altered volcanic lithic particle from A1. Symbols: void (v), Silica (Si), Feldspar (Fsp), Kaolin (Kao), Pyrophyllite (Prl) (e) In A1, the lithic particle's plagioclase phenocrysts and interstitial groundmass (Mx) suffered strong alteration to be replaced by clays (Prl) and silica, and the porphyritic texture remains visible. Magnetite grains (Mgt) are unaffected. (f) Mafic crystals such as clinopyroxene, orthopyroxene (Opx), and magnetite enclosed in the lithic particle of A2 are resistant to alteration to retain their forms and compositions. (g) In A1, a compact aggregate composed of silica, orthoclase (Or), and kaolin clay (Kao) occurred as epithermal products of local volcanism. (h) Alkali feldspar (Fsp), plagioclase, pyroxene (Cpx, Opx), hornblende (Hbl), silica, kaolin, and pyrophyllite (or chlorite) in the lithic grain of A2 were cumulated in random-orientation as well-sorted sediments</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 9** Secondary characteristics of altered materials and free crystals in samples A1, A2, and A3 under the binocular microscope: (a) Sample A1. Altered materials are usually oxidized to turn rusty-colored in appearance. Felsic and lithic particles were easily eroded during hydrothermal alteration to become porous in texture. Mafic crystals resisted hydrothermal alteration better than felsic ones did so they tended to remain in ordinary shapes, luster, and color. In contrast, plagioclase crystals were altered to epithermal phases, e.g. kaolin minerals, and silicon oxides, in varied degrees. (b) Sample A2. Altered materials and free crystals are quite the same as those of the A1 sample in ash componentry and morphologies. (c) Sample A3. Altered materials in A3 are similar to those in A1 and A2 regarding their components. Both A1 and A3 contain altered particles with exceptionally good roundness</figcaption></figure>

poly-crystalline form) and anhedral orthoclase, as free
particles or lithic particles, which could be related to the
quartzose units in the Mushan or Wutsishan Formation
under the TVG cover. These particles are considered
"accidental tephra", resulting from the fragmentation of
pre-volcanic basements. The volcanic components of
the samples are referred to as "accessory tephra" since no

juvenile or fresh lithic particles are present, and the lithic
particles display ubiquitous alteration of various degrees,
which denies the possibility of a cognate origin.

Tephra ejected during volcanic eruptions provides vital clues to the types of eruptions characterized by the relative proportions of juvenile and host rock fragments (Benet et al. 2021). The eruptive types are broadly defined as the following four styles: magmatic, phreatomagmatic, phreatic eruptions, and hydrothermal explosions, by the roles of magma and water played in the triggering mechanisms (Miyagi et al. 2020; Browne and Lawless 2001). For the mechanism generating the ashy components in the samples, the absence of juvenile particles denies the possibility of explosive-magma-driven fragmentation but favors the mechanism of over-heated aquifer depressurization (Stix and de Moor 2018; Miyagi et al. 2020; Alvarado et al. 2016). The samples suggest that wall rock was the only source of the ashy materials (if this is the case). It also implied that fresh eruptible magma had already been discharged and insufficient in supply rate to produce explosivity but heat. Componenty also indicates that hydrothermal activity was dominant in the volcanic system and may have played a significant role in the late explosive activity of the source region. The samples could be sourced from phreatic/hydrothermal explosions or purely hydrothermal activities. The former is related to eruptive events, while the latter may not.

Phreatic and hydrothermal explosion styles are differ-
entiated on the basis of their eruption-triggering mech-
anisms. Phreatic eruptions are typically driven by the
heating and flushing of groundwater by a magma reser-
voir, whereas hydrothermal explosions occur without
a magma heat source and are triggered by heat loss and
phase changes in a circulating hot water/steam-domi-
nated hydrothermal system (Browne and Lawless 2001).

In our case, it is challenging to determine the respon-
sible triggering mechanism for the ashy deposits.
Therefore, we have examined hydrothermal mineral
assemblages in the studied samples to constrain the
hydrological conditions of their source region. This
approach helps identify the possible source or eruption
style of the ashy deposits (see Sect. 6.2).

## 6.2 Hydrothermal mineralogy and corresponding volcanic-hydrothermal activities

Hayashi (1973) proposed criteria for distinguishing
between epithermal deposits of three volcanic alteration
zones (Quartz-Cristobalite, Kaolin-Alunite, and Smec-
tite-other clays) and non-volcanic sediments, as the fol-
lowing subsections 6.2.1~6.2.4, based on hydrothermal
mineral assemblages. By characterizing hydrothermal
mineral assemblages in samples, it is possible to evaluate
the temperature, pressure, and acidity levels of the region
<figure><img src="image_15.png" /><figcaption>**Fig. 10** Representative shape parameters (SPs) of sample particles - solidity and convexity, as two axes of scatter graph describing distinctive petrologic properties of each ash sample set: (a) Ash particles of specific eruptive styles (phreatic or magmatic) illustrate their spatial preferences in the solidity-to-convexity diagram. Reference ashes from known eruptions as follows: TP (purple circle) – Tangkuban Perahu hydrothermal explosion in 2013, Ma (blue square) – Marapi Sumatra hydrothermal explosion in 2015, Iw (green triangle) – Iwodake hydrothermal explosion in 2013, Ra (orange diamond) – Rabaul Volcanian eruption in 2014, KeD (pink circle) – Kelud Dieng Plinian eruption in 2014, KeS (red circle) – Kelud Solo Plinian eruption in 2014. (The ash samples provided by Dr. D. Nurfiani & Prof. Bouvet de Maisonneuve of NTU, Sg) (b) Particles of samples A1 and A2 sit largely within the practical range of phreatic (blue closed line) and magmatic eruption (red closed line) ashes as noted in (c). Samples A1 and A2 are more similar to the phreatic ash for their dense and right-leaning distribution pattern, less like the magmatic ash, except for their high convexity values in part exceeding the phreatic range</figcaption></figure>

from which the tephra originated, e.g. the reservoir and
wall rock.

6.2.1 **Quartz-cristobalite assemblage**

Among altered particles in all samples, some plagioclase
laths were highly corroded, leading to internal vugs or
relict casts (Figs. 6d and 8e). Their surrounding ground-
mass was replaced with silica (Figs. 6d and 8a). These
findings are consistent with the results of andesite leach-
ing experiments (Fang 2002). Under strongly acidic con-
ditions, most element compounds, especially Na, Ca,
and Al, become mobile in the acidic fluid (Nogami and
Yoshida 1995). However, silica, being relatively immo-
bile, remains in the cristobalite phase as a leached min-
eral relict or epithermal fluid precipitates as amorphous
opal, subsequently recrystallizing to cristobalite or quartz
(Chen 1961; Hedenquist et al. 2000; Fang 2002).

Under very acidic conditions in the epithermal envi-
ronment, cristobalite typically forms under 120 °C, while
hydrothermal quartz occurs above 100 °C (Fig. 11). Based
on the geological setting (Yen et al. 1984; Huang, 2005;
Song et al. 2011) and petrographic observations (grain
size, accompanying orthoclase), we suppose that the
majority of quartz grains in the samples were derived
from the quartzose unit of the underlying sedimentary
basement (the Mushan or Wutsishan Formation). Only a
small percentage of quartz as microcrystalline aggregates
came from hydrothermal precipitation or recrystalliza-
tion of cristobalite and opal. As such, the coexistence of
cristobalite and quartz in the samples indicates a stable
temperature range of around 100–120 °C (Fig. 11), as

summarized by Hayashi (1973) and Hedenquist et al.
(2000).

6.2.2 Kaolin-alunite assemblage

Halloysite, a disordered form of kaolin mineral, is present
in all samples as groundmass replacement and vug infill-
ing of the altered lithic particles. Kaolinite, a common
form of kaolin mineral, is generally more abundant than
halloysite alone in the same epithermal environment.
Kaolinite dominates kaolin minerals from around 200 °C
to below 100 °C in acidic conditions (Hayashi 1973;
Hedenquist et al. 2000), while halloysite hardly exists
above 90 °C (Hedenquist et al. 2000) (Fig. 11). Halloysite
has a preference to form under higher silica activity and
lower temperature compared to kaolinite (Kawano and
Tomita 1995) and is often accompanied by amorphous
silica deposited from altered potassic feldspar (Tsuzuki
1976; Toshio and Susumu 1978; Angelica and Santama-
rina 2005). Although halloysite and kaolinite form under
similar temperature and pH conditions, halloysite tends
to form in the early stage of hydrothermally-altered feld-
spar, followed by kaolinite when the temperature rises
(Ching 2005).

Alunite and natroalunite, the K and Na end members
of the alunite solid solution, were occasionally found as
alteration products with silica and kaolin minerals in
all samples, or formed aggregates in sample A2. They
can be precipitated in sulfuric acid conditions over a
wide range of temperatures, from below 100 °C to above
300 °C (Hayashi 1973; Hedenquist et al. 2000) (Fig. 11).
In such a strongly acidic environment, the only immobile
<table><caption>**Fig. 11** Thermal stability of various hydrothermal minerals in the epithermal environment under acid- to alkaline-pH conditions. (modified from Hayashi 1973; Taguchi et al. 2018, Hedenquist et al. 2000). Samples A1, A2, and A3 present highly-overlapping mineral constitutes (refer to Table 1) which reflect diverse stable fields from very strong acidic- to neutral-pH, below 120 °C conditions (marked by gray squares), respectively. The inconsistent conditions of mineral stability ranges presented by samples A1, A2, and A3 imply that these samples are post-eruptive mixtures by supergent processes</caption><thead><tr><th rowspan="2">Type</th><th rowspan="2">pH</th><th rowspan="2">Immobile comp.</th><th rowspan="2">Typical mineral</th><th colspan="3">Temperature (°C)</th></tr><tr><th>100</th><th>200</th><th>300</th></tr></thead><tbody><tr><td>I</td><td>Very Strongly Acidic</td><td>None</td><td>Cristobalite<br>Quartz</td><td></td><td></td><td></td></tr><tr><td>II</td><td>Sulfuric Acid</td><td>SiO<sub>2</sub></td><td>Jarosite<br>Alunite</td><td></td><td></td><td></td></tr><tr><td>III</td><td>Acidic</td><td>SiO<sub>2</sub><br>Al<sub>2</sub>O<sub>3</sub></td><td>Halloysite<br>Kaolinite<br>Dickite<br>Pyrophylite<br>Diaspore<br>Anatase<br>Rutile</td><td></td><td></td><td></td></tr><tr><td>IV</td><td>Weakly Acidic<br>Neutral</td><td>SiO<sub>2</sub><br>Al<sub>2</sub>O<sub>3</sub><br>Fe<sub>2</sub>O<sub>3</sub><br>CaO</td><td>Pyrite<br>Marcasite<br>Smectite<br>Chl/Smc<br>III/Smc<br>Chlorite<br>Illite<br>Biotite</td><td></td><td></td><td></td></tr><tr><td>V</td><td>Neutral<br>Alkaline</td><td>SiO<sub>2</sub><br>Al<sub>2</sub>O<sub>3</sub><br>Fe<sub>2</sub>O<sub>3</sub><br>CaO<br>MgO</td><td>Adularia<br>Calcite<br>Laumonite<br>Wairakite<br>Epidote</td><td></td><td></td><td></td></tr></tbody></table>

compound in primary deposits is silica. Relatively, the mobile alkaline metals and Al tend to form alunite (at median K⁺ activities) or kaolinite (at low K⁺ activities) with oxidized sulfuric acid in volcanic fluid (Schoen et al. 1974). However, their abundance should have been much more in the TVG as expected in a sulfuric fluid-dominated system in other volcanic regions (Dill 2001). It is believed that the presence of sulfate minerals such as alunite could have been diminished due to prolonged supergene processes, e.g. chemical weathering.

### 6.2.3 Smectite-other clays association

Smectite present in all samples prefers to grow under neutral pH conditions in the epithermal environment (Hayashi 1973; Hedenquist et al. 2000). At temperatures above 150 °C, smectite tends to transform into mixed-layer clays, such as illite/smectite (100–200 °C) or chlorite/smectite (> 150 °C), in weakly acidic to neutral conditions (Fig. 11). This transformation depends on whether K or Mg is dominant in parent rock

compositions (Moore and Reynolds 1997). However, no mixed-layer clays were found in our samples, indicating that the smectite-bearing particles did not experience temperatures higher than 150 °C (Fig. 11).

At temperatures below 150 °C, kaolinite-smectite is a common alteration assemblage of 100–120 °C epithermal deposits, and it is indicative of paleo-surface or paleo-water table as a hypogene-type (magmatic-hydrothermal) product (Hedenquist et al. 2000). Our samples are in the temperature-pH stability field of kaolinite-smectite assemblage. In our samples, the kaolin mineral occurred in halloysite form rather than kaolinite. It suggests that the smectite in our samples may not be related to a hypogene source. In similar cases, smectite is formed by the supergene process (subsequent weathering) in a hydrothermal zone away from an epithermal alteration core (Ece and Schroeder 2007). Therefore, we attribute the smectite in our samples to source region weathering.
### 6.2.4 Other mineral species in question

Apart from the quartz-attached orthoclase suggesting a sedimentary source, anhedral patches of alkali feldspar (Or-Ab) with Or mol% of 47–86 in A1, 60–90 in A2, and 60–80 in A3 are prevalent among the altered particles. They are not native to the TVG andesitic host rock with known mineral constitutions reported by Song (Song et al. 2011). The probable explanation for their presence is volcanic-hydrothermal fluid precipitation (Hedenquist et al. 2000). In such a scenario, they should be termed adularia, a kind of K-feldspar with a dominant {110} prism form (Nesse 1991). Adularia typically forms at temperatures above 150 °C under neutral to alkaline conditions, whereas it also develops between 80 and 150 °C (Hayashi 1973; Hedenquist et al. 2000) (Fig. 11). Calcite clasts are rare in A1 and A2, and they represent alkaline pH conditions at varying temperatures (Hedenquist et al. 2000). The neutral to alkaline pH conditions indicated by adularia and calcite contradict the acidic environment implied by the quartz-cristobalite and kaolin-alunite epithermal mineral assemblages (Fig. 11).

From the mineralogy evidence, we conclude that the studied samples have a reaction temperature between 80 and 120 °C, under strongly acidic to neutral fluid conditions. However, the contradictory fluid conditions raise doubts about the presumption of the event layers being monogenetic tephra from a single eruptive event. The alteration mineral assemblages in the samples indicate varied epithermal environments. The components and shapes of the sample particles reveal their non-magmatic nature. These samples possibly underwent subsequent supergene modification, which changed their textures and compositions and eliminated their precipitation textures (e.g. colloform or banded precipitation texture: described in Imura et al. 2019; Minami et al. 2016; made of common epithermal minerals such as sulfur, pyrite, anhydrite, gypsum, alunite, hydroxides, silica, kaolinite, pyrophyllite). By supergene modification, most sulfuric compounds may have been leached away.

## 6.3 Sedimentological clues to the formation of the ashy layers

The mineral composition of the mud units reveals the characteristics of their sediment sources. The mud units are highly similar to the ashy layers in major components (quartz, feldspar, smectite, halloysite, etc.), suggesting that the mud units were derived from the same volcanic hydrothermal environment. Illite is a common constituent mineral in sedimentary lithologies. It is abundant in the clastic rocks under the TVG and occurs strictly in the mud units but not in the ashy layers. We infer that the illite in the mud units is attributed to the clastic rock sources underlying the volcano. The illite selective deposition was likely driven by grain size winnowing of

water flows. Slow water flows, eroding landforms from the upstream provenance, brought illite to the study site, where it accumulated as mud units. The intercalated ashy layers (A1, A2, A3) were deposited by more energetic water flows that flushed the loose tephra or sediment away. The coarse and uniform grain size of the event layers reflects the nature of the high-energy water flow. Coarse-grained fragments of feldspar and quartz were preferentially deposited in the ashy layers, while finer-grained illite clasts were carried away. Other clay minerals from hydrothermal sources were still preserved for their widely present in the altered particles.

The convexity parameters of our samples are systematically biased towards higher values than known phreatic/hydrothermal ashes (Fig. 10b). The higher convexity values result from smoother outlines and higher roundness for particles, which could be attributed to the rounding effect during particle transportation to re-sedimentation. In other words, these particles are reworked sediments that settled down in the paleo-pond. Post-eruption hydrothermal alteration on the samples also promoted surface smoothing of the raw particles by subsequent clay replacement and led to a convexity values increase. However, hydrothermally-altered volcanic bedrocks nearby could be a possible source of the raw particles to exhibit similar shape features.

## 6.4 Petrogenesis implications on the ashy layers

Unlike typical ash, the ashy samples have distinguishing features, including a well-sorted, laminated texture and a normal grading structure composed of highly altered particles (Fig. 3). Additionally, the particles have high roundness and low roughness in shape. These characteristics indicate that the ashy layers conformably embedded within the parallel-laminated lacustrine mud deposits are not in-situ ash-fall tephra. Instead, they are attributed to sediment gravity flow inputs between calm water deposition. The ashy samples contained diatom-rich clasts eroded from a freshwater lake (Fig. 7). Based on ash componentry and shape analysis, these samples are highly similar to lithic particles of phreatic explosions (Fig. 10). They were likely sourced from a long-lasting lake (in the late Pleistocene) that supplied sediments to the studied locality, as evidenced by the hydrothermal mineral assemblages and displaced diatoms.

If the ashy samples are tephra from the vicinity of the sample locality, two possible non-magmatic eruption styles could have yielded tephra of epithermal characteristics similar to the samples: phreatic eruption and hydrothermal explosion. Phreatic eruptions are triggered by hot water and steam interacting with intrusive magma, while hydrothermal eruptions are triggered by the pressure buildup within a hydrothermal system (Miyagi et al. 2020; Browne and Lawless 2001).
<figure><img src="image_16.png" /><figcaption>**Fig. 12** Phase relations for water under variable pressure and temperature conditions. (modified from Mayer et al. 2015). The possible evolving stages of a proposed shallow hydrothermal reservoir in the TVG region during hydrothermal explosions is marked by solid squares **A**, **B**, and **C**. The dashed arrows show the evolving paths from **A** to **B**, and from **B** to **C**. It is assumed that groundwater in a near-surface hydrothermal system is heated to 120 °C and pressurized to 5 MPa with heat from a magma chamber in a hydrothermal explosion scenario, whose status changes from **A** to **B**. When the fluid pressure in the system overcomes lithostatic pressure, the cap rock fractures and causes a sudden pressure drop which allows water to flash to steam (whose status changes from **B** to **C**), resulting in the fragmentation of overlying and surrounding strata. Therefore, it forms a steam explosion mixing hot steam with host rock fragments</figcaption></figure>

The youngest lava flow from the Shamou dome (Song et al. 2017; Lee et al. 2023) suggests that the lava extrusion in the TVG lasted until 20 ka. Therefore, the magmatic intrusion underneath likely remained in the shallow crust during the very late Pleistocene (Song et al. 2017). The young intrusive magma body could have heated shallow hydrothermal systems and triggered phreatic eruptions or hydrothermal explosions, producing piles of hydrothermal-related debris deposits that were further resedimented.

When considering the pressure-temperature conditions for steam explosion (phreatic or hydrothermal), it is noted that the estimated depth for water to flash to steam at 120 °C (the highest alteration temperature of our samples) is very close to the ground surface (White et al. 1971; Browne and Lawless 2001; Mayer et al. 2015) and far from the expected deep hydrothermal reservoir, approximately 1–2 km in depth (Wen et al. 2012; Murase et al. 2014; Huang et al. 2017; Lin et al. 2019; Pu et al. 2021). The possible evolving stages of a proposed shallow fluid reservoir during hydrothermal explosions are illustrated as Fig. 12, which demonstrates the compression-heating (from **A** to **B**) and decompression-fragmentation (from **B** to **C**) processes on the phase diagram of water. Therefore, the proposed depth herein (tens of meters: White et al. 1971; Morgan et al. 2009) is not only inconsistent with the deep hydrothermal reservoir reported by previous studies but also insufficient to trigger a phreatic eruption. Instead, a hydrothermal explosion (if

it occurred) could have been initiated nearly at ground surface level and penetrated downwards to a great depth causing sudden pressure lowering and further brecciation of bedrock (White et al. 1971; Browne and Lawless 2001; Morgan et al. 2009).

Volcanic deposits may vary in composition or texture during hypogene or supergene alteration processes which eliminate original minerals such as sulfates, sulfides, and silicon oxides in specific conditions. Such minerals are also indicative of temperature and acidity levels (Hedenquist et al. 2000). If tephra deposits were modified by weathering between eruptions, weathering-induced oxidation/alteration could overprint existing mineral assemblages to create a simplified supergene assemblage with immobile compositions (Ece and Schroeder 2007).

## 7 Concluding remarks

Previous studies used the ashy layers at the southern foot of Mt. Shamao as volcanic tephra to depict phreatic eruptions during 17–13 ka by radiocarbon dating on finely dispersed organic materials (Belousov et al. 2010). This study argues against that the ashy layers are situ tephra, which is not indicative of volcanic eruption activity.

We propose two possibilities for the ashy layers’ origin:

1. The ashy layers are resedimented volcanoclastic deposits (McPhie et al. 1993) from epithermal alteration of volcanic bedrock. If so, they are not indicative of volcanic eruption activity.

2. The ashy layers are reworked tephra, i.e. volcanogenic sedimentary deposits (McPhie et al. 1993). The well-sorted grain size distributions and highly rounded shapes of the ashy samples suggest that the ashy particles were transported and redeposited. They could have formed by either syn-eruptive or post-eruptive floods.

The acidity ranges of the fluids equilibrated with the hydrothermal mineral assemblages in the ashy samples are inconsistent. It implies that the ashy samples do not characterize syn-eruptive but post-eruptive deposits. Therefore, we suppose that the ashy layers are reworked tephra undergoing weathering. The initial events forming the raw tephra must be older than the radiocarbon dating ages of the current resedimented deposits. In conclusion, it is unachievable to determine the ages of the eruptive activities related to the ashy layers.

## Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1007/s44195-025-00104-x.
Supplementary Material 2

Acknowledgements

This study was initiated by the 2019 Advanced Institute on Asian Consortium of Volcanology - 4th Training Camp Programme. We acknowledge Dr. Dini Nurfiani (Research Center for Geological Disaster, National Research and Innovation Agency, Indonesia) and Dr. Caroline Bouvet de Maisonneuve (Nanyang Technological University, Singapore) for kindly sharing reference ash samples used in this study. We thank Dr. Chun-Jung Chen (National Museum of Natural Science, Taichung, Taiwan) who conducted XRD experiments for our mineral identification. Beneficial discussion with Emeritus Prof Cheng-Hong Chen (National Taiwan University), Prof. Wei-Teh Jiang, Prof. Huai-Jen Yang (National Cheng Kung University) and their comments are appreciated. Constructive review comments from Editor Prof. Pei-Ling Wang, two reviewers Prof. Mei-Fei Chu and an anonymous one have improved the presentation of various aspects of this manuscript.

Author contributions

GTH: Conceptualization, Methodology, Investigation, Formal Analysis, Data Curation, Writing—Original Draft, Writing—Review & Editing. BMD: Methodology, Formal Analysis, Data Curation, Writing—Review & Editing. KLW: Writing—Review & Editing, Supervision. FC: Writing—Review & Editing, Supervision. LCW: Investigation, Formal Analysis, Data Curation. All authors read and approved the final manuscript.

Data availability

All data generated or analysed during this study are included in this published article and its supplementary information files. Any datasets used in this study but not included above are also available from the corresponding author on reasonable request.

Declarations

Competing interests

A potential conflict of interest exists due to one of the co-authors (KLW) serving as an editor for this journal. This dual role could potentially compromise the objectivity of the review and decision-making process. The journal policy related to submission with the competing interest is therefore strictly followed during the handling of the manuscript. The other authors declare that they have no competing interests.

Received: 11 September 2024 / Accepted: 10 April 2025

Published online: 17 April 2025

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