Eruption types and vesicularities from pumice clasts of Member N-5 in the Maljandeung tuff, Ulleung Island, Korea

We analyzed several microtextures and measured densities and vesicularities to interpret its eruption type, and fragmentation type and mechanism in the Member N-5 of the Maljandeung Tuff, Ulleung Island. Occurrence of wet pyroclast materials, such as accretionary lapilli and armoured lapilli in the Member N-5A and D, suggests that the volcanic activity at the Maljandeung during 18.8 ka B.P. originates from 'wet' explosive eruptions, and reflects the environment in which magma may encounter external water. It means that the explosive eruption is a 'wet' phreatomagmatic eruption than 'dry' magmatic eruptions, as considering water-rich wet plumes and abundance of finer grains. Considering the occurrence of charred woods in the member, the external water is considered to be groundwater that can come into contact with magma. Mean vesicularities in pumiceous lapillistones of the Member U-5B range from 54.5 to 63.3%, indicating that the level of the phreatomagmatic fragmentation surface in which magma comes into contact with aquifer is lower than level of the hypothetical magmatic fragmentation surface, because the level where magma can cause magmatic fragmentation in the conduit is where the magma rises and expands at a vesicularity of 75 to 83%. Thus, the phreatomagmatic fragmentation was driven by volumetric expansion of groundwater after it was rapidly heated by contact with magma in the conduit during 18.8 ka B.P. Exsolving magmatic volatiles may also contribute to expansion and fragmaentation during the phreatomagmatic event. At this time, the phreatomagmatic fragmentation resulted in a higher gas pressure caused by groundwater than the magmatic volatiles. The blocky grains dominant the Member resemble shattered glass or glass that has been partially twisted and deformed while still plastic during the fragmentation. These fragments form a fragmentation mechanism that stress waves propagate through a melt and produced strain rates that exceed its bulk modulus due to quenching during the fragmentation.