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Accepted Manuscript Geological Society, London, Special Publications

Volcano-air-sea interactions in a coastal ring, Jeju , Korea

Young Kwan Sohn, Chanwoo Sohn, Woo Seok Yoon, Jong Ok Jeong, Seok- Hoon Yoon & Hyeongseong Cho

DOI: https://doi.org/10.1144/SP520-2021-52

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Received 15 March 2021 Revised 23 May 2021 Accepted 31 May 2021

© 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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Volcano-air-sea interactions in a coastal tuff ring, , Korea

Young Kwan Sohn1,*, Chanwoo Sohn2, Woo Seok Yoon3, Jong Ok Jeong4, Seok- Hoon Yoon3, Hyeongseong Cho1

1 Department of 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 and Marine Sciences, Jeju National University, Jeju 63243, Republic of Korea 4 Center for Research Facilities, Gyeongsang National University, Jinju 52828, Republic of Korea *Correspondence: [email protected] Abstract:

The 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, which 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 erosion and reworking by marine processes because of post-eruption changes of the sea level. Dramatic changes of 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 batch, related to the chemical change, could not be linked with either the Earth and ocean or the meteorological event. However, the changes of the pyroclasts texture suggest a sudden change of the diatreme fill from waterACCEPTED-undersaturated to supersaturated because MANUSCRIPT of an increased supply of external 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. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

(Introduction)

Surtseyan and phreatomagmatic eruptions, produced by magma-water interactions in either surface or subsurface environments are one of the commonest eruption styles on Earth (White and Houghton, 2000; Houghton et al., 2015). These eruptions commonly last days to months (Simkin and Siebert, 1984; Simkin and Siebert, 2000) and result in accumulation of rings or cones around the vent that are tens of meters to over a hundred meters high. The deposition rate of tephra is therefore incomparable with that of ordinary sedimentary deposits. Because of the high sedimentation rates, some volcaniclastic deposits contain the records of the Earth-surface processes and environments in unusual detail (Sohn et al., 2002; Jeong et al., 2008; Sohn and Yoon, 2010; Sohn and Sohn, 2019b). Craters in -diatreme volcanoes also act as new accommodation space for sediment accumulation where unusual details of the changing environments can be preserved (White, 1989; White, 1990; White, 1992). These studies suggest that much more information can be drawn from the study of Surtseyan and phreatomagmatic deposits, regarding a variety of processes acting on the Earth’s surface.

In this paper, we introduce a coastal tuff ring or maar-diatreme volcano, named Songaksan, in Jeju Island, Korea (Fig. 1), which has been studied by one of the authors (YKS) since the late 1980s, together with some other examples of hydrovolcanic deposits on the island. Past and ongoing studies of this volcano suggest that the volcano preserves the geological records of marine and atmospheric processes in unusual detail, including fair-weather to stormy-weather sea levels, tides, waves, and post-eruption sea-level changes. The volcano also experienced dramatic changes in eruption behavior during a storm event, possibly having a connection with the processes in the sea and the . Songaksan is thus regarded to be an example of a coastal volcano, which underwent volcano-air-sea interactions during the eruption. In this paper, we review the previous and ongoing research of this volcano in terms of the interactions between the volcano and the environment, especially ocean tides and storms. TerminologyACCEPTED MANUSCRIPT Using the classic distinction between 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 Ross, 2011). We

cannot see the crater floor of the tephra ring at Songaksan because it is filled by later

cones and ponded (Fig. 1c). However, sea cliff exposures show clearly that the inner Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

crater wall of the tephra ring extends below the pre-eruption surface (Fig. 2) (also see Fig. 3a

of Sohn et al., 2002). The abundance of accidental materials in the tephra ring also argues for

the formation of a diatreme beneath the crater of Songaksan, which is estimated to be over

500 m deep (Go et al., 2017). It should be noted that the crater of a tuff ring at or above the

pre-eruption surface is not the evidence for the absence of a diatreme in the subsurface

because the crater of a tuff ring can be filled by later volcanic deposits.

We are doubtful whether a ‘true’ tuff ring does exist in , which was produced by

entirely above the pre-eruption surface and is therefore devoid of a diatreme in the

subsurface. A ‘true’ tuff ring should be devoid of accidental particles excavated explosively

from the country rocks if the explosions persisted above the pre-eruption surface without

diatreme formation throughout the eruption. But the authors are not aware of any ‘true’ tuff

ring that is composed solely of juvenile tephra. We presume that tuff rings also have

diatremes in the subsurface and can be included in the category of maar-diatreme volcanoes.

Therefore, we do not consider maars and tuff rings to be distinct volcano types. In addition,

the term ‘tuff ring’ has been used for decades for Songaksan, and we prefer to use the term

‘Songaksan tuff ring’ here for historical continuity.

Volcaniclastic terms are used according to the definition of White and Houghton (2006),

which was slightly modified by Sohn and Sohn (2019a) regarding the secondary

volcaniclastic deposits. A concise summary of the volcaniclastic terminology is given in FigureACCEPTED 3. MANUSCRIPT Geological setting

Jeju Island is an intraplate alkali basaltic volcano, 7433 km in area, and the highest peak (Mt. Hallasan) rises to 1950 m a.s.l. The island was built on the ca. 100 m-deep continental shelf of the southeastern Yellow Sea, ca. 650 km away from the nearest zone of the Nankai Trough (Brenna et al., 2015; Koh et al., 2017) (Fig. 1a). The surface of the island is covered by shield-forming, basaltic to trachytic and hundreds of monogenetic volcanic cones that have formed throughout the Quaternary (Brenna et al., 2012a; Brenna et al., 2012b; Koh et al., 2013) (Fig. 1b), whereas the Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

subsurface geology is characterized by extensive hydrovolcanic deposits and quartzose shelf sediments that accumulated under the influence of fluctuating Quaternary sea levels (Sohn et al., 2008). Songaksan is the youngest volcanic center on the island, which formed along the present shoreline after the Holocene transgression, ~3.7 ka BP (Sohn et al., 2002; Cheong et al., 2007; Ahn et al., 2015; Sohn et al., 2015). The volcano consists of a basaltic tephra ring with a rim diameter of 800 m, a nested scoria cone and a ponded lava () inside the crater (Sohn et al., 2002) (Fig. 1c). The accidental componentry of the tephra ring suggests that the volcano is underlain by a diatreme as deep as ~600 m (Go et al., 2017). Vertical crustal motion is regarded to have been negligible in the southeastern Yellow Sea area because the area is located in an intraplate setting that is tectonically more stable than other regions in East Asia (Hamdy et al., 2005). The is semidiurnal, and the tidal range is 1.7 m at the southern coast of the island. Typhoons make landfall 3.1 times a year (averaged over 107 years), mainly between July and September (National Typhoon Center, 2011). The paths of the typhoons that have hit the Korean peninsula are mostly adjacent to Jeju Island (Fig. 1a).

Paleo-sea level in shoaling-to-emergent volcanic successions

Volcanoes that began to grow underwater and then emerged above the water commonly show subaqueous-to-subaerial, or shoaling-to-emergent, facies transitions because of the changing eruptive conditions at the vents and the changes in surface environments at the depositional sites (Sohn, 1995; White, 1996b; Smellie and Hole, 1997; White, 2001; Schmidt and Schmincke, 2002). The approximate locations of paleo-sea levels or levels in shoaling-to-emergent volcanic successions can be inferred from the surf zone facies intercalated between subaqueous and subaerial deposits (Ayres et al., 1991) or from the deposit architecture of ‘passage zones’, which develop at the subaerial to subaqueous transition zone of lava-fed deltas (Jones and Nelson, 1970; Skilling, 2002; Smellie et al., 2013) or tephra cones (Russell et al., 2013). Finding the records of past sea levels or lake levels is crucial for paleoenvironmental reconstruction of a volcanic succession because they mark major transitions in depositional environments. In the case of Jeju Island, the traces of paleo- sea levelACCEPTEDs have also been used to guess the appro MANUSCRIPTximate eruption ages of some coastal volcanoes prior to dating, i.e., if the eruption of a volcano occurred before or after the Holocene transgression. Those volcanoes with subaqueous-to-subaerial facies transitions, such as Songaksan tuff ring and Ilchulbong tuff cone (Fig. 1d), were later dated to be middle to late Holocene in age, whereas those that lack such features, such as Suwolbong tuff ring and Udo tuff cone, were dated to be pre- Holocene (Cheong et al., 2006; Cheong et al., 2007; Koh et al., 2013; Ahn et al., 2015; Lim et al., 2015; Sohn et al., 2015). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

Recent reexamination of the subaqueous-to-subaerial facies transitions in Songaksan (Yoon et al., 2017) with the use of a GPS surveying unit (South S82T RTK) shows that the elevations of the subaqueous-to-subaerial facies transitions in these volcanoes coincide with the current high tide level. In Songaksan, the high tide level is inferred from the level of the intertidal to supratidal facies transition. The intertidal facies is characterized by alternations of pyroclastic and wave-worked volcaniclastic deposits (Fig. 4). The pyroclastic deposits consist of 1) megaripple-bedded or planar- stratified () tuffs commonly with normal or inverse grading of lapilli and 2) accretionary lapilli- bearing, crudely stratified and -bedded, and commonly fine-grained tuffs. They are interpreted to have been emplaced by pyroclastic surges and fall, respectively, when the depositional site was exposed above sea level at low tides (Chough and Sohn, 1990; Yoon et al., 2017). On the other hand, the wave-worked volcaniclastic deposits are ripple cross-laminated, and commonly intercalated with or capped by mm-thick to paper-thin drapes. These deposits are interpreted to have been wave-worked above the fair-weather wave base when the depositional site was submerged under water at high tides (Yoon et al., 2017). The mud drapes are interpreted to have resulted from settling of suspended fines during slack at high tides. The reworked deposits show gradual upward fining of mean grain size, suggesting suppression of orbital wave- generated motion of the water as the depositional surface approached the supratidal level (Yoon et al., 2017).

Above the supratidal level, the deposits have features that are not likely to form or be preserved in subaqueous settings, such as footprints of a bird, prod/bounce marks produced by the impact of falling lapilli on the bed, and raindrop impressions (Yoon et al., 2017). Preservation of the gradual intertidal to supratidal facies transition in such unusual detail was probably possible because of (1) the fine grain size of tephra (medium to fine ash), which made reworking or reprocessing (see Fig. 3 for these terms) of tephra by fair-weather marine processes possible, (2) continual supply and subhorizontal layer-by-layer accumulation of pyroclasts at both high and low tides, and (3) the fair- weather conditions of the sea, which prevented removal of the deposits by erosion.

EffectsACCEPTED of storm surges and tides MANUSCRIPT

Storms are weather systems that accompany strong surface winds, and can affect both subaerial and marine environments. In the former, they can erode and transport freshly deposited hundreds of kilometers from the volcano and prolong the impacts of volcanic eruptions (Arnalds et al., 2013). In the latter, storm-related processes are particularly pronounced in coastal to shelf areas because strong winds drive ocean currents and generate large waves that can affect much deeper parts of the sea than fair-weather waves. Storms can also cause a storm surge, which can inundate Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

coastal areas by raising the sea level above the normal tidal level. Because the majority (~80%) of modern shelves are storm-dominated (Johnson and Baldwin, 1996), tuff rings/cones and maars, which are particularly common in coastal to shelf settings, are likely to be affected by storm winds, waves, or surges during their eruptions. In spite of this possibility, the records of paleostorms have not been sought from coastal volcaniclastic deposits except for a few studies addressing the role of storm activity in post-eruptive reshaping of submarine volcanic edifices (Cas et al., 1989; Andrews, 2003; Sorrentino et al., 2014).

It was recently found that Songaksan preserves the records of a paleostorm, ~3.7 ka BP, which left volcano-wide erosion surfaces associated with wave-worked deposits in the distal sequence of the tuff ring (Sohn and Sohn, 2019b). These features were recognized in the 1980s, but were overlooked and attributed to rain flushing during volcanic quiescence (Chough and Sohn, 1990). However, they were recently reinterpreted to have resulted from a storm (typhoon) event in the middle of the eruption of Songaksan (Sohn and Sohn, 2019b). This interpretation is based on three interbeds of horizontally laminated, low-angle inclined stratified, ripple cross-laminated to hummocky/swaly cross-stratified deposits together with mud drapes, which are intercalated with primary tuff beds of or fall origin (Fig. 5). These interbeds occur up to an altitude of ~5.6 m, i.e., ~4.6 m higher than normal high tide level. Their exclusive occurrence below ~5.6 m and the prevalence of wave-formed structures with both seaward-dipping and landward-dipping cross strata negate the possibility of reworking by rainfall-induced surface runoff or tsunami-generated coastal inundation. Instead, the distal margins of the tuff ring are interpreted to have been submerged underwater repetitively and subjected to wave activity in a swash to surf zone because the sea level rose several meters above the normal high-tide level as a result of a storm surge event combined with ocean tides. The triple intercalation of the wave-worked deposits between primary tuff beds indicates repetitive submergence and emergence of the depositional site, which is attributed to sea-level fluctuations due to tides during a storm event that lasted 1.5 day, i.e., three tidal cycles (Sohn and Sohn, 2019b) (Fig. 5b).

TheACCEPTED three interbeds of wave-worked deposits MANUSCRIPT also show vertical facies changes, which probably reflect waxing and waning of wave intensity during the storm event. The lower interbed (unit R1) comprises a mm-thick to 1 cm-thick mud drape above a shallow erosion surface (Fig. 5) or without erosion at the base (Fig. 6), suggesting inundation of the tuff ring margin and suspension settling of fines without significant wave erosion. The mud drape is composed of mainly nonvolcanic components, such as illite, , and biotite. It is postulated that there was a forerunner surge, i.e., a water level rise ~12 hours in advance of the landfall of the typhoon, as exemplified by the 1900 Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

and the 1915 Galveston Hurricanes and the 2008 Hurricane Ike (Kennedy et al., 2011). The middle interbed (unit R2) is characterized by a prominent erosion surface at the base (Fig. 5) and deformation of the underlying tuff bed (Fig. 6), suggesting strong shear and normal stresses on the by strong pounding waves during flood. The erosion surface is overlain by a mud drape, deposited probably at high tide. The upper interbed (unit R4) comprises a hummocky to swaly cross- stratified deposit, which is sandwiched between horizontally laminated deposits without erosion at the base (Fig. 4). The sequence of structures in Unit R4 suggests deepening and shallowing of the depositional site from swash zone to surf zone and then to swash zone, while the intensity of the storm was weakening (Sohn and Sohn, 2019b). The changing characteristics of the three wave- worked interbeds thus appear to represent the waxing and waning of wave intensity during the storm event, providing the most complete record of an ancient storm event ever reported (Sohn and Sohn, 2019b).

The storm wave-worked deposits of Songaksan demonstrate that the tide can play a significant role in determining the stratal characteristics, i.e., the triple intercalation, of storm deposits (tempestites) even in microtidal settings because the tidal range (1.7 m in Songaksan area) can be as large as the wave runup (estimated to be 2.8 m in Songaksan; Sohn and Sohn, 2019b) and the storm surge height (~1 m) in most marginal marine settings. Multiple tempestite beds can therefore be produced by a single storm event because of tidal fluctuations even in a microtidal setting. This finding thus provides significant implications regarding the interpretation of paleotempest deposits in marginal marine environments.

Songaksan also demonstrates that tuff rings/cones and maars in coastal to shelf settings can record past storm events in unusual detail because of their extremely rapid sedimentation and burial by later deposits, preventing post-depositional erosion, reworking or bioturbation. This study therefore highlights the potential significance of coastal to marine tuff rings/cones and maars in paleotempestology because these volcanoes are second only to scoria cones in abundance on Earth and particularly common in coastal to shelf areas, and can be affected by storms in regions of frequentACCEPTED storms in spite of their short eruption MANUSCRIPT duration. These volcanoes can therefore be potential sources of high-resolution proxy records of past storm activity, which have been overlooked to date, but are worthy of close examination in the future (Sohn and Sohn, 2019b).

Storm wave vs. tsunami

Coasts are exposed to hazards associated with either or both of tsunamis and storms, depending on tectonic and climatic settings (Adger et al., 2005), as exemplified by the 2004 Indian Ocean tsunami Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

(Lay et al., 2005), the 1900 Galveston Hurricane (Horowitz, 2015) and the 2005 Hurricane Katrina (Ashley and Ashley, 2008). Because of the growing demand for reliable hazard management in coastal areas, geological studies of onshore to offshore deposits related to either tsunamis or storms have increased rapidly in recent years. One of the main problems facing these studies has been to distinguish between tsunami and storm deposits because both are high- events, and their deposits may have many characteristics in common, including grain size, grain componentry, spatial distribution of deposits, and sedimentary structures (Nanayama et al., 2000; Goff et al., 2004; Tuttle et al., 2004; Kortekaas and Dawson, 2007; Morton et al., 2007; Engel et al., 2016). One of the most distinguishing features of tsunami deposits is multiple mud drapes or mud caps intercalated with commonly normally graded sandy subunits. This association is interpreted to result from suspension settling of fines between tsunami waves, which arrive at the shore with an interval of tens of minutes, thereby representing the number of tsunami waves. On the other hand, storm deposits are generally devoid of such mud laminae because of the constant action of waves during a storm.

The mud drapes within the storm wave-worked deposits of Songaksan (Fig. 5) are therefore unusual. The possibility of a tsunami is negated, however, because of the prevalence of wave- formed structures in the deposits. The maximum tsunami height that can be produced by the largest possible in the Nankai Trough is also estimated to be smaller than 0.5 m along the coastline of Jeju Island (Kim et al., 2016). The mud drapes are therefore interpreted to have been deposited during slack waters at high tides, combined with a forerunner surge (Kennedy et al., 2011) before the arrival (or landfall) of the typhoon, or between long-period infragravity waves, which hit the shoreline with the dominant period of 80 to 300 s (Ardhuin et al., 2014) after the landfall of the typhoon (Sohn and Sohn, 2019b).

A recent study showed that the coastal setup caused by storm waves can oscillate with the incidence of large and small wave groups and can steepen into a tsunami-like wave (Roeber and Bricker, 2015), suggesting that the distinction between storm and tsunami deposits can be difficult in spite of ongoing studies (e.g., Nanayama et al., 2000; Goff et al., 2004; Tuttle et al., 2004; Kortekaas and Dawson,ACCEPTED 2007; Morton et al., 2007; Engel etMANUSCRIPT al., 2016). Further investigation of tsunami and storm deposits in diverse settings and the role of long-period waves such as infragravity waves or surf beats thus seems necessary for a better understanding of extreme depositional events in coastal regions and how they are imprinted in sedimentary proxy records.

Post-eruption sea-level change Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

Phreatomagmatic and Surtseyan volcanic eruptions in coastal to shelf settings result in a sporadic increase in volcanic sediment supply to nearby subaerial to marine environments because of the ease of erosion and resedimentation of the commonly fine-grained tephra produced by these eruptions. These eruptions can therefore create stratigraphic records in an otherwise sediment- starved , e.g., a lava-dominated area (Sohn et al., 2002). On Jeju Island, two volcanogenic sedimentary formations are worth noting in this respect, the Hamori Formation exposed along the eastern and western coasts of the Songaksan tuff ring (Fig. 1c) and the Sinyangri Formation exposed along the southern coast of the Ilchulbong tuff cone (Fig. 1d). These formations are composed almost entirely of the volcanic components that were derived from the nearby tuff ring/cone and were deposited in a high-energy nearshore environment. These formations contain abundant mollusk shells at the base probably because of rapid burial of mollusks by volcanic sediment. The lower age limits of these formations could be therefore constrained by radiocarbon and 230Th/234U age dating of mollusk shells. The radiocarbon ages of the Hamori Formation range between 4090  90 and 3900  100 yrs BP (Sohn et al., 2002), 3862 ± 35 and 2995 ± 35 yrs BP (Cho et al., 2005) and 3840  40 yrs BP (Cheong et al., 2006). 230Th/234U ages are between 3670  63 and 4345  38 yrs BP (Cheong et al., 2006). These ages are almost identical to the eruption age (~3.7 ka BP) of Songaksan (Sohn et al., 2015). The radiocarbon and 230Th/234U ages of mollusk shells from the Sinyangri Formation range between 4400  100 and 1570  90 yrs BP (Kim et al., 1999) and 4980  40 and 3793  30 yrs BP (Cheong et al., 2006), respectively, also suggesting a Middle Holocene age of the formation.

The Hamori Formation comprises 1) swash zone facies (inner planar facies of Clifton et al., 1971) composed of planar- to low-angle inclined-stratified pebbly (Fig. 7a) rarely with ripple marks and desiccation cracks on the bedding planes (Fig. 7b), 2) inner rough facies composed of seaward- or landward-dipping trough cross-stratified pebbly sandstones (Fig. 7c), formed within the transition between surf and swash zones, 3) surf zone facies (outer planar facies of Clifton et al., 1971) composed of planar-stratified, low-angle inclined-stratified, or gently swaley cross-stratified sandstonesACCEPTED locally intercalated with mud drapes MANUSCRIPT (Fig. 7d), 4) outer rough facies composed of large- scale cross-stratified sandstones produced by landward-migrating megaripples or dunes in the zone of wave build-up (Fig. 7e), and 5) supratidal beach deposits composed of pebbly coarse sandstones, which are interpreted to have formed by storm waves above mean high tide level (Fig. 7f). Lateral changes of these facies in the Hamori Formation indicate zoned wave activities in a high- energy nearshore environment (Sohn et al., 2002). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

The basal surface of the Hamori Formation occurs up to an altitude of 5.2 m, overlying the pyroclastic deposits of Songaksan with erosion (Fig. 7a). The upper surface of the formation is inferred to occur at least a meter above that altitude, when excluding the supratidal storm beach ridge deposits. The occurrence of the intertidal and deeper marine deposits up to that altitude suggests that there was a period of sea level being higher than the present and at the time of the eruption of Songaksan, ~3.7 ka BP. In addition, Sohn et al. (2002) described several subunits in the formation, bounded by laterally continuous erosion surfaces and marked by sharp grain size contrasts and lateral shift of facies between these subunits. They interpreted these characteristics as evidence for sea-level fluctuations during deposition of the formation, and proposed that the formation records high-frequency (millennial-scale) and meter-scale sea-level fluctuations during the late Holocene. This study therefore attests to the role of coastal volcanoes in causing short-lived but abundant supply of sediment to nearby areas and in creating high-resolution stratigraphic records of Earth surface processes in otherwise sediment-starved, lava-dominated volcanic fields. The Sinyangri Formation, which occurs several meters above present sea level at the eastern margin of Jeju Island (Fig. 1d), also records sea-level fluctuations in the late Holocene, consisting of alternating swash zone and surf zone facies (Han et al., 1987), although the resolution is poorer.

External forcing of volcanic eruptions

Unit T1 and the underlying tuff beds and unit T3 and the overlying tuff beds (Fig. 5) have distinctly different characteristics in chemistry, accidental componentry, and ash-accretion texture, suggesting dramatic changes in the eruption behavior of Songaksan 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 particles and contains abundant (~40 modal %) accidental 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.ACCEPTED On the other hand, unit T3 consist sMANUSCRIPT of high-Mg (MgO content > 4.00 wt%) juvenile particles and very rare accidental 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. Pyroclasts of Unit T2 between these units have intermediate 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 Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

amount (~55 modal %) of accidental quartz of all the pyroclastic deposits of Songaksan (Go et al., 2017).

These changes in tephra characteristics, including the chemical composition of juvenile tephra, contents of accidental particles, and the aggregation-related features of tephra, are dramatic because these tuff units are interpreted to have been erupted in close succession without a break in eruption. As for the tephra composition, units T1 and T3, as well as the over- and underlying tuff beds, are interpreted to have been derived from chemically distinct magma batches (Brenna et al., 2011). Unit T2 intercalated between these two tuff units is interpreted to have resulted from the mixing of the two in the feeder dike. 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 suggests that the eruption of the earlier magma batch was immediately followed by the eruption of the later magma batch through the same feeder dike and vent without a break in eruption (Go et al., 2017). The dramatic changes in the content of accidental particles and the aggregation-related features of tephra are interpreted to be related to changes in diatreme conditions and eruption processes, as explained below.

1) Unit T1 and earlier tuff beds resulted when there was an abundant supply of accidental particles probably because of active collapse and/or downward excavation of the diatreme. The abundance of ash-coated particles and ash aggregates, which are interpreted to have formed mostly by granular mixing in the diatreme, suggests that the diatreme was filled with wet and cohesive materials undersaturated with water (Fig. 9a). 2) Before the eruption of Unit T2, the diatreme-filling was almost completely removed (Fig. 9b) because unit T2 is almost completely devoid of ash-coated particles and low-Mg from the earlier magma batch. Unit T2 thus contained accidental particles newly supplied from the diatreme walls and uncoated with fine ash as well as uncoated juvenile particles of (Fig. 8c). The high abundance of accidentalACCEPTED particles in unit T2 also suggests MANUSCRIPT massive collapse of the diatreme wall prior to the eruption of the unit (Fig. 9b). 3) Afterwards, the supply of accidental particles to the diatreme was virtually cut off (Fig. 9d). Unit T3 and the overlying tuff beds contain very small amounts of accidental particles, implying that the conduit walls had stabilized. In addition, the diatreme fill is interpreted to have been water-saturated, and the erupted materials comprised abundant liquid water to form a vesiculated tuff (Go and Sohn, 2021). The vesiculated tuff suggests that the tephra was so wet that it could form a coherent, water-saturated and airtight Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

medium upon deposition that could retain a gas phase (Lorenz, 1970; Lorenz, 1974). Near absence of ash-coated particles or ash aggregates in Unit T3 and the overlying tuff beds is also interpreted to be due to inhibition of ash accretion in the water-saturated diatreme fill. All these changes in the characteristics of the tuff beds and the eruptive behavior occurred during a storm event. We are therefore obliged to assess the possible role of external forcing on Songaksan eruptions. A study of the intervals between eruptions at the in Yellowstone National Park provides some implications for the triggering and modulation of volcanic eruptions by external forces (Hurwitz et al., 2014). This study concluded that the eruption intervals of the geysers are insensitive to periodic stresses induced by barometric pressure variations because the subsurface water column is decoupled from the atmosphere. Neither are the eruption intervals modulated by solid Earth tides. Therefore, the changes in barometric pressure associated with a storm or the stresses induced by the Earth and ocean tides together with the storm surge are not likely to have triggered the ascent of new magma or the changes in eruption behavior at Songaksan. The possibility of tidal forcing on volcanic activity cannot, however, be completely ruled out because a number of studies have provided evidence for tidal modulation of eruption frequency and/or intensity (Sottili and Palladino, 2012; Girona et al., 2018; Petrosino et al., 2018 and references therein). The ascent of new magma in Songaksan could have been modulated by the tide combined with a storm surge, but is more likely due to internal forcing, working in the deep magmatic feeding system. The causes of the diatreme-emptying eruptions are unresolvable and cannot be related to either intrinsic or extrinsic factors with confidence. However, the changes in the accretion-related features of the tephra might have been caused by an extrinsic cause, i.e., intense rainfall accompanying the storm.

Numerous examples exist regarding rainfall-induced activity of volcanoes, including the 1979 of Karkar volcano, (McKee et al., 1981), the eruptions of Mount St. Helens, USA (Mastin, 1994), Unzen, Japan (Yamasato et al., 1997), Merapi, (Voight et al., 2000)ACCEPTED, the 2000 and 2001 eruptions of Soufrie're MANUSCRIPT Hills Volcano, (Matthews et al., 2002; Carn et al., 2004), and the 2018 eruption at Kilauea, (Farquharson and Amelung, 2020). Although the precise causal mechanisms remain controversial, rainfalls are considered to modulate the eruption processes in various ways. In the case of tuff rings/cones and maars, rainwater may fall directly into the crater or flow into the conduit or diatreme through aquifers, thereby increasing the mass ratio of water to magma. The dramatic change in water saturation of the diatreme fill, inferred from the changes in tephra characteristics, is therefore interpreted to have resulted from the abrupt addition of water into the diatreme during the storm event. High Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

permeability of the substrate beneath Songaksan and the fluctuation of the groundwater according to the amount of precipitation in this area (Koh, 1997; Won et al., 2006) suggest that the groundwater table could rise rapidly and facilitate more efficient supply of groundwater into the diatreme.

Conclusions

The Songaksan tuff ring, Jeju Island, Korea, shows that tuff rings/cones and maars can preserve the records of Earth surface processes and environments in exceptional detail because of unusually rapid sedimentation of relatively fine-grained (ash-size) tephra, which can be easily reprocessed or reworked by ordinary Earth surface processes, compared with scoriaceous to pumiceous lapilli- dominated tephras from magmatic eruptions. Detailed sedimentological observations of both primary and secondary volcaniclastic deposits show that the tuff ring preserves the records of fair- weather sea level at the time of the eruption of the tuff ring (Yoon et al., 2017), stormy-weather sea level raised by a storm surge event during the eruption of the tuff ring (Sohn and Sohn, 2019b), and post-eruption changes of the sea level (Sohn et al., 2002). Songaksan thus demonstrates that tuff rings or maars, which are second only to scoria cones in abundance on Earth and particularly common in coastal areas, can be potential sources of accurately levelled and dated data points for the Quaternary sea-level curve, which has so far been constructed based mainly on the study of fossil terraces and oxygen isotopic ratios.

An important consequence of rapid sedimentation of volcaniclastic deposits in coastal environments is that they can preserve the records of low-magnitude Earth surface processes, such as daily tides and fair-weather waves, which are rarely preserved in geological records because they are likely to be eroded and redeposited by bigger events (Sohn and Yoon, 2010). Volcaniclastic deposits can therefore be used to interpret ordinary or non-catastrophic processes in a depositional environment and can not only allow more accurate interpretation of the depositional environments but also help assess the biases in the stratigraphic records, which preferentially record rare, bigger, and extraordinaryACCEPTED events (Doyle et al., 2001). MANUSCRIPT Modulation of eruptions, e.g., the timing of the ascent of a new magma batch, by the Earth and ocean tides combined with meteorological events is difficult to prove at Songaksan in spite of the temporal linkage between the magma ascent and the storm event. However, heavy rainfall during the storm event is considered to have modulated the eruption processes by changing the water saturation in the diatreme. Almost complete removal and renewal of the diatreme fill during the storm event, inferred from the study of tephra characteristics (Go et al., 2017), is currently Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

unresolvable and cannot be related to either intrinsic or extrinsic factors, necessitating further investigation of the relationship between volcanoes and their environments.

Acknowledgements and Funding

This work was funded by the National Research Foundation of Korea (NRF-2020R1A2B5B02001660). We thank Gerardo Carrasco-Núñez and Jocelyn McPhie for their careful reviews, which helped to improve and clarify the manuscript.

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Figure captions

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).

Fig. 2. Intracrater exposure of Songaksan, showing steeply inward-dipping (ventward-dipping) tuff beds with a prominent internal truncation surface. The tuff sequence is overlain by dark gray ponded trachybasalt lava and reddish scoria deposits. See Figure 1c for the location of the photograph.

Fig. 3. Summary of volcaniclastic terminology after White and Houghton (2006) and Sohn and Sohn (2019a).

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.

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.

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 mm-thick and continuous mud drape, and is barely visible. A photo scale is at the center 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 overlyingACCEPTED coarse sandy deposit. (c) At a more proximalMANUSCRIPT locality than (a), unit R1 is composed of a single mm-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 the location of the photographs.

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 Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

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 for scale is 28 cm long. (f) Supratidal beach ridge deposit at the top of the Hamori Formation, which occurs up to an altitude of ~10 m. The deposit is composed of very coarse 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.

Fig. 8. Backscattered electron (BSE) images of undisturbed tuff samples. (a) Both juvenile (sideromelane, S) and accidental (quartz, Q) particles of unit T1 occur as aggregates or ash-coated particles. Large interconnected pores reflect loose packing of cohesive ash aggregates or ash-coated particles. (b) Unit T3 is almost completely devoid of accidental quartz particles. The 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. (c) Unit T2 has the highest accidental quartz content of all the pyroclastic facies of Songaksan. The unit also lacks ash aggregates. Ash-coating is also poorly developed.

Fig. 9. Changing diatreme conditions during the storm event (modified after Go et al., 2017 by permission from Springer Nature, Bulletin of Volcanology, copyright 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 . (b) The diatreme is inferred to have been emptied by the eruption of unit T1 because unit T2 is almost completely devoidACCEPTED of ash-coated particles and low-Mg juvenile MANUSCRIPT 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 Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

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).

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