Abstract
The Misaki tuff cone, located on Oki-Dōgo Island within the Oki Islands UNESCO Global Geopark, Japan, provides an exceptional record of phreatomagmatic processes associated with alkaline monogenetic volcanism. This study reconstructs the eruptive history of Misaki, which is exceptionally revealed by a natural cross-section that exposes the volcano’s internal stratigraphy, using detailed field logging and lithofacies analysis. The sequence begins with Surtseyan-style activity, where tephra jets and continuous uprush deposited disorganised lapilli tuffs with minor tractional features, followed by base surges that generated thinly to crudely stratified lapilli tuffs in medial to distal settings. A transition to drier eruption conditions is marked by deposition of a dark, juvenile-rich lapilli tuff, characterised by disorganised proximal facies grading downslope into stratified surge deposits. Mass failure within the crater produced lapillistone units, while the final eruptive phase excluded external water and generated effusive lava flows that capped the edifice, forming broad sheetflows. Post-eruptive hydrothermal alteration led to pervasive palagonitisation of the lower succession.
The lower, extensively palagonitised succession is overall finer-grained, with juvenile material dominated by sharp glass shards and few vesicular lapilli. In contrast, the overlying darker tuff unit is dominated by moderately vesicular juvenile lapilli with relatively sharp ash-sized glass shards, reflecting a shift in fragmentation style from dominantly phreatomagmatic to a combination of phreatomagmatic and magmatic fragmentation.
Whole-rock geochemical analyses reveal a compositionally restricted suite, with variations largely attributable to palagonitisation rather than primary magmatic processes. The Misaki and the underlying Otokoyama basalt units are petrogenetically linked, with evidence for fractional crystallisation dominated by olivine, minimal crustal contamination, and mantle sources consistent with a mixture of garnet peridotite and pyroxenite lithologies. Cr-spinel compositions suggest either small-scale mantle heterogeneity or partial melting processes in a garnet-bearing source region.
Together, the volcanological and geochemical evidence indicates that the Misaki tuff cone records a relatively simple but well-preserved monogenetic eruptive history, offering new insights into magma–water interaction, eruptive transitions, and post-eruptive alteration in small alkaline intraplate volcanoes.