Volcano-air-sea interactions in a coastal tuff ring, Jeju Island, Korea
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Downloaded from http://sp.lyellcollection.org/ by guest on October 30, 2021 Volcano–air–sea interactions in a coastal tuff ring, Jeju Island, Korea Young Kwan Sohn1*, Chanwoo Sohn2, Woo Seok Yoon3, Jong Ok Jeong4, Seok-Hoon Yoon3 and Hyeongseong Cho1 1 Department of Geology and Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Korea 2 Research Institute of Basic Sciences, Seoul National University, Seoul 08826, Republic of Korea 3 Department of Earth and Marine Sciences, Jeju National University, Jeju 63243, Republic of Korea 4 Center for Research Facilities, Gyeongsang National University, Jinju 52828, Republic of Korea YKS, 0000-0002-1811-0545; CS, 0000-0002-8346-0955; WSY, 0000-0002-1417-092X; JOJ, 0000-0002-7005-9329; S-HY, 0000-0002-7691-2538; HC, 0000-0003-4596-9870 *Correspondence: yksohn@gnu.ac.kr Abstract: The Holocene tuff ring of Songaksan, Jeju Island, Korea, is intercalated with wave-worked deposits at the base and in the middle parts of the tuff sequence. They are interpreted to have resulted from fair-weather wave action at the beginning of the eruption and storm wave action during a storm surge event in the middle of the eruption, respectively. The tuff ring is overlain by another marine volcaniclastic formation, suggesting ero- sion and reworking by marine processes because of post-eruption changes in sea-level. Dramatic changes in the chemistry, accidental componentry and ash-accretion texture of the pyroclasts are also observed between the tuff beds deposited before and after the storm invasion. The ascent of a new magma batch, related to the chem- ical change, could not be linked with either the Earth and ocean tides or the meteorological event. However, the changes in the texture of the pyroclasts suggest a sudden change in the diatreme fill from water-undersaturated to supersaturated because of an increased supply of external water into the diatreme. Heavy rainfall associated with the storm is inferred to have changed the water saturation in the diatreme. Songaksan demonstrates that there was intimate interaction between the volcano and the environment. Surtseyan and phreatomagmatic eruptions, produced suggest that much more information can be drawn by magma–water interactions in either surface or from the study of Surtseyan and phreatomagmatic subsurface environments, are one of the commonest deposits regarding a variety of processes acting on eruption styles on Earth (White and Houghton 2000; the Earth’s surface. Houghton et al. 2015). These eruptions commonly In this paper, we introduce a coastal tuff ring or last days to months (Simkin and Siebert 1984, maar–diatreme volcano, named Songaksan, in Jeju 2000) and result in the accumulation of tephra Island, Korea (Fig. 1), which has been studied by rings or cones around the vent that are tens of metres one of the authors (YKS) since the late 1980s, to over 100 m high. The deposition rate of tephra is together with some other examples of hydrovolcanic therefore incomparable with that of ordinary sedi- deposits on the island. Past and ongoing studies of mentary deposits. Because of high sedimentation this volcano suggest that it preserves the geological rates, some volcaniclastic deposits contain the records of marine and atmospheric processes in records of the Earth-surface processes and environ- unusual detail, including fair-weather to stormy- ments in unusual detail (Y.K. Sohn et al. 2002; weather sea-levels, tides, waves and post-eruption Jeong et al. 2008; Y.K. Sohn and Yoon 2010; sea-level changes. The volcano also experienced dra- C. Sohn and Y.K. Sohn 2019b). Craters in maar– matic changes in eruption behaviour during a storm diatreme volcanoes also act as new accommodation event, possibly having a connection with the pro- space for sediment accumulation where unusual cesses in the sea and the atmosphere. Songaksan is details of the changing environments can be pre- thus regarded as an example of a coastal volcano, served (White 1989, 1990, 1992). These studies which underwent volcano–air–sea interactions From: Di Capua, A., De Rosa, R., Kereszturi, G., Le Pera, E., Rosi, M. and Watt, S. F. L. (eds) Volcanic Processes in the Sedimentary Record: When Volcanoes Meet the Environment. Geological Society, London, Special Publications, 520, https://doi.org/10.1144/SP520-2021-52 © 2021 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
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Y. K. Sohn et al.
Fig. 1. Location of study area. (a) Location of Jeju Island. Tracks of the five largest typhoons that have hit the
Korean Peninsula in the last half century are indicated (after Sohn and Sohn 2019b). 1, Sarah, 1959; 2, Rita, 1972; 3,
Rusa, 2002; 4, Maemi, 2003; 5, Chaba, 2016. (b) Simplified geological map of Jeju Island (modified after Park et al.
2000b). (c) Geological map of the Songaksan area (after Park et al. 2000a). (d) Geological map of the Ilchulbong
area (after Sohn et al. 2002).
during the eruption. In this paper, we review the pre-
vious and ongoing research on this volcano in terms
of the interactions between the volcano and the envi-
ronment, especially ocean tides and storms.
Terminology
Using the classic distinction between maars and tuff
rings based on the position of the crater floor relative
to the pre-eruption surface (Lorenz 1973; Fisher and
Schmincke 1984; Cas and Wright 1987), Songaksan
is a maar or a maar–diatreme volcano (White and
Fig. 2. Intracrater exposure of Songaksan, showing
Ross 2011). We cannot see the crater floor of the steeply inward-dipping (ventward-dipping) tuff beds
tephra ring at Songaksan because it is filled with with a prominent internal truncation surface. The tuff
later scoria cones and ponded lava (Fig. 1c). How- sequence is overlain by dark grey ponded trachybasalt
ever, sea cliff exposures show clearly that the inner lava and reddish scoria deposits. See Figure 1c for the
crater wall of the tephra ring extends below the location of the photograph.
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Volcano–air–sea interaction in a coastal volcano
Fig. 3. Summary of volcaniclastic terminology after White and Houghton (2006) and Sohn and Sohn (2019a).
pre-eruption surface (Fig. 2; also see fig. 3a of Sohn subduction zone of the Nankai Trough (Brenna
et al. 2002). The abundance of accidental materials et al. 2015; Koh et al. 2017; Fig. 1a). The surface
in the tephra ring also argues for the formation of a of the island is covered by shield-forming, basaltic
diatreme beneath the crater of Songaksan, which is to trachytic lavas and hundreds of monogenetic vol-
estimated to be over 500 m deep (Go et al. 2017). canic cones that have formed throughout the Quater-
It should be noted that the crater of a tuff ring at or nary (Brenna et al. 2012a, b; Koh et al. 2013;
above the pre-eruption surface is not evidence for Fig. 1b), whereas the subsurface geology is charac-
the absence of a diatreme in the subsurface because terized by extensive hydrovolcanic deposits and
the crater of a tuff ring can be filled with later volca- quartzose shelf sediments that accumulated under
nic deposits. the influence of fluctuating Quaternary sea-levels
We are doubtful whether a ‘true’ tuff ring does (Sohn et al. 2008). Songaksan is the youngest volca-
exist in nature, produced by explosions entirely nic centre on the island, which formed along the pre-
above the pre-eruption surface and therefore devoid sent shoreline after the Holocene transgression, c.
of a diatreme in the subsurface. A ‘true’ tuff ring 3.7 ka BP (Sohn et al. 2002, 2015; Cheong et al.
should be devoid of accidental particles excavated 2007; Ahn et al. 2015). The volcano consists of a
explosively from the country rocks if the explosions basaltic tephra ring with a rim diameter of 800 m, a
persisted above the pre-eruption surface without nested scoria cone and a ponded lava (trachybasalt)
diatreme formation throughout the eruption. How- inside the crater (Sohn et al. 2002; Fig. 1c). The acci-
ever, the authors are not aware of any ‘true’ tuff dental componentry of the tephra ring suggests that
ring that is composed solely of juvenile tephra. We the volcano is underlain by a diatreme as deep as c.
presume that tuff rings also have diatremes in the 600 m (Go et al. 2017). Vertical crustal motion is
subsurface and can be included in the category of regarded as having been negligible in the southeast-
maar–diatreme volcanoes. Therefore, we do not con- ern Yellow Sea area because the area is located in an
sider maars and tuff rings to be distinct volcano intraplate setting that is tectonically more stable than
types. In addition, the term ‘tuff ring’ has been other regions in East Asia (Hamdy et al. 2005). The
used for decades for Songaksan, and we prefer to tide is semidiurnal, and the tidal range is 1.7 m at the
use the term ‘Songaksan tuff ring’ here for historical southern coast of the island. Typhoons make landfall
continuity. an average of 3.1 times a year (averaged over
Volcaniclastic terms are used according to the 107 years), mainly between July and September
definition of White and Houghton (2006), slightly (National Typhoon Center 2011). The paths of the
modified by Sohn and Sohn (2019a) regarding the typhoons that have hit the Korean peninsula are
secondary volcaniclastic deposits. A concise sum- mostly adjacent to Jeju Island (Fig. 1a).
mary of the volcaniclastic terminology is given in
Figure 3.
Palaeosea-level in shoaling-to-emergent
volcanic successions
Geological setting
Volcanoes that began to grow underwater and then
Jeju Island is an intraplate alkali basaltic volcano, emerged above the water commonly show
74 × 33 km in area, and the highest peak (Mt Halla- subaqueous-to-subaerial, or shoaling-to-emergent,
san) rises to 1950 m a.s.l. The island was built on the facies transitions because of the changing eruptive
c. 100 m-deep continental shelf of the southeastern conditions at the vents and changes in surface envi-
Yellow Sea, c. 650 km away from the nearest ronments at the depositional sites (Sohn 1995;
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Y. K. Sohn et al.
White 1996, 2001; Smellie and Hole 1997; Schmidt high tide level is inferred from the level of the inter-
and Schmincke 2002). The approximate locations of tidal to supratidal facies transition. The intertidal
palaeosea-levels or lake levels in shoaling-to-emer- facies is characterized by alternations of pyroclastic
gent volcanic successions can be inferred from the and wave-worked volcaniclastic deposits (Fig. 4).
surf zone facies intercalated between subaqueous The pyroclastic deposits consist of (1) megaripple-
and subaerial deposits (Ayres et al. 1991) or from bedded or planar-stratified (lapilli) tuffs commonly
the deposit architecture of ‘passage zones’, which with normal or inverse grading of lapilli and (2)
develop at the subaerial to subaqueous transition accretionary lapilli-bearing, crudely stratified and
zone of lava-fed deltas (Jones and Nelson 1970; mantle-bedded, and commonly fine-grained tuffs.
Skilling 2002; Smellie et al. 2013) or tephra cones They are interpreted to have been emplaced by pyro-
(Russell et al. 2013). Finding the records of past sea- clastic surges and fall, respectively, when the depo-
levels or lake levels is crucial for palaeoenvironmen- sitional site was exposed above sea-level at low
tal reconstruction of a volcanic succession because tides (Chough and Sohn 1990; Yoon et al. 2017).
they mark major transitions in depositional environ- On the other hand, the wave-worked volcaniclastic
ments. In the case of Jeju Island, traces of palaeosea- deposits are ripple cross-laminated, and commonly
levels have also been used to guess the approximate intercalated with or capped by millimetre-thick to
eruption ages of some coastal volcanoes prior to dat- paper-thin mud drapes. These deposits are inter-
ing, i.e. if the eruption of a volcano occurred before preted to have been wave-worked above the fair-
or after the Holocene transgression. Those volcanoes weather wave base when the depositional site was
with subaqueous-to-subaerial facies transitions, such submerged under water at high tides (Yoon et al.
as Songaksan tuff ring and Ilchulbong tuff cone 2017). The mud drapes are interpreted to have
(Fig. 1d), were later dated to be middle to late Holo- resulted from settling of suspended fines during
cene in age, whereas those that lack such features, slack waters at high tides. The reworked deposits
such as Suwolbong tuff ring and Udo tuff cone, show gradual upward fining of mean grain size, sug-
were dated to be pre-Holocene (Cheong et al. gesting suppression of orbital wave-generated
2006, 2007; Koh et al. 2013; Ahn et al. 2015; Lim motion of the water as the depositional surface
et al. 2015; Sohn et al. 2015). approached the supratidal level (Yoon et al. 2017).
Recent re-examination of the subaqueous-to- Above the supratidal level, the deposits have fea-
subaerial facies transitions in Songaksan (Yoon tures that are not likely to form or be preserved in
et al. 2017) with the use of a GPS surveying unit subaqueous settings, such as the footprints of birds,
(South S82 T RTK) shows that the elevation of the prod/bounce marks produced by the impact of fall-
subaqueous-to-subaerial facies transition in this vol- ing lapilli on the bed and raindrop impressions
cano coincides with the current high tide level. The (Yoon et al. 2017). Preservation of the gradual
Fig. 4. Intertidal facies of the Songaksan tuff ring, composed of alternations of megaripple-bedded or accretionary
lapilli-bearing tuffs and ripple cross-laminated deposits. The photo scale is 5 cm long. See Figure 1c for the location
of the photograph.
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Volcano–air–sea interaction in a coastal volcano
intertidal to supratidal facies transition in such tuff rings/cones and maars, which are particularly
unusual detail was probably possible because of (1) common in coastal to shelf settings, are likely to be
the fine grain size of tephra (medium to fine ash), affected by storm winds, waves or surges during
which made reworking or reprocessing (see Fig. 3 their eruptions. In spite of this possibility, the records
for these terms) of tephra by fair-weather marine pro- of palaeostorms have not been sought from coastal
cesses possible, (2) continual supply and subhori- volcaniclastic deposits except for a few studies
zontal layer-by-layer accumulation of pyroclasts at addressing the role of storm activity in post-eruptive
both high and low tides, and (3) the fair-weather con- reshaping of submarine volcanic edifices (Cas et al.
ditions of the sea, which prevented removal of the 1989; Andrews 2003; Sorrentino et al. 2014).
deposits by erosion. It was recently found that Songaksan preserves
the records of a palaeostorm, c. 3.7 ka BP, which
left volcano-wide erosion surfaces associated with
Effects of storm surges and tides wave-worked deposits in the distal sequence of the
tuff ring (Sohn and Sohn 2019b). These features
Storms are weather systems that accompany strong were recognized in the 1980s, but were overlooked
surface winds and can affect both subaerial and and attributed to rain flushing during volcanic quies-
marine environments. In the former, they can erode cence (Chough and Sohn 1990). However, they were
and transport freshly deposited volcanic ash hun- recently reinterpreted as having resulted from
dreds of kilometres from the volcano and prolong a storm (typhoon) event in the middle of the erup-
the impacts of volcanic eruptions (Arnalds et al. tion of Songaksan (Sohn and Sohn 2019b). This
2013). In the latter, storm-related processes are par- interpretation is based on three interbeds of
ticularly pronounced in coastal to shelf areas because horizontally laminated, low-angle inclined stratified,
strong winds drive ocean currents and generate large ripple cross-laminated to hummocky/swaley cross-
waves that can affect much deeper parts of the sea stratified deposits together with mud drapes, which
than fair-weather waves. Storms can also cause a are intercalated with primary tuff beds of pyroclastic
storm surge, which can inundate coastal areas by surge or fall origin (Fig. 5). These interbeds occur up
raising the sea-level above the normal tidal level. to an altitude of c. 5.6 m, i.e. c. 4.6 m higher than
Because the majority (c. 80%) of modern shelves normal high tide level. Their exclusive occurrence
are storm-dominated (Johnson and Baldwin 1996), below c. 5.6 m and the prevalence of wave-formed
Fig. 5. (a) Photograph and (b) graphic column of three storm-wave-worked units (R1, R2 and R4) intercalated with
pyroclastic surge (unit T1, T2, T4b and T5) or fall deposits (unit T4a), which were deposited during three tidal cycles.
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Y. K. Sohn et al.
structures with both seaward-dipping and landward- Sohn and Sohn 2019b) and the storm surge height
dipping cross strata negate the possibility of rework- (c. 1 m) in most marginal marine settings. Multiple
ing by rainfall-induced surface runoff or tsunami- tempestite beds can therefore be produced by a single
generated coastal inundation. Instead, the distal mar- storm event because of tidal fluctuations even in a
gins of the tuff ring are interpreted to have been sub- microtidal setting. This finding thus provides sig-
merged underwater repetitively and subjected to nificant implications regarding the interpretation
wave activity in a swash to surf zone because the sea- of palaeotempest deposits in marginal marine
level rose several metres above the normal high-tide environments.
level as a result of a storm surge event combined with Songaksan also demonstrates that tuff rings/
ocean tides. The triple intercalation of the wave- cones and maars in coastal to shelf settings can
worked deposits between primary tuff beds indicates record past storm events in unusual detail because
repetitive submergence and emergence of the depo- of their extremely rapid sedimentation and burial
sitional site, which is attributed to sea-level fluctua- by later deposits, preventing post-depositional ero-
tions owing to tides during a storm event that sion, reworking or bioturbation. This study therefore
lasted 1.5 days, i.e. three tidal cycles (Sohn and highlights the potential significance of coastal to
Sohn 2019b; Fig. 5b). marine tuff rings/cones and maars in palaeotempes-
The three interbeds of wave-worked deposits also tology because these volcanoes are second only to
show vertical facies changes, which probably reflect scoria cones in abundance on Earth and particularly
the waxing and waning of wave intensity during the common in coastal to shelf areas, and can be affected
storm event. The lower interbed (unit R1) comprises by storms in regions of frequent storms in spite of
a millimetre-thick to 1 cm-thick mud drape above a their short eruption duration. These volcanoes can
shallow erosion surface (Fig. 5) or without erosion therefore be potential sources of high-resolution
at the base (Fig. 6), suggesting inundation of the proxy records of past storm activity, which have
tuff ring margin and suspension settling of fines been overlooked to date, but are worthy of close
without significant wave erosion. The mud drape is examination in the future (Sohn and Sohn 2019b).
composed of mainly non-volcanic components,
such as illite, quartz and biotite. It is postulated
that there was a forerunner surge, i.e. a water level Storm wave versus tsunami
rise c. 12 h in advance of the landfall of the typhoon,
as exemplified by the 1900 and the 1915 Galveston Coasts are exposed to hazards associated with either
hurricanes and the 2008 Hurricane Ike (Kennedy or both tsunamis and storms, depending on tectonic
et al. 2011). The middle interbed (unit R2) is charac- and climatic settings (Adger et al. 2005), as exempli-
terized by a prominent erosion surface at the base fied by the 2004 Indian Ocean tsunami (Lay et al.
(Fig. 5) and deformation of the underlying tuff bed 2005), the 1900 Galveston Hurricane (Horowitz
(Fig. 6), suggesting strong shear and normal stresses 2015) and the 2005 Hurricane Katrina (Ashley and
on the seabed by strongly pounding waves during Ashley 2008). Because of the growing demand for
flood. The erosion surface is overlain by a mud reliable hazard management in coastal areas, geolog-
drape, deposited probably at high tide. The upper ical studies of onshore to offshore deposits related to
interbed (unit R4) comprises a hummocky to either tsunamis or storms have increased rapidly in
swaley cross-stratified deposit, which is sandwiched recent years. One of the main problems facing
between horizontally laminated deposits without these studies has been distinguishing between tsu-
erosion at the base (Fig. 4). The sequence of struc- nami and storm deposits because both are high-
tures in Unit R4 suggests deepening and shallowing energy events, and their deposits may have many
of the depositional site from swash zone to surf zone characteristics in common, including grain size,
and then to swash zone, while the intensity of the grain componentry, spatial distribution of deposits
storm was weakening (Sohn and Sohn 2019b). The and sedimentary structures (Nanayama et al. 2000;
changing characteristics of the three wave-worked Goff et al. 2004; Tuttle et al. 2004; Kortekaas and
interbeds thus appear to represent the waxing and Dawson 2007; Morton et al. 2007; Engel et al.
waning of wave intensity during the storm event, 2016). One of the most distinguishing features of tsu-
providing the most complete record of an ancient nami deposits is multiple mud drapes or mud caps
storm event ever reported (Sohn and Sohn 2019b). intercalated with commonly normally graded sandy
The storm-wave-worked deposits of Songaksan subunits. This association is interpreted to result
demonstrate that the tide can play a significant role from suspension settling of fines between tsunami
in determining the stratal characteristics, i.e. the tri- waves, which arrive at the shore with an interval of
ple intercalation, of storm deposits (tempestites) tens of minutes, thereby representing the number of
even in microtidal settings because the tidal range tsunami waves. On the other hand, storm deposits
(1.7 m in Songaksan area) can be as large as the are generally devoid of such mud laminae because
wave runup (estimated to be 2.8 m in Songaksan; of the constant action of waves during a storm.
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Volcano–air–sea interaction in a coastal volcano
Fig. 6. Outcrop features of storm-wave-worked deposits (units R1 and R2) and adjacent pyroclastic deposits. (a) Unit
R2 shows lenticular geometry because it was ponded in the trough of the underlying megaripple bedform of unit T2.
A mud drape occurs at the base of the unit, upon the eroded unit T2. The mud is injected into the cracks of the
underlying tuff bed. Unit R1 is composed of a single millimetre-thick and continuous mud drape and is barely visible.
A photo scale is at the centre of the photograph. (b) Close-up of the boxed area in (a), showing load and flame
structures in unit R2, suggesting liquefaction of the underlying water-saturated silty deposit by the load of the
overlying coarse sandy deposit. (c) At a more proximal locality than (a), unit R1 is composed of a single
millimetre-thick and continuous mud drape, overlying unit T1 without erosion. The crest of the megaripple of unit T2
was eroded and overlain by the mud drape of unit R2, which was later injected into the underlying tuff bed. All photo
scales are 3 cm long. See Figure 1c for location of the photographs.
The mud drapes within the storm-wave-worked because of the prevalence of wave-formed structures
deposits of Songaksan (Fig. 5) are therefore unusual. in the deposits. The maximum tsunami height that
The possibility of a tsunami is negated, however, can be produced by the largest possible earthquake
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Y. K. Sohn et al.
in the Nankai Trough is also estimated to be smaller 2006). 230Th/234U ages are between 3670 + 63
than 0.5 m along the coastline of Jeju Island and 4345 + 38 years BP (Cheong et al. 2006).
(Kim et al. 2016). The mud drapes are therefore These ages are almost identical to the eruption age
interpreted to have been deposited during slack (c. 3.7 ka BP) of Songaksan (Sohn et al. 2015).
waters at high tides, combined with a forerunner The radiocarbon and 230Th/234U ages of mollusc
surge (Kennedy et al. 2011) before the arrival (or shells from the Sinyangri Formation range between
landfall) of the typhoon, or between long-period 4400 + 100 and 1570 + 90 years BP (Kim et al.
infragravity waves, which hit the shoreline with the 1999) and 4980 + 40 and 3793 + 30 years BP
dominant period of 80–300 s (Ardhuin et al. 2014) (Cheong et al. 2006), respectively, also suggesting
after the landfall of the typhoon (Sohn and Sohn a Middle Holocene age of the formation.
2019b). The Hamori Formation comprises (1) swash zone
A recent study showed that the coastal setup facies (inner planar facies of Clifton et al. 1971)
caused by storm waves can oscillate with the inci- composed of planar- to low-angle inclined-stratified
dence of large and small wave groups and can steepen pebbly sandstones (Fig. 7a) rarely with ripple marks
into a tsunami-like wave (Roeber and Bricker 2015), and desiccation cracks on the bedding planes
suggesting that differentiating between storm and (Fig. 7b), (2) inner rough facies composed of
tsunami deposits can be difficult in spite of ongoing seaward- or landward-dipping trough cross-stratified
studies (e.g. Nanayama et al. 2000; Goff et al. pebbly sandstones (Fig. 7c), formed within the tran-
2004; Tuttle et al. 2004; Kortekaas and Dawson sition between surf and swash zones, (3) surf zone
2007; Morton et al. 2007; Engel et al. 2016). Further facies (outer planar facies of Clifton et al. 1971)
investigation of tsunami and storm deposits in composed of planar-stratified, low-angle inclined-
diverse settings and the role of long-period waves stratified or gently swaley cross-stratified sandstones
such as infragravity waves or surf beats thus seems locally intercalated with mud drapes (Fig. 7d), (4)
necessary for a better understanding of extreme dep- outer rough facies composed of large-scale cross-
ositional events in coastal regions and how they are stratified sandstones produced by landward-
imprinted in sedimentary proxy records. migrating megaripples or dunes in the zone of
wave build-up (Fig. 7e) and (5) supratidal beach
ridge deposits composed of pebbly coarse sand-
Post-eruption sea-level change stones, which are interpreted to have formed by
storm waves above mean high-tide level (Fig. 7f).
Phreatomagmatic and Surtseyan volcanic eruptions Lateral changes in these facies in the Hamori Forma-
in coastal to shelf settings result in a sporadic tion indicate zoned wave activities in a high-energy
increase in volcanic sediment supply to nearby sub- nearshore environment (Sohn et al. 2002).
aerial to marine environments because of the ease of The basal surface of the Hamori Formation
erosion and resedimentation of the commonly fine- occurs up to an altitude of 5.2 m, overlying the pyro-
grained tephra produced by these eruptions. These clastic deposits of Songaksan with erosion (Fig. 7a).
eruptions can therefore create stratigraphic records The upper surface of the formation is inferred to have
in an otherwise sediment-starved volcanic field, occurred at least a metre above that altitude, when
e.g. a lava-dominated area (Sohn et al. 2002). On excluding the supratidal storm beach ridge deposits.
Jeju Island, two volcanogenic sedimentary forma- The occurrence of the intertidal and deeper marine
tions are worth noting in this respect, the Hamori deposits up to that altitude suggests that there was
Formation exposed along the eastern and western a period with sea-levels higher than the present lev-
coasts of the Songaksan tuff ring (Fig. 1c) and the els, including the time of the eruption of Songaksan,
Sinyangri Formation exposed along the southern c. 3.7 ka BP. In addition, Sohn et al. (2002)
coast of the Ilchulbong tuff cone (Fig. 1d). These for- described several subunits in the formation, bounded
mations are composed almost entirely of the volcanic by laterally continuous erosion surfaces and marked
components that were derived from the nearby tuff by sharp grain size contrasts and lateral shift of facies
ring/cone and were deposited in a high-energy near- between these subunits. They interpreted these char-
shore environment. These formations contain abun- acteristics as evidence for sea-level fluctuations dur-
dant mollusc shells at the base, probably because ing deposition of the formation and proposed that the
of rapid burial of molluscs by volcanic sediment. formation records high-frequency (millennial-scale)
The lower age limits of these formations could there- and metre-scale sea-level fluctuations during the
fore be constrained by radiocarbon and 230Th/234U late Holocene. This study therefore attests to the
age dating of mollusc shells. The radiocarbon ages role of coastal volcanoes in causing a short-lived
of the Hamori Formation range between 4090 + but abundant supply of sediment to nearby areas
90 and 3900 + 100 years BP (Sohn et al. 2002), and in creating high-resolution stratigraphic records
3862 + 35 and 2995 + 35 years BP (Cho et al. of Earth surface processes in otherwise sedi-
2005) and 3840 + 40 years BP (Cheong et al. ment-starved, lava-dominated volcanic fields. The
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Volcano–air–sea interaction in a coastal volcano
(a) (b)
Swash zone facies
of the Hamori Fm.
Songaksan tuff
(c) (d)
(e) (f)
Fig. 7. Deposit features of the Hamori Formation. (a) The Hamori Formation, composed of low-angle
inclined-stratified deposits (swash zone facies), overlies the distal tuff beds of Songaksan with erosion. The photo
scale is 5 cm long. (b) Ripple marks with superimposed mudcracks on the upper bedding plane of swash zone facies,
suggesting periodic exposure of the depositional surface in the intertidal zone. The photo scale is 5 cm long. (c) Inner
rough facies consisting of trough cross-stratified, very coarse sandy to gravelly deposits formed within the transition
between surf and swash zones. The coin is 2.3 cm in diameter. (d) Planar- to wavy-bedded coarse sandy deposits,
which are internally massive to low-angle cross-stratified. They are interpreted to be deposits from strong waves in
the surf zone. The pen is 15 cm long. (e) Large-scale, landward-dipping cross-stratification produced by
landward-migrating megaripples in the wave build-up zone. Seaward-migrating countercurrent ripples are observed at
the base of the cross-stratified set. The hammer is 28 cm long. (f ) Supratidal beach ridge deposit at the top of the
Hamori Formation, which occurs up to an altitude of c. 10 m. The deposit is composed of very coarse sand and
rounded granules and fine pebbles, and shows crude stratification and openwork texture. The photo scale is 5 cm
long. See Figure 1c for the locations of the photographs.
Sinyangri Formation, which occurs several metres External forcing of volcanic eruptions
above present sea-level at the eastern margin of
Jeju Island (Fig. 1d), also records sea-level fluctua- Unit T1 and the underlying tuff beds and unit T3
tions in the late Holocene, consisting of alternating and the overlying tuff beds (Fig. 5) have distinctly
swash zone and surf zone facies (Han et al. 1987), different characteristics in chemistry, accidental
although the resolution is poorer. componentry and ash-accretion texture, suggesting
Downloaded from http://sp.lyellcollection.org/ by guest on October 30, 2021
Y. K. Sohn et al.
dramatic changes in the eruption behaviour of Son-
gaksan across the boundary between these units.
As for the tephra chemistry, MgO content was useful
for distinguishing juvenile particles from different
tuff units (Sohn and Sohn 2019a). Unit T1 consists
of low-Mg (MgO content ,3.33 wt%) juvenile par-
ticles and contains abundant (c. 40 modal %) acci-
dental particles (mostly detrital quartz grains) (Go
et al. 2017). Both juvenile and accidental particles
occur mostly as aggregates or ash-coated particles
(Fig. 8a). Unit T1 as well as the underlying tuff
beds thus have large interconnected pores probably
because of loose packing of cohesive ash aggregates
or ash-coated particles upon deposition. On the other
hand, unit T3 consists of high-Mg (MgO content
.4.00 wt%) juvenile particles and very rare acciden-
tal particles (Fig. 8b). Almost none of these particles
is aggregated or ash-coated. Unit T3, as well as the
overlying tuff beds, therefore, has relatively low
porosity, suggesting denser packing of non-
aggregated tephra particles upon deposition. Pyro-
clasts of Unit T2 between these units have intermedi-
ate MgO contents between 3.33 and 4.00 wt% (Sohn
and Sohn 2019a) and are scarcely aggregated or
coated by fine ash (Fig. 8c). The unit also contains
the highest amount (c. 55 modal %) of accidental
quartz of all of the pyroclastic deposits of Songaksan
(Go et al. 2017).
These changes in tephra characteristics, including
the chemical composition of juvenile tephra, con-
tents of accidental particles and the aggregation-
related features of tephra, are dramatic because
these tuff units are interpreted to have erupted in
close succession without a break in the eruption.
As for the tephra composition, units T1 and T3, as
well as the over- and underlying tuff beds, are inter-
preted to have been derived from chemically distinct
magma batches (Brenna et al. 2011). Unit T2 inter-
calated between these two tuff units is interpreted
to have resulted from the mixing of the two magmas
in the feeder dyke. About 55 vol% of low-Mg
magma and about 45 vol% of high-Mg magma are
estimated to have contributed to form unit T2
(Sohn and Sohn 2019a). The magma mixing sug-
gests that the eruption of the earlier magma batch
was immediately followed by the eruption of the Fig. 8. Backscattered electron images of undisturbed
later magma batch through the same feeder dyke tuff samples. (a) Both juvenile (sideromelane, S) and
and vent without a break in eruption (Go et al. accidental (quartz, Q) particles of unit T1 occur as
2017). The dramatic changes in the content of acci- aggregates or ash-coated particles. Large interconnected
dental particles and the aggregation-related features pores reflect loose packing of cohesive ash aggregates
of tephra are interpreted to be related to changes in or ash-coated particles. (b) Unit T3 is almost
diatreme conditions and eruption processes, as completely devoid of accidental quartz particles. The
explained below. unit also lacks ash aggregates and ash-coated particles.
The relatively low porosity of the unit is interpreted to
be due to denser packing of non-aggregated pyroclasts.
(1) Unit T1 and earlier tuff beds resulted when (c) Unit T2 has the highest accidental quartz content of
there was an abundant supply of accidental all of the pyroclastic facies of Songaksan. The unit also
particles, probably because of active collapse lacks ash aggregates. The ash coating is also
and/or downward excavation of the diatreme. poorly developed.Downloaded from http://sp.lyellcollection.org/ by guest on October 30, 2021
Volcano–air–sea interaction in a coastal volcano
The abundance of ash-coated particles and ash (Fig. 9b) because unit T2 is almost completely
aggregates, which are interpreted to have devoid of ash-coated particles and low-Mg
formed mostly by granular mixing in the diat- tephras from the earlier magma batch. Unit
reme, suggests that the diatreme was filled with T2 thus contained accidental particles newly
wet and cohesive materials undersaturated supplied from the diatreme walls and uncoated
with water (Fig. 9a). with fine ash as well as uncoated juvenile par-
(2) Before the eruption of Unit T2, the diatreme- ticles of intermediate composition (Fig. 8c).
filling debris was almost completely removed The high abundance of accidental particles in
Fig. 9. Changing diatreme conditions during the storm event (modified after Go et al. 2017, by permission from
SpringerNature, Bulletin of Volcanology, ©2017). (a) Before the storm, ash-coated and quartz-rich tephra was ejected
from the diatreme, which was filled with ash-coated pyroclasts undersaturated with water. The magma was low in
magnesium. (b) The diatreme is inferred to have been emptied by the eruption of unit T1 because unit T2 is almost
completely devoid of ash-coated particles and low-Mg juvenile tephras. The diatreme is also inferred to have been
filled by quartzose materials by the collapse of the quartz-rich diatreme wall rocks. (c) The fresh (ash-uncoated)
quartzose diatreme fill, newly derived from the diatreme wall rocks, was almost completely ejected by the eruption of
unit T2. The magma had an intermediate Mg content because of mixing of the earlier low-Mg magma with the newly
arrived high-Mg magma. (d) Unit T3 and the overlying tuff beds are almost devoid of accidental quartz grains and
ash aggregates or ash-coated particles. This suggests almost complete removal of the quartzose diatreme fill by the
previous eruption of unit T2 and the cutoff of further supply of accidental materials from the diatreme walls. The lack
of ash aggregates or coated particles suggest supersaturation of the diatreme fill with water, inhibiting adhesion of
tephra particles in the diatreme. Deposition of a vesiculated tuff (unit T4b) also suggests that the tephra was
extremely wet and could form a water-saturated and airtight medium upon deposition (Go and Sohn 2021).Downloaded from http://sp.lyellcollection.org/ by guest on October 30, 2021
Y. K. Sohn et al.
unit T2 also suggests massive collapse of the Numerous examples exist regarding the rainfall-
diatreme wall prior to the eruption of the unit induced activity of volcanoes, including the 1979
(Fig. 9b). phreatic eruption of Karkar volcano, Papua New
(3) Afterwards, the supply of accidental particles Guinea (McKee et al. 1981), the eruptions of
to the diatreme was virtually cut off (Fig. 9d). Mount St Helens, USA (Mastin 1994), Unzen,
Unit T3 and the overlying tuff beds contain Japan (Yamasato et al. 1997), Merapi, Indonesia
very small amounts of accidental particles, (Voight et al. 2000), the 2000 and 2001 eruptions
implying that the conduit walls had stabilized. of Soufrière Hills Volcano, Montserrat (Matthews
In addition, the diatreme fill is interpreted to et al. 2002; Carn et al. 2004), and the 2018 eruption
have been water-saturated, and the erupted at Kilauea, Hawaii (Farquharson and Amelung
materials comprised abundant liquid water to 2020). Although the precise causal mechanisms
form a vesiculated tuff (Go and Sohn 2021). remain controversial, rainfall is considered to modu-
The vesiculated tuff suggests that the tephra late the eruption processes in various ways. In the
was so wet that it could form a coherent, water- case of tuff rings/cones and maars, rainwater may
saturated and airtight medium upon deposition fall directly into the crater or flow into the conduit
that could retain a gas phase (Lorenz 1970, or diatreme through groundwater aquifers, thereby
1974). The near absence of ash-coated particles increasing the mass ratio of water to magma. The
or ash aggregates in Unit T3 and the overlying dramatic change in water saturation of the diatreme
tuff beds is also interpreted to be due to inhibi- fill, inferred from the changes in tephra characteris-
tion of ash accretion in the water-saturated tics, is therefore interpreted to have resulted from
diatreme fill. the abrupt addition of water into the diatreme during
the storm event. High permeability of the substrate
All of these changes in the characteristics of the beneath Songaksan and the fluctuation of the
tuff beds and the eruptive behaviour occurred dur- groundwater table according to the amount of precip-
ing a storm event. We are therefore obliged to assess itation in this area (Koh 1997; Won et al. 2006) sug-
the possible role of external forcing on Songaksan gest that the groundwater table could rise rapidly and
eruptions. A study of the intervals between erup- facilitate more efficient supply of groundwater into
tions at the geysers in Yellowstone National Park the diatreme.
provides some implications for the triggering and
modulation of volcanic eruptions by external forces
(Hurwitz et al. 2014). This study concluded that the Conclusions
eruption intervals of the geysers are insensitive to
periodic stresses induced by barometric pressure The Songaksan tuff ring, Jeju Island, Korea shows
variations because the subsurface water column is that tuff rings/cones and maars can preserve the
decoupled from the atmosphere. Neither are the records of Earth surface processes and environments
eruption intervals modulated by solid Earth tides. in exceptional detail because of unusually rapid sed-
Therefore, the changes in barometric pressure asso- imentation of relatively fine-grained (ash-size)
ciated with a storm or the stresses induced by the tephra, which can be easily reprocessed or reworked
Earth and ocean tides together with the storm by ordinary Earth surface processes, compared with
surge are not likely to have triggered the ascent of scoriaceous to pumiceous lapilli-dominated tephras
new magma or the changes in eruption behaviour from magmatic eruptions. Detailed sedimentological
at Songaksan. The possibility of tidal forcing on observations of both primary and secondary volcani-
volcanic activity cannot, however, be completely clastic deposits show that the tuff ring preserves the
ruled out because a number of studies have provided records of fair-weather sea-level at the time of the
evidence for tidal modulation of eruption frequency eruption of the tuff ring (Yoon et al. 2017), stormy-
and/or intensity (Sottili and Palladino 2012; Girona weather sea-level raised by a storm surge event dur-
et al. 2018; Petrosino et al. 2018 and references ing the eruption of the tuff ring (Sohn and Sohn
therein). The ascent of new magma in Songaksan 2019b) and post-eruption changes in sea-level
could have been modulated by the tide combined (Sohn et al. 2002). Songaksan thus demonstrates
with a storm surge, but is more likely to be due to that tuff rings or maars, which are second only to sco-
internal forcing, working in the deep magmatic ria cones in abundance on Earth and particularly
feeding system. The causes of the diatreme-empty- common in coastal areas, can be potential sources
ing eruptions are unresolvable and cannot be related of accurately levelled and dated data points for the
to either intrinsic or extrinsic factors with confi- Quaternary sea-level curve, which has so far been
dence. However, the changes in the accretion- constructed based mainly on the study of fossil
related features of the tephra might have been coral reef terraces and oxygen isotopic ratios.
caused by an extrinsic cause, i.e. intense rainfall An important consequence of rapid sedi-
accompanying the storm. mentation of volcaniclastic deposits in coastalDownloaded from http://sp.lyellcollection.org/ by guest on October 30, 2021
Volcano–air–sea interaction in a coastal volcano
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