Progress in Earth andPlanetary Science

Takeuchi et al.
Progress in Earth and Planetary Science(2024) 11:47
https://doi.org/10.1186/s40645-024-00652-9

Open Access

RESEARCH ARTICLE

Unsinkable, long‑drifting,millimeter‑sized pumice of the 2021 eruptionof Fukutoku‑Oka‑no‑Ba submarine volcano
Shingo Takeuchi1* , Kosuke Ishige1, Shimpei Uesawa1 and Yukiko Suwa2

Abstract
Understanding the relationship between pumice formation and long-term floatability in seawater is becoming increasingly important in terms of eruption dynamics, material cycles, biological and environmental effects,
and ocean hazards. Pumice rafts were produced during the 2021 eruption of the Fukutoku-Oka-no-Ba submarine
volcano in the Pacific Ocean, far from the Japanese archipelago. The pumice rafts reached the Amami-Ōshima
and Okinawa Islands approximately two months after the eruption and continued to cover the sea surface at several
bays and ports, providing a great opportunity to study the characteristics of raft pumices. Sieve analysis of the floating pumice indicated that the pumice rafts characteristically contained several millimeter-sized particles with a peakat 2–4 mm. This evidence raises an important question why the millimeter-sized, Fukutoku-Oka-no-Ba 2021 pumice particles were able to float for over two months, which exceeds the floatation time shown in previous studies.
To answer this question, a porosity measurement technique for millimeter-to-centimeter-sized pumice particles
was established and applied to drifting pumice that erupted during the Fukutoku-Oka-no-Ba 2021 eruption. The
total, connected, and isolated porosities (including errors) were acquired for floating and sunken pumice particles.
As studied for the floating pumice from past submarine eruptions, most floating pumice particles contain a highamount of isolated porosity (> 30 vol%) and are thus unsinkable even when the pumice size is in the millimeterscale. This study emphasizes that skeletal density is a useful measure for the determination of pumice floatability(sinkable or unsinkable). As the particle size decreases, crystals are lost from the floating pumice particles, suggesting that the particle size of the floating pumice is affected by its petrological properties (crystal content and size).
A comparison with natural pumices from subaerial eruptions and experimental pumices from magma decompression
experiments suggests that the Fukutoku-Oka-no-Ba 2021 pumices contain abundant isolated pores due to the suppression of expansion after fragmentation by quenching in seawater, and that the relatively low to moderate crystal
content in the magma (< 17 vol%) may contribute to favorable conditions to produce abundant, millimeter-sized,
unsinkable pumice.
Keywords 2021 Fukutoku-Oka-no-Ba volcano eruption, Pumice raft, Porosimetry, Connected and isolated porosity,
Unsinkable pumice*Correspondence:
Shingo Takeuchi
takeshin@criepi.denken.or.jp
1
Nuclear Risk Research Center, Central Research Institute of ElectricPower Industry, 1646 Abiko, Abiko, Chiba 270‑1194, Japan
2
CERES, Inc., SEC Building 4F, 5‑6‑13 Namiki, Abiko, Chiba 270‑1165, Japan

1 Introduction
Pumice is a highly porous, glassy volcanic rock produced by magma vesiculation and quenching during volcanic eruptions. Pumice sometimes has a lower density
than water, producing floatability in water. In addition
to volcanic ash dispersion in the atmosphere, pumicetransport by water flow on land and in the ocean causes© The Author(s) 2024. 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 theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother 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 thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

widespread dispersion of volcanic materials to extensive areas. Recent advances in satellite imagery haverevealed the common occurrence of drifting pumicerafts that have erupted from submarine volcanoes (e.g.,
Jutzeler et al. 2014, 2020). An analysis of old document
records also has revealed the history of drifting pumice washed ashore along the Nansei Islands, Southwest
Japan, since the eighteenth century (Oikawa et al. 2023).
In some cases, drifting pumices float on the sea surface
for distances of thousands of kilometers. (e.g., Bryan et al.
2012). Therefore, understanding the relationship between
pumice formation and long-term floatability in seawater
is becoming increasingly important in terms of eruption
dynamics, material cycles, biological and environmental
effects, and ocean hazards.
Drifting pumice rafts occurred during the 2021 eruption of Fukutoku-Oka-no-Ba (hereafter referred to as
FOB) submarine volcano in the Pacific Ocean, far fromPage 2 of 19

the Japanese archipelago (Fig. 1a). The 2021 FOB eruption and drifting pumice rafts have been extensively
studied. Despite the submarine volcanoes being far fromland, the time evolution of the 2021 eruption has beenrecorded using high-resolution satellite images (Maeno
et al. 2022; Fauria et al. 2023), infrasound remote observations (Maeno et al. 2022), and ionospheric response
observations (Heki and Fujimoto 2022). Pumice rafts
drifting on the sea surface have been observed using
high-resolution satellite images (Kaneta and Chiba 2022;
Maeno et al. 2022; Fauria et al. 2023; Iskandar et al.
2023). The pumice rafts drifted thousands of kilometers
and reached the Amami-Ōshima and Okinawa Islands
approximately two months after the eruption and thenreached the coast of Southeast Asia several months later(Yoshida et al. 2022a). The arrival times of drifting pumice washed onto beaches have been recorded in detail inseveral previous studies (Konoo et al. 2022; Usami and

Fig. 1 a Location of the Fukutoku-Oka-no-Ba volcano in the Izu–Bonin Islands and Okinawa main and Amami-Ōshima Islands. b Sampling localities
in Okinawa main and Amami-Ōshima Islands

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Shinjo 2022; Yoshida et al. 2022a, b, c). Awareness of thisphenomenon has motivated simulation studies on drifting pumice rafts from the FOB and other volcanoes (Shiraki et al. 2022; Asami and Takahashi 2023; Chang et al.
2023; Nishikawa et al. 2023; Iskandar et al. 2023). Petrological studies on pumice of FOB 2021 have focused on
the eruption process from the magma chamber to theatmosphere (Maeno et al. 2022; Yoshida et al. 2022b,
2023a). Drifting pumice has impacted coastal environments and human societies. The effects of drifting andbeached pumices on coastal vegetation and aquatic life
have also been observed (Ohno et al. 2022; Akiyama et al.
2022; Sato et al. 2023). Vessels and fishing industries havesuffered severe damage from drifting pumices. The highproportion of millimeter-sized pumice particles, which
is a notable feature of the pumice rafts of FOB 2021 asshown in Sect. 6.1, were sucked into the cooling systemof vessels or into the bodies of cultured fish that mistook
the pumice particles for bait (Ohno et al. 2022). Fishing
was halted to avoid vessel breakdown, increasing economic damage (Ryukyu Shimpo 2021). The behavior offloating pumice in ports with various types of breakwaters has been observed from the viewpoint of port engineering (Nagayama and Izaki 2022; Nakata et al. 2022;
Noshi et al. 2022; Takahashi et al. 2022). Floating pumicein ports were removed to reduce the impact on vessels,
and the removal cost exceeded 1.6 billion yen as of February 2022 (Ryukyu Shimpo 2022).
Previous experimental and modeling studies on floating pumices have shown that the floatation time is related
to the particle size (Whitham and Sparks 1986; Manvilleet al. 1998; Fauria et al. 2017). The larger the pumice particles, the longer they float. Manville et al. (1998) revealed
that < 4 mm-sized pumice particles can sink within twomonths. This knowledge raises an important question
why the millimeter-sized, FOB 2021 pumice particles
were able to float for over 2 months. The key to solving
this question lies in the porosimetric characteristics ofthe floating pumice. Previous studies on drifting pumices
have revealed that they contain abundant isolated pores,
which enable them to float eternally (Kato 1987, 1988;
Manga et al. 2018; Mitchell et al. 2021), hereafter referred
to as unsinkable pumice. This feature significantly differsfrom pumices from subaerial explosive eruptions, whichhave been the main experimental samples for previous
studies on pumice floatation (Whitham and Sparks 1986;
Manville et al. 1998; Fauria et al. 2017). Although pumices from subaerial eruptions have high porosity, theyalso have high pore connectivity on average, resulting
in low isolated porosity (Colombier et al. 2017a). Basedon these previous studies, drifting pumice from the FOB
2021 eruption is a valuable case to study particle size andfloating features for precise porosimetry.

Page 3 of 19

In this study, we sampled floating pumice on the sea
surface near the land and quantified their particle size
characteristics. Subsequently, we developed methodsof porosity measurement for millimeter- to centimetersized pumice particles and obtained the total, connected,
and isolated porosities over a wide range of particle sizes.
Finally, we discuss the floatability of the pumice and its
relationship with petrological and magmatic features.

2 Definitions of terms
In this study, we discuss various pumice states. To facilitate clarity and understanding, we have defined the terms
used in this study as follows. Because this study focuseson long-drifting pumice, these definitions are for cold
pumice.
Pumice raft: Pumice particles floating in clusters on thesea surface.
Floating pumice: Pumice particles floating on the sea
surface, including both unsinkable and sinkable pumice.
Sunken pumice: Pumice particles that have sunken in
the seawater.
Beached pumice: Pumice particles washed ashore anddeposited on the beach.
Unsinkable pumice: Pumice particles floating on thesea surface and unsinkable when their connected poresare saturated because of their high isolated porosity.
Sinkable pumice: Pumice particles that can eventually sink in seawater, including floating pumice, due totrapped air in the connected pores.
The terms, floating pumice and sunken pumice, aredefined as referring to the state at the time of observation. In contrast, the terms, unsinkable pumice andsinkable pumice, are defined as the state in which theconnecting pore is saturated with seawater.
3 The 2021 eruption at the Fukutoku‑Oka‑no‑Ba
submarine volcano
The FOB is a submarine volcano in the Izu–Bonin volcanic arc, approximately 1300 km south of Tokyo(Fig. 1a), and forms the central cone of the North Fukutoku Caldera (Ito et al. 2011; Minami and Tani 2023).
Eruptions, pumice rafts, and discolored water have beenfrequently observed at the FOB since the beginning of
the twentieth century (Minami and Tani 2023). The FOB
1986 eruption was at a large scale and caused drifting
pumice that reached the Ryukyu Islands (Kato 1988).
In 2010, a seafloor survey revealed that the crater wasapproximately 30 m below sea level (Ito et al. 2011).
Maeno et al. (2022) analyzed and summarized atime series of the FOB 2021 eruption based on satellite
imagery and airborne observations. The eruption began
at 05:55 JST on August 13, 2021. The eruption is classified into four phases. Phase 1 produced a continuous

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

high eruption column with a height of 16 km for approximately 14 h. The plume was notably white. Phase 1 was
the most intense, ejecting most of the eruptive product,
and the following stages shifted to a relatively less intense
pulsating, Surtseyan activity. Pumice rafts were identified
in Phase 1 and drifted west–northwest, carried by ocean
currents. The estimated volume of the eruptive productfrom the pumice rafts and new islands around the eruptive vent, likely formed in Phase 1, is 0.1–0.4 ­km3 (0.03–
0.1 ­km3 dense rock equivalent; Maeno et al. 2022).

4 Drifting pumice of the Fukutoku‑Oka‑no‑Ba
2021 eruption
There is a record of observation of drifting raft pumice
made by a research vessel Keifu-Maru of the Japan Meteorological Agency (Japan Meteorological Agency, 2021a,b). The raft pumice was sampled on August 22, 2021,
at a location ca. 300 km northwest of the FOB volcano.
Although no particle size distribution data are available,
it is reported that millimeter- to centimeter-sized pumice
was main size in the pumice raft. According to Yoshida
et al. (2022b), on October 5, 2021, the pumice rafts
drifted west–northwest ca. 1040 km to the Daito Islands
and further west–northwest to the Amami-Ōshima andOkinawa Islands after October 10, 2021. In the summerof 2022, pumice drifted to the Pacific Ocean and Sea ofPage 4 of 19

Japan, and beached pumices were reported (Yoshida et al.
2022a). The route taken by the pumice rafts drifting fromthe FOB volcano to the Amami-Ōshima and Okinawa
Islands and the Taiwan Islands was revealed using satellite image analysis (Iskandar et al. 2023).
In the following section, we describe the appearanceof pumice rafts observed on the shore. Millimeter-sized
pumice particles were spread over the sea surface in alarge amount. Underwater and coarser pumice particles
were observed underneath a layer of millimeter-sizedparticles (Yomiuri Shinbun 2021). A similar grading
structure was observed when the pumice sample wasfloated in a laboratory tank (Fig. 2a).
A pumice raft, not constrained by its surroundings,
appears to have a relatively lower thickness. The motionof the thin pumice raft followed the wave movements on
the sea surface. However, the motion of a pumice raft,pushed onto the shore by the wind and/or ocean currents, is more resistant to wave movement. This resistive behavior is due to the high thickness of the pumice
raft, which is increased by lateral stresses from wind and/or currents. A similar response of pumice rafts to wave
motion, dependent on layer thickness, has also been
observed in high-resolution satellite images for the FOB
2021 eruption (Fauria et al. 2023), and 7 August 2019
pumice raft from the Tonga Arc (Jutzeler et al. 2020).

Fig. 2 a Grading structure of the floating pumice particles in pumice rafts reproduced in a laboratory tank. b Grading structure of the beachedpumice deposit at the coast facing Haneji-Naikai in the Okinawa main Island. c Floating and sunken pumices in a container with artificial
seawater. This image was taken on December 15, 2023, more than two years after it was sampled from the sea surface and began to be stored
in the container with seawaterTakeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Under lateral stress, the upper part of the pumice raft islifted above the sea surface because of the buoyancy of
the thicker underwater part. The particles in the upper
part of the raft were in contact with each other in the
air. The more intense mechanical interaction between
the pumice particles in the air can produce a higher yieldstrength in the upper part of the pumice raft. The yieldstrength of thick pumice rafts is evidenced by the presence of small objects on them, such as crabs (Akiyama
et al. 2022).
The appearance of beached pumice deposits depends
on the amount of pumice, beach orientation, and direction of wind and currents (Yoshida et al. 2022b). Crosssections of a massive, beached pumice deposit in theinland sea, where the waves were calm, showed a grading structure (Fig. 2b) similar to that observed in thesea (Yomiuri Shinbun 2021) and in the tank (Fig. 2a).
Beached pumice deposits returned to the sea during hightide. Therefore, the sampled pumices from the sea surface
near the shore in this study might not float on the sea
surface for the entire period from eruption to sampling
and have been washed ashore several times.
Pumice rafts from the FOB 2021 eruption contain various petrological types of pumice. Yoshida et al. (2022b)
classified pumice into six types based on color and texture. The most abundant type (> 90%) was gray pumice.
The other minor types were black, brown, pale gray,amber, and streaky. The gray pumice consists of a grayvesicular groundmass and phenocrysts.

5 Methods
5.1 Sampling

In this study, pumices were sampled from the sea surface to determine the particle size distribution of floating pumice in pumice rafts. The sampling locations
and dates are shown in Fig. 1b and Table 1. Two methods were adopted to collect floating pumice samples:
scooping pumice particles with seawater using a container (IKB1, SNB1&2, and SNB3), and scooping pumice particles using a sieve or pole net (IKB2, YGC1, and
YGC3). In the former method, a submerged container
was quickly lifted to sample floating pumice on the sea
surface of shallow water near the shore. In the lattermethod, a collector lowered a sieve with a 500–800 µm
mesh opening and a diameter of 18.5 cm from a bridgeover an inland sea. The sieve was submerged in the sea
and quickly lifted to sample the floating pumice. Insteadof the sieve, a pole net with a 200–350 µm mesh opening
and a diameter of 27 cm was extended from the shore to
sample floating pumice on the sea surface. Some pumice samples were collected from floating pumice (GGAand MKY) or beached pumice deposits (ABU, SNB4,

Page 5 of 19

Table 1 Sampling locality and date, sample appearance andsampling method
Name

Latitude Longitude SamplingdateAppearance

MethodaABU

26.5384

128.0921

19-Oct-21

BeachedHS

IKB1

26.3877

127.9910

20-Oct-21

Floating

C

IKB2

26.3877

127.9910

20-Oct-21

Floating

PN

KMT

28.3197129.5356

27-OctFloating
to 1-Nov-21

PSb

YGC1

26.6445128.0327

01-Nov-21

Floating

S

YGC3

26.6445128.0327

01-Nov-21

Floating

S

SNB1&2 26.1763

127.830703-Nov-21

Floating

C

SNB3

127.830703-Nov-21

Floating

C26.1763

SNB4

26.1763127.830703-Nov-21

BeachedHS

MKY

26.6373

128.0348

08-Mar-22

Floating

HS

GGA​

26.6318

128.0117

08-Mar-22

Floating

HS

SNB5

26.1763127.8307

10-Mar-22

BeachedHSa

Pumice scooped by a container with seawater (C), a hand shovel (HS), a polenet (PN), a power shovel (PS) and a sieve (S)bPumice removed for port restoration was acquired from port officials

and SNB5) to obtain sufficient samples or large particles.
A sample of pumice removed for port restoration was
acquired from port officials (KMT). The samples werelifted with the seawater (IKB1, SNB1&2, and SNB3) andstored with seawater in the laboratory. We reproduced
the floating of a sample lifted using a pole net (IKB2)
in a laboratory tank containing artificial seawater. The
pumice particles floated at the time of sampling; however, over time, a small number of pumice particles sankto the bottom of the container (Fig. 2c). Such pumices
were termed sunken pumices and were also analyzed.
Samples of IKB1, SNB1&2, SNB3 and IKB2 were stored
in containers with seawater for the duration of 16, 15, 13
and 35 days, respectively. Pumice particles in these samples were then determined whether floating or sunken
and prepared for density measurements. GGA, MKY
and YGC3 were sampled as floating pumice at the timeof sampling in the field. All particles used for the measurements in this study were gray pumice, as classified
by Yoshida et al. (2022b) and Konoo et al. (2022). Density measurements were performed on samples in thenatural state of the pumice particles without shaping orprocessing.
Large pumice particles for the polished sections (11035C-GP1, AB3a-Tim1_V, and AB3a-Tim1_P) were sampled from beached pumice deposits (SNB5 and ABU) in
addition to the sections (AM12-1 and AM12-2) of pumices provided by port officials (KMT). After resin impregnation to pores by using a vacuum, the polished sectionsof pumice were produced.

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

5.2 Particle size analysis

Sieving of the sampled floating pumices was performed to obtain their particle size distributions on a
Φ scale at an interval of 0.5 Φ. The Φ scale is definedas Φ = − ­log2D/D0, where D and D­ 0 are the particle
diameter (mesh opening) in millimeters and the reference diameter (1 mm), respectively (Krumbein 1934;
McManus 1963). Before sieving, the samples were
soaked in tap water for one to several days and dried at
110 °C in an oven. A low quantity of impurities (plantfragments and seaweed) were removed during soaking and drying. Any particles that sank during soaking
in tap water were collected to avoid loss. Dried samples
with an initial mass of 40–500 g were manually sieved.
Completeness of sieving down to − 1 Φ (2 mm in particle diameter) was confirmed by the naked eye. The
samples were then sieved using a shaking sieve systemwith an electromagnetic drive (Vibratory Sieve Shaker
ANALYSETTE 3 PRO, Fritsch GmbH), with the smallest
vibration amplitude (0.5 mm). Through a time run withseveral time intervals, ranging from several tens of seconds to several minutes, the end of the sieve analysis wasdetermined by checking for a mass change of each sieveclass within 1% of the initial sample mass.
5.3 Porosimetry

In this section, the density measurement methods arebriefly summarized. The details of the measurements
using gas and pseudofluid displacement methods aredescribed in the Supplementary Materials.
5.3.1 Definition and calculation of porosity

Considering the porosimetric properties of pumice particles, the total porosity, connected porosity, and isolated
porosity of pumice particles are defined (e.g., Yokoyama
and Takeuchi 2009). Pores that are opened to the outsideand closed inside a pumice particle are called connected
and isolated pores, respectively. The volume fractions of
the connected and isolated pores in the volume of thepumice particle are defined as connected and isolated
porosities, respectively. The total porosity is the sum ofconnected and isolated porosities.
Porosity was calculated based on the measured densities of the samples. For porosity calculation, the density of the gas filling the pores was assumed to be zero
because it is negligible compared to the solid density.
The solid density, ρs, is the density of the solid phase in a
pumice particle consisting of crystals and glass. Bulk density, ρb, is the density of a pumice particle consisting of
the solid phase and the total pores. Skeletal density, ρs+i,
is the density of the combined portions of solid phase andisolated pores inside the pumice particle (e.g., Michol
et al. 2008). The skeletal density is useful because it is the

Page 6 of 19

primary density output in the measurement of the gas
displacement method (Sect. 5.3.3).
Following Michol et al. (2008), the total porosity, φt, iscalculated using the bulk and solid densities as follows:φt = 1 −

ρb
ρs

(1)

The connected porosity, φc, is calculated using the bulk
and skeletal densities as follows:

φc = 1 −

ρb
ρs+i

(2)

The following relationship exists among total, connected, and isolated porosities, φi:φt = φc + φi(3)

5.3.2 Measurement of bulk density

Volume measurement is a significant hurdle in obtaining the bulk density of pumice particles. The Archimedes’
method is the simplest for measuring the volume of an
object. However, the Archimedes’ method cannot measure the volume of pumice particles with connected pores
into which water can infiltrate. To overcome this technical hurdle, several methods for the surface coating of
pumice particles have been proposed (e.g., Houghton andWilson 1989). The pseudofluid displacement method is
also effective in measuring the volume of porous materials such as pumice. In the pseudofluid displacement
method, a sample is placed in a container with a pseudofluid such as glass beads. The volumes change before
and after sample placement are measured. The glass
beads method (Sasaki and Katsui 1981) is well known in
volcanology and is used to measure the bulk density of
pyroclasts (Shimano and Nakada 2006; Nakamura et al.
(2008); Colombier et al. 2017b). It was assumed that thepseudofluid does not infiltrate the connected pores ofthe pumice particles. Recently, a precise volume measurement instrument (GeoPyc 1365, Micromeritics) has
enabled us to measure volumes using the pseudofluid
displacement method and has been used in volcanology (e.g., Pardo et al. 2012; Heinrich et al. 2020). In thisstudy, the volume of the pumice particles was measured
using a GeoPyc 1365 installed at the Central ResearchInstitute of Electric Power Industry. A dedicated DryFlo
powder (40–280 µm in particle diameter) and FGB40
glass beads (355–500 µm in particle diameter) were usedas the working pseudofluids, whose details are describedin Section S1.2 in the Supplementary Materials. Whilemeasurements in previous volcanological studies haveoften been conducted on centimeter-sized particles, wehere measured millimeter-sized particles. VolumetricTakeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

measurement methods were divided into measurements
for a single-particle sample if the volume of a particle
was > 0.1 ­cm3 and measurements for a multiparticle sample if the volume of a particle was < 0.1 ­cm3. In the multiparticle sample measurements, the average volume anddensity of the multiparticles were measured. The validityof the measurements using GeoPyc 1365 was examinedbased on a comparison with values from the Archimedes’
method, and thus, non-porous reference objects of various shapes and volumes from millimeters to centimetersin size were measured (Tables S2 and S3). Based on thiscomparison, we corrected the systematic errors observedin the measurements of a single-particle sample with < 1­cm3 and multiparticles. Eventually, we concluded that thebulk density measurements in this study had maximum
relative errors of 3 and 5% for single- and multiparticle
samples, respectively (Figs. S1 and S2). The larger the
sample volume, the smaller the relative error in the bulk
density measurement of 1.5%.
5.3.3 Measurement of skeletal density

In volcanology, gas displacement method (gas pycnometry) is the standard method for measuring the skeletal
density of porous volcanic materials. The skeletal volume,
defined as the sample mass divided by the skeletal density measured by the gas displacement method, does notinclude the connected pores through which the working
gas can enter and exit. In this study, the skeletal volumeof the pumice particles was measured using a Quantachrome Penta-Pycnometer at the Central ResearchInstitute of Electric Power Industry. Helium was used as
the working gas. As well as the pseudofluid displacement
method, we examined the validity of density measurements in the gas displacement method for small objects
by comparing the volume values of non-porous, small
objects obtained using the Archimedes’ method. We
evaluated the volume detection limit to 0.1 ­cm3 (Fig.
S3a). The maximum relative error of the skeletal density
measurements was 40% for 0.1 c­ m3 in skeletal volume.
The larger the skeletal volume, the smaller the relative
error in skeletal density measurement at < 1% (Fig. S3a,b and c). The error in gas displacement method depends
mostly on the volume of sample compared to the volumeof the cell used. The same criteria for bulk density measurements were used to classify measurements of singleand multiparticle samples.

Page 7 of 19

were the same as those prepared for X-ray fluorescence(XRF) analyses. Solid density measurements were difficult to obtain for sunken pumice because a sufficientamount of sieved samples could not be obtained. Therefore, the relationship between the solid density and particle volume of floating pumice was also applied to sunken
pumice. The relative error of the solid density measurement was evaluated based on the sample volume using
the same functions as for the skeletal density measurement using the gas displacement method (Section S1.3).
5.4 Petrological analysis

As shown in Sect. 6.4 later, we observed a change in solid
density with particle size. To examine the change in solid
density in terms of change in crystal content, we estimated crystal size and content in pumice by image analysis of elemental and electronic images, and mass balancecalculations of the ­K2O content among the bulk rock,glass, and crystals, respectively. Methods for image analysis for pumice sections and chemical analysis for bulk
rock, glass and crystals are described in the Supplementary Materials.

6 Results
6.1 Particle size distribution

The particle size distributions of the floating pumicesamples were obtained from four sites (IKB1, SNB1&2,
YGC1, and YGC3; Table 1). There was no significant difference between the sampling methods, particularly withrespect to smaller particle sizes (Fig. 3). Results show thatthe number of particles that escaped through the sieve
mesh was negligible. Data from all sites showed a primary peak of –1 Φ to − 2 Φ, that is, 2–4 mm in diameter(Fig. 3). A decrease on the finer side down to 1 Φ (0.5 mm

5.3.4 Measurement of solid density

Solid density was measured for a sufficiently pulverized
powder of pumice particles using the gas displacement
method, a Quantachrome Penta-Pycnometer at the Central Research Institute of Electric Power Industry. The
powdered samples used for solid density measurements

Fig. 3 Particle size distributions of the floating pumice samples

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

in diameter) was also common. By contrast, samples
with or without a second peak on the coarser side were
present.
6.2 Bulk density

The bulk densities of all floating pumice particles were
below that of the density of seawater (Fig. 4a, Table 2
and Table S1). Floating pumices > 0.1 ­cm3, measured asa single-particle sample, have a wide range of bulk densities from 0.2 to 0.8 g/cm3 at an average of 0.56 g/cm3.
Floating pumices <  − 2.5 Φ (5.6 mm in diameter and0.095 ­cm3 in equivalent sphere volume), measured as amultiparticle sample, have bulk density values decreasingfrom 0.7 to 0.5 g/cm3 with decreasing particle volume.
However, this decrease was within the density range ofparticles > 0.1 ­cm3. Sunken pumices, measured as singleparticle samples > 0.2 ­cm3, have bulk densities around
0.6 g/cm3 overlapping those of floating pumices. Sunken

Page 8 of 19

pumices <  − 3.0 Φ (8 mm in diameter and 0.27 c­ m3 inequivalent sphere volume), measured as multiparticle
samples, have higher bulk densities (0.7–0.8 g/cm3) thanthose of floating pumice particles with a similar particle
volume.
6.3 Skeletal density

The skeletal densities of most floating pumice particles
were lower than that of the seawater (Fig. 4b, Table 2
and Table S1). Most floating pumice particles > 0.1 ­cm3
have a range of skeletal densities from 0.7 to 1.0 g/cm3
at an average of 0.88 g/cm3. A large floating pumice
particle with a skeletal density of 1.2 g/cm3 higher thanthat of seawater was found. Floating pumice <  − 2.5 Φ(5.6 mm in diameter and 0.095 ­cm3 in equivalent spherevolume) tends to have a non-variable skeletal densitywith decreasing particle volume, which is lower thanthe density of seawater. Sunken pumices, measured as asingle-particle sample, > 0.2 ­cm3, have skeletal densitiesof 1.1–1.3 g/cm3; however, some data have large errors
owing to the small skeletal volume. These skeletal densities were higher than those of floating pumice particles and seawater. Sunken pumice particles, measured
as a multiparticle sample, <  − 3.0 Φ (8 mm in diameterand 0.27 c­ m3 in equivalent sphere volume) tend to haveincreasing skeletal densities from 1.1 to 1.4 g/cm3 withdecreasing particle volume. However, notably, the sunken
pumice with a diameter of 0 to − 1 Φ (1–2 mm in diameter and 5.2 × ­10−4 to 4.2 × ­10−3 ­cm3 in equivalent spherevolume) has a large error owing to the small skeletalvolume.
6.4 Solid density

Solid densities of the floating pumice monotonously
increase from 2.5 to 2.8 g/cm3 with increasing particle size from 1.0 to − 5.5 Φ (0.5–45.8 mm in diameter)(Fig. 5a and Table 3). Yoshida et al. (2022b) classified the
pumices of FOB 2021 as trachyte (alkaline magma) basedon their bulk rock composition. Although there is a large
difference in the chemical variation between the alkaline
and sub-alkaline magmas, the solid density of the trachyte of FOB 2021 is close to that of andesitic to dacitic
rocks (sub-alkaline magma), which have a similar bulk
­SiO2 content (Takeuchi et al. 2021). Fitting a linear function with this relationship between solid densities andsize in the Φ scale (Fig. 5a), we obtain an empirical linear function that can be used to calculate the relationshipbetween particle volume and solid density for the FOB
floating pumice as follows:
Fig. 4 a Bulk density and b Skeletal density vs. particle volume.
A horizontal blue line indicates an assumed range in the densityof seawater (1.02–1.03 g/cm3), based on Wadachi (1987)ρs = −0.0413� + 2.51

(4)where Φ is the particle diameter in the Φ scale (Sect. 5.2).
In the calculation of solid density using Eq. (4) forTakeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Page 9 of 19

Table 2 Porosimetrical data of floating and sunken pumice erupted in the Fukutoku-Oka-no-Ba 2021 eruption
Name

ParticlevolumeBulkdensity

SkeletaldensitySoliddensityTotalporosity

Connected
porosity

Isolated
porosity

Pore connectivity

Saturation
density(cm3)(g/cm3)(g/cm3)(g/cm3)(vol%)(vol%)(vol%)(vol%)(g/cm3)0.25–0.74(0.58)

0.68–1.22(0.89)

2.52–2.77(2.62)

73–91 (78)

20–80 (34)

11–58 (44)27–88 (43)0.76–1.06(0.93)

Floating
pumicea
IKB1-flt
4 mm

0.06b

0.62

0.91

2.60

76

32

44

42

0.94

IKB1-flt
2 mm

0.007b

0.63

0.91

2.56

76

32

44

42

0.95

IKB1-flt
2 mm-20.007b

0.62

0.91

2.56

76

32

44

42

0.95

IKB1-flt
1 mm

0.0009b

0.55

0.87

2.52

78

37

41

47

0.93

IKB2-flt1

0.52

0.57

0.91

2.65

78

37

41

47

0.95

IKB2-flt4

0.61

0.64

0.93

2.65

76

31

45

41

0.96

IKB2-flt5

0.55

0.44

0.81

2.65

83

45

38

55

0.91

IKB2-flt7

0.43

0.64

0.91

2.64

76

30

46

39

0.95

IKB2-flt8

0.33

0.67

0.83

2.64

75

20

55

27

0.87

IKB2-flt14

0.15

0.61

0.84

2.62

77

27

50

35

0.89

YGC3-flt
4 mm

0.06b

0.66

0.93

2.60

75

29

46

39

0.95

YGC3-flt
4 mm-2

0.06b

0.65

0.93

2.60

75

30

45

40

0.96

YGC3-flt
2 mm

0.007b

0.63

0.90

2.56

75

30

46

39

0.93

YGC3-flt
1 mm

0.0009b

0.54

0.86

2.52

78

37

42

47

0.92

YGC3-flt
1 mm-20.0009b

0.53

0.86

2.52

79

39

40

49

0.93

SNB1&2-flt
4 mm

0.06b

0.65

0.90

2.60

75

28

47

38

0.94

SNB1&2-flt
2 mm

0.007b

0.63

0.89

2.56

75

29

46

38

0.93

SNB1&2-flt
1 mm

0.0009b

0.55

0.91

2.52

78

39

39

50

0.96

SNB3-flt1

1.28

0.53

0.68

2.67

80

22

58

27

0.76

SNB3-flt3

7.11

0.60

0.78

2.70

78

22

55

29

0.83

SNB3-flt2

0.30

0.68

0.86

2.64

74

21

53

28

0.90

SNB3-flt5

0.55

0.51

0.75

2.65

81

32

49

40

0.84

SNB3-flt6

10.00

0.74

0.99

2.71

73

25

47

35

1.00

MKY-flt1

8.42

0.51

0.99

2.70

81

49

32

60

1.01

MKY-flt2

2.73

0.64

0.90

2.68

76

29

47

39

0.94

GGA-flt1

20.25

0.25

1.22

2.72

91

80

11

88

1.06

GGA-flt2

18.23

0.51

0.73

2.72

81

30

51

37

0.82

GGA-flt3

27.87

0.46

0.83

2.73

83

45

38

54

0.92

GGA-flt4

26.61

0.48

1.00

2.73

82

52

30

63

1.02

0.56―0.75(0.64)

1.05―1.35(1.17)

N.D.c

71―79 (75)

30―55 (44)24―40 (31)43―70 (59)1.04―1.19(1.10)Sunkenpumicea
IKB1-snk1

0.40

0.63

1.29

2.64

76

51

25

67

1.15

IKB1-snk2

0.23

0.58

1.10

2.63

78

48

31

61

1.06

IKB1-snk4

0.25

0.56

1.24

2.63

79

55

24

70

1.12

IKB2-snk2

0.27

0.57

1.18

2.63

78

52

27

66

1.10

SNB3
snkLb-1

0.96

0.61

1.14

2.66

77

46

31

60

1.09

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Page 10 of 19

Table 2 (continued)
Name

ParticlevolumeBulkdensity

SkeletaldensitySoliddensityTotalporosity

Connected
porosity

Isolated
porosity

Pore connectivity

Saturation
density(cm3)(g/cm3)(g/cm3)(g/cm3)(vol%)(vol%)(vol%)(vol%)(g/cm3)

SNB3 snkLb
4mm-1

0.09b

0.70

1.05

2.61

73

34

40

46

1.04

SNB3 snkLb
4mm-2

0.09b

0.69

1.06

2.61

74

36

38

48

1.05

SNB3 snkLb
2mm

0.01b

0.75

1.08

2.57

71

30

40

43

1.06

SNB3 snkLb
1mm

0.001b

0.70

1.35

2.53

72

48

24

66

1.19

a

Lines in italics represent ranges and averages in a bracketbParticle volume calculated from sphere volume equivalent to median Φ scale in the sieve classc

Not determined

multiparticle samples, the median Φ values of the sieveclass were adopted. The error of solid density was simply and conservatively set to 0.1 g/cm3 for all particles,
considering scattered data with errors in Fig. 5a. Equation (4) was also applied to calculate the solid density ofthe sunken pumice.
6.5 Total, connected and isolated porosities

Fig. 5 a Solid density and b crystal content vs. particle sizeof the floating pumice particles

The total and connected porosities of the pumice were
calculated by applying the bulk (Sect. 6.2), skeletal(Sect. 6.3) and solid (Sect. 6.4) densities to Eq. (1) and (2),respectively (Table 2 and Table S1). By applying the total
and connected porosities to Eq. (3), the isolated porosityof the pumice can be calculated (Table 2 and Table S1).
The total porosities of floating and sunken pumice
are 73–91 vol%, with an average of 78 vol%, and 71–79
vol%, with an average of 75 vol%, respectively (Fig. 6a).
The connected porosities of floating and sunken pumice are 20–80 vol%, with an average of 34 vol%, and
30–55 vol%, with an average of 44 vol%, respectively(Fig. 6b). Owing to the decrease in the skeletal volume,the errors in the connected porosity measured as a single-particle sample increased with decreasing particle
volume. The isolated porosities of floating and sunken
pumice are 11–58 vol%, with an average of 44 vol%, and
24–40 vol%, with an average of 31 vol%, respectively(Fig. 6c). A particle with uniquely high total porosityof 91 vol% and connected porosity of 79 vol%, corresponding to the only floating pumice particle with askeletal density higher than that of seawater (Fig. 4b)
was observed.
6.6 Bulk rock composition

single-particle samples, the particle diameter was calculated from its measured volume assuming a sphere withequivalent volume, and converted to the Φ scale. For

All bulk rock composition (Table S4) is shown on an­K2O-SiO2 diagram normalized on an N
­ a2O-free basis(Fig. 7a) and ­K2O on an N
­ a2O-free basis-Φ diagram

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Page 11 of 19

Table 3 Solid density and crystal content of sieved samples of floating pumice erupted in the Fukutoku-Oka-no-Ba 2021 eruption
Name

Particle size

Massa

Volumea

Solid density

Error

Crystal content

Error

(Φ)

(g)

(cm3)

(g/cm3)(g/cm3)

(wt%)

(wt%)

SNB5flt − 5.0φ

− 5.0 to − 5.5

4.34

1.57

2.76

0.08

15

4

SNB5flt − 5.0φ

− 5.0 to − 5.5

4.05

1.49

2.71

0.08

14

4

SNB5flt − 5.0φ

− 5.0 to − 5.5

4.35

1.57

2.77

0.08

14

4

SNB1&2flt − 4.0φ

− 4.0 to − 4.5

4.86

1.82

2.67

0.07

18

4

SNB1&2flt − 3.0φ

− 3.0 to − 3.5

3.10

1.17

2.66

0.10

25

4

SNB1&2flt − 3.0φ − 2− 3.0 to − 3.5

4.48

1.69

2.65

0.07

20

4

YGC3flt − 3.0φ

− 3.0 to − 3.5

3.51

1.32

2.66

0.09

21

4

IKB1flt − 2.5φ

− 2.5 to − 3.0

3.71

1.41

2.64

0.08

15

4

SNB1&2flt − 2φ

− 2.0 to − 3.0

3.16

1.22

2.59

0.10

16

4

YGC3flt − 2.0φ

− 2.0 to − 2.5

2.94

1.14

2.58

0.10

19

4

IKB1flt − 2.0φ

− 2.0 to − 2.5

2.93

1.15

2.55

0.10

12

4

SNB1&2flt − 1φ

− 1.0 to − 2.0

2.99

1.17

2.55

0.10

11

4

YGC3flt − 1.0φ

− 1.0 to − 1.5

2.92

1.14

2.57

0.10

12

4

IKB1flt − 1.0φ

− 1.0 to − 1.5

2.89

1.13

2.55

0.10

8

4

YGC3flt − 0.5φ

− 0.5 to − 1.0

3.87

1.49

2.59

0.08

9

4

YGC1flt − 0.5φ

− 0.5 to − 1.0

4.95

1.93

2.56

0.06

8

4

SNB1&2flt 0φ

0.0 to − 1.0

3.07

1.21

2.53

0.09

8

4

SNB1&2flt 1φ

1.0 to 0.0

2.98

1.18

2.52

0.10

4

4

a

Mass and volume of powder samples used in the solid density measurements(Fig. 7b). The normalization on an N
­ a2O-free basis wasapplied to reduce the effects of inadequate cleaning of sea
salt in the samples (Section S2.2 in the Supplementary
Materials). Data on the FOB gray pumice from Yoshida
et al. (2022b) were shown with the same normalization
on an N
­ a2O-free basis. Data on pumice particles with adiameter of − 2 to − 5 Φ (from 4 to 32 mm in diameter)overlap with or are close to those of Yoshida et al. (2022b).
As the particle size decreases from − 2 to 1 Φ (from 4 to
0.5 mm in diameter), the ­K2O content increases with the­SiO2 content and Φ (Fig. 7).
6.7 Crystal and groundmass glass composition

The average chemical compositions of crystal phaseswith modal fractions in whole crystals and assumed
crystal densities to convert area fraction to weightfraction are shown in Table S5. Although there was no­K2O-rich phase, plagioclase contained trace amountof ­K2O because of its high albite content. The compositions of the groundmass glasses are listed inTable S5. The crystal and groundmass compositions
were normalized on an ­Na2O-free basis for mass balance calculations to estimate the crystal content. The
whole crystal and groundmass compositions are alsoplotted on the S­ iO2–K2O diagram on an ­Na2O-freebasis as a graphical expression of the mass balance(Fig. 7a).

6.8 Crystal size and content

We estimated the typical crystal size by analyzing backscattered electron and elemental images of pumice crosssections with an area of 6.2–7.2 c­m2. The area of theindividual crystal was approximated as an ellipse using
the ImageJ software, and the crystal size was defined as
the median value between the major and minor diameters of the ellipse. The size of the aggregated crystals
was regarded as a grain of crystal. The crystal size distributions of the five gray pumice samples are shown in
Fig. 8. Crystals with median diameters of 100 to 3000 µm
account for > 90% of the total crystals, indicating thatmost crystals should be called phenocrysts in the light ofthe definition summarized in Takeuchi et al. (2021).
The crystal content was based on a mass balance calculation for the K­ 2O content on an N
­ a2O free basis among
bulk rock, glass, and crystals (Fig. 5b). Results showedthat the crystal content decreased from 25 to 5 wt% as
the particle size of the floating pumice decreased. Owing
to the slightly higher outlier data at − 3.0 Φ (8 mm indiameter), it appears as if the maxima of the crystal content (ca. 25 wt%) were considered at − 3.0 Φ. In addition,
two − 3.0 Φ data points with a lower crystal content (ca.
20 wt%) were present. Considering errors in crystal content, the higher data at − 3.0 Φ may be due to the heterogeneity in the crystal content visible at pumice particle
sizes close to the maximum crystal size.

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Fig. 6 a Total, b connected, and c isolated porosities versus particle volume

Page 12 of 19

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Page 13 of 19

Fig. 8 Crysal size distribution in the centimeter-sized FOB pumices.
Median diameter indicates the median value between the longand short axes of ellipses fitted to crystals in the sections

Fig. 7 a ­K2O vs. S­ iO2 contents on an ­Na2O-free basis. This figure
shows mass balance among the bulk rock composition of the sieved
FOB 2021 floating pumice (open circles), averaged compositionof groundmass glass (cross), and averaged composition of wholecrystals (square). Solid circles represent bulk rock compositionof the gray pumices from Yoshida et al. (2022b). b ­K2O content
on an ­Na2O free basis vs. particle size of sieved floating pumice.
Shaded range represent bulk rock composition of the gray pumicesfrom Yoshida et al. (2022b)

7 Discussion
7.1 Mostly unsinkable and only a few sinkable pumicesin the floating pumice

Based on the measured porosities, the saturation density
was calculated to examine the floatability of the pumicein seawater (Fig. 9). The saturation density is defined as
the density assuming that the connected pores of thepumice are saturated with seawater, assuming that thedensity of seawater is 1.025 g/cm3, based on Wadachi(1987). Most floating pumice particles have saturation densities lower than that of seawater, and are thus
unsinkable. One floating pumice particle with a saturation density higher than that of seawater is expected toeventually sink once the air trapped in the connected
pores is lost (Fauria et al. 2017), and is thus sinkable. The
proportion of unsinkable and sinkable pumices in floating pumice particles is difficult to quantify precisely, andcan only be roughly estimated. During the sample selection of a floating pumice particle, it is unknown whether
the pumice is sinkable or unsinkable. Therefore, the sample selection in this study was random. Under the random selection conditions, only one sinkable pumice wasfound among the 26 samples. This suggests that, in thestudied particle volume range, only a small quantity ofthe sinkable pumice floated with the help of trapped air
in the connected pore.
The relationship between skeletal density and seawater
density essentially determines the unsinkable or sinkable pumice (Fig. 4b). If a pumice particle has a skeletal
density less than that of seawater, it should be consideredunsinkable. Therefore, the determination of unsinkableor sinkable pumice can be performed based solely on
the skeletal density, as measured by the gas displacement
method. Despite the availability of connected porosity
data in many previous studies, skeletal density data are
yet to be explicitly stated. Therefore, we emphasize thatskeletal density should be explicitly shown as an effective measure of eventual pumice floatability (sinkable orunsinkable).
7.2 Influence of crystals on pumice floatability and size

Porosimetric and petrological data indicate that petrological properties influence the relationship between floatability and pumice size. When the pumice size becomes
down and close to the typical crystal size (Fig. 8), theTakeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Page 14 of 19

Fig. 9 Saturation density vs. particle volume. A horizontal blue line indicates an assumed range in the density of seawater (1.02 to 1.03 g/cm3),
based on Wadachi (1987)

crystal content of floating pumice decreases with pumice
particle size (Fig. 5b). We interpreted that pumice containing fewer crystals and a more pumiceous groundmass
glass could selectively survive as floating pumice on thesea surface when the pumice size approaches the typicalcrystal size. The pumiceous groundmass glass plays animportant role in reducing the skeletal density as isolated
pores exist in the glass. In contrast, we interpreted thatcrystal-rich, millimeter-sized pumice particles or mostly
crystal fragments with still pumiceous glass around sankinto the sea, and were not observed as floating pumice
particles. The almost absence of floating pumice smaller
than 1.0 Φ (0.5 mm in diameter) (Fig. 3) suggests that
almost all pumices smaller than 1.0 Φ sank due to losing
isolated pores and were not observed as floating pumice
particles.
7.3 Comparison with other natural and experimental
pumicesComparison with existing porosity data for natural
pumice originating from subaerial and submarine eruptions and for experimental pumice from decompression

experiments provides an interpretation that rapid
quenching of the FOB 2021 magma with relatively low
to moderate crystal content by the seawater produced
unsinkable pumice with abundant isolated porosity.
Figure 10a shows the relationship between total
porosity and pore connectivity for pumices from the
FOB 2021 and other submarine eruptions. Pore connectivity is defined as the connected porosity divided
by the total porosity (Colombier et al. 2017a). The permeable porosity measured using water pycnometry in
Kato (1987, 1988) was regarded as connected porosity.
The FOB 2021 floating pumice particles have high total
porosity and low pore connectivity, similar to part ofthe beached pumice of the Iriomote 1924 (Kato 1987)
and FOB 1986 eruptions (Kato 1988), and the raft pumice (RP) of the Havre 2012 eruption (Manga et al. 2018;
Mitchell et al. 2021). However, sinkable pumice particles with high total porosity and pore connectivity werecommon in the raft pumice of the Havre 2012 eruption.
The FOB 2021 sunken pumice particles were plottedin the region between the raft pumice and ash-lapilliblock deposits (ALB) sampled on the sea floor proximal

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

to the vent of the Havre 2012 eruption. Incorporating
the compiled data (subaerial pumice and scoria) from
Colombier et al. (2017a), Fig. 10a shows that most subaerial pumice and scoria have high pore connectivity ofmore than 70% and are plotted in the region of sinkable
pumice and quite different from the FOB 2021 unsinkable pumice and other unsinkable pumices from submarine eruptions, as noted in a study of the Havre 2012
eruption (Mitchell et al. 2021).
Data of experimental pumices quenched in the decompression experiments of vesiculating magmas are alsoplotted on the total porosity-pore connectivity diagramwith data on pumice of FOB 2021 (Fig. 10b). The total
and connected porosities were measured for quenchedsamples of vesiculated, hydrous silicic melts without
crystals (Takeuchi et al. 2009) or with crystals (Graham
et al. 2023). The crystal-free samples with 70–80 vol%total porosity (Takeuchi et al. 2009) had pore connectivity in the region of unsinkable pumice, which is close todata on pumice of FOB 2021 (Fig. 10b). The pore connectivity of crystal-free samples rapidly increased at
approximately 80 vol% total porosity, and those with > 80
vol% total porosity were plotted in the region of sinkable pumice. Figure 10b also suggests the influence of
crystals in the magma on the floatability of the pumice.
Colombier et al. (2022) predicted the influence of crystals
on pore connectivity by a model analysis. Graham et al.
(2023) performed experiments on crystal-bearing rhyolitic magma with up to 40 vol% crystals. For crystal-richpumices (40 vol% crystal content), the pore connectivity
gradually increased with an increase in the total porosity
from 20 to 70 vol%. Graham et al. (2023) interprets thatthe increase in crystal content causes the reduction ininterstitial melt available for bubble expansion, resulting
in the forced connection among bubbles (pores) at lower
total porosity. The total porosity-pore connectivity relationship for crystal-rich pumices varied in the conditionof producing sinkable pumice and can not enter the condition of unsinkable pumice. For pumice with moderatecrystal content (20 vol% crystal content), the relationshipbetween total porosity and pore connectivity approaches

Page 15 of 19

the boundary separating the condition and appears toenter the region of unsinkable pumice. Considering thatthe crystal content of FOB 2021 pumices >  − 4 Φ (16mm in diameter), larger compared with the typical crystal size shown in Sect. 6.8, shows a more representative
crystal content in the magmas, the magmas of the FOB
2021 eruption have a crystal content of < 20 wt% (Fig. 5b).
Assuming that the glass and whole crystal density are
2.5 and 3.0 g/cm3, based on solid density of 1Φ (Table 3)
and modal fractions in whole crystals (Table S5), respectively, a crystal content of approximately 20 wt% can beconverted to approximately 17 vol%. This low to moderate crystal content in the FOB 2021 magma, together
with adequate porosimetric properties, may contributeto favorable conditions to produce abundant, millimetersized, unsinkable pumice.
Generally, in decompression experiments, the sample is
kept at a high temperature and pressure and then decompressed to the targeted pressure at various decompression
rates. The decompressed magmas undergo vesiculation,
including bubble nucleation, growth, coalescence, andshape relaxation. Finally, the samples are quenched fora much shorter time than that during a natural eruption
process. Based on a study of natural subaerial pumices,
Gardner et al. (1996) concluded that the delayed expansion of pumice occurs in the range of melt viscosities insubaerial eruptions. Delayed expansion can cause theisolated pores to expand and transform into connected
pores. In contrast, submarine eruptions may have effectively quenched vesicular magmas, as in decompression
experiments, resulting in producing more isolated poresin the pumice because delayed expansion after fragmentation can be suppressed (Rotella et al. 2015). Figure 10
shows that pumices from subaerial eruptions can float as
air-trapped floating pumices (sinkable pumices) owing
to their high connected porosity. Such subaerial pumices
are expected to sink after a finite floatation time, depending on the pore structure and particle size (Manville et al.
1998; Fauria et al. 2017). Maeno et al. (2022) interpreted
that most of the FOB 2021 pumice were not transported
by eruption plume in the atmosphere but rose from the(See figure on next page.)
Fig. 10 a Pore connectivity vs. total porosity. Open and blue solid circles represent floating and sunken pumices, respectively. Crosses, open
squares, solid squares, open diamonds, and open triangles represent pumice data for the Havre 2012 ALB (ash-lapilli block) sampled on the seafloor(Mitchell et al. 2021), the Havre 2012 RP (raft pumice; Manga et al. 2018; Mitchell et al. 2021), Shinkai pumice sampled on the seafloor (Kato 1987),
the Iriomote 1924 beached pumice (Kato 1987), and the Fukutoku-Oka-no-Ba 1986 beached pumice (Kato 1988), respectively. b Pore connectivity
vs. total porosity. Open and blue solid circles represent floating and sunken pumices, respectively. Red crosses, open squares, open triangles, open
diamonds, gray solid squares, and gray solid triangles represent data for the decompression experiments at a crystal-free condition (Takeuchi et al.
2009), and wollastonite 20%, corundum and wollastonite 20%, corundum 20%, wollastonite 40%, and corundum and wollastonite 40% conditions(Graham et al. 2023). In (a) and (b), gray dots indicate pore connectivity and total porosity of subaerial pumice and scoria compiled by Colombier
et al. (2017a). In (a) and (b), solid and broken lines separate the conditions of unsinkable and sinkable pumices with a solid density of 2.8 and 2.5 g/
cm3, respectively, assuming that the density of seawater is 1.025 g/cm3, based on Wadachi (1987)

Takeuchi et al. Progress in Earth and Planetary ScienceFig. 10 (See legend on previous page.)(2024) 11:47

Page 16 of 19

Takeuchi et al. Progress in Earth and Planetary Science(2024) 11:47

Page 17 of 19

submarine vent to the sea surface. This interpretation isconsistent with the porosimetric characteristics of floating pumices of FOB 2021 containing abundant isolated
pores, which suggests well-quenching and suppression ofdelayed expansion after fragmentation.
Considering these experimental results, the conditions
for producing unsinkable pumice may be narrowly limited in the case of submarine eruption by magmas with
low to moderate crystal content that are quenched at awater depth where high total porosity and relatively low
pore connectivity is achieved. Crystal-rich, experimental pumices have a higher pore connectivity at the same
total porosity (Fig. 10b), suggesting that sinkable pumicewith high pore connectivity can be produced even duringsubmarine eruptions. Consistently with this, the magmathat produced large-magnitude pumice rafts in the Havre2012 eruption was of a crystal-poor nature (Carey et al.
2018).

It is also a future theme to clarify the difference in porosimetric characteristics between the deposited and longdrifting pumice.

8 Conclusions
A porosity measurement technique for millimeter-tocentimeter-sized pumice particles was established andapplied to drifting pumice that erupted during the FOB
2021 eruption. The total, connected, and isolated porosities with errors were obtained for floating and sunken
pumice particles. As for the floating pumices from submarine eruptions studied previously, most floating pumice particles are rich in isolated porosity (> 30 vol%) andare thus unsinkable, even when the pumice size reaches
the millimeter scale. This study emphasized that the skeletal density is a useful measure of pumice floatability(sinkable or unsinkable). As the particle size decreases,
crystals are lost from the floating pumice particles, suggesting that the particle size of the floating pumice isaffected by the petrological properties (crystal content
and size). A comparison with natural pumices from subaerial eruptions and experimental pumices from magma
decompression experiments suggests that FOB 2021
pumices contain abundant isolated pores due to the suppression of expansion after fragmentation by quenching in seawater, and that the relatively low to moderatecrystal content (< 17 vol%) may contribute to favorable
condition to produce abundant, millimeter-sized, unsinkable pumice. The floatability of unsinkable pumice will
be changed if its pore structure is changed. For example,
splitting, abrasion, and weathering of pumice can change
the isolated pores to connected pores, producing sinkable pumice. However, these processes remain poorly
known and are thus important targets in future field andexperimental studies. After the 2021 eruption, a dredge
survey in a research cruise revealed pyroclastic materials
that were deposited on the sea floor in the vicinity of the
FOB volcano without long-drifting (Yoshida et al. 2023b).

Acknowledgements
We thank Ito, H. and Toshida, K. for supporting our field work and valuable
discussion, and officials of Kominato fishing port at the Amami-Ōshima Islandfor providing pumice sample. We thank Editage (www.​edita​ge.​jp) for English
language editing. We also thank Colombier, M. and an anonymous reviewer
for giving comments to improve the manuscript and Nakagawa, M. for editorial handling.

Abbreviations
FOB	Fukutoku-Oka-no-Ba
XRF	X-ray fluorescence
WDS	Wavelength dispersive spectroscopy

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​024-​00652-9.
Additional file 1.
Additional file 2.
Additional file 3.
Additional file 4.
Additional file 5.
Additional file 6.

Author contributions
ST designed the study, conducted field work, collected samples, performed
sieve analysis and density measurements, and wrote the manuscript; KI conducted field work, collected samples, performed sieve analysis, density measurements and petrological analysis; SU conducted field work and collected
samples; YS conducted field work, collected samples, performed density
measurements and petrological analyses. All authors discussed the results andapproved the final manuscript.
Funding
This work was funded by Central Research Institute of Electric Power Industry.
Availability of data and materials
The datasets supporting the conclusions of this article are included within thearticle and its additional files.

Declarations
Competing interests
The author declares no competing interests.
Received: 21 December 2023 Accepted: 21 August 2024

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