Pyroclast textures of the Ilchulbong ‘wet’ tuff cone, Jeju Island, South Korea

Abstract

Jeju Island is a Quaternary shield volcano situated approximately 95 km south of the Korean Peninsula, comprising Holocene–Late Pleistocene phreatomagmatic deposits and shield-forming basaltic to trachytic lavas that overlie Early–Mid Pleistocene marine sedimentary and hydrovolcanic deposits. Ilchulbong tuff cone is among the youngest hydrovolcanic centres on the island, formed along the shoreline from a Surtseyan-style eruption involving interaction of magma with external water. We have analysed lapilli from selected sites on the volcanic edifice, in terms of vesicle population, texture, shape and size, and quantitatively, with reference to bulk vesicularity, vesicle number density, vesicle dimensions and distributions. The lapilli range in total vesicularity from low (15–35%) to high (50–80%) values, and show a modest range of vesicle number densities (N V, number of vesicles per unit rock volume) between 2.26 × 10 3 and 6.97 × 10 3 vesicles mm − 3 . Such vesicle number densities are higher than those typical of Strombolian (N V between 9.3 × 10 1 and 1.3 × 10 3 vesicles mm − 3 , Stromboli 2002, Lautze and Houghton, 2007) and Hawaiian eruptions, and more similar to those of Plinian (N V between 1.5 and 2.5 × 10 3 vesicles mm − 3 , Tarawera, Sable et al., 2009) eruptions. The lapilli studied were not strongly quenched, and we infer that the high vesicle number densities reflect a late-nucleated bubble population related in origin and formed in response to high levels of microlite growth in the Ilchulbong magma prior to final ascent.