Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Accepted Manuscript Geological Society, London, Special Publications Volcano-air-sea interactions in a coastal tuff ring, Jeju Island, 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 To access the most recent version of this article, please click the DOI URL in the line above. When citing this article please include the above DOI. 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 Manuscript version: Accepted Manuscript This is a PDF of an unedited manuscript that has been accepted for publication. 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Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 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, 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 *Correspondence: [email protected] 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, 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 magma batch, related to the chemical change, could not be linked with either the Earth and ocean tides 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 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. 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 tephra 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 maar-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 atmosphere. 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 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 Ross, 2011). We cannot see the crater floor of the tephra ring at Songaksan because it is filled by later scoria cones and ponded lava (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 nature, which was produced by explosions 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, 7433 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 subduction 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 lavas 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 (trachybasalt) 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 tide is semidiurnal, and the tidal range is 1.7 m at the southern coast of the island.