Relationship between eruptive style and vesicularity of juvenile clasts during eruptive episode A of Towada Volcano, Northeast Japan

It has been reported that juvenile pumice lapilli found in plinian eruptions have high vesicularity, while those found in phreatoplinian eruptions have low vesicularity. However, juvenile glass shards from phreatoplinian eruptions consist of large, expanded bubbles such as bubble wall-type glass. These glass shards seem to possess high vesicularity, unlike the pumice lapilli. This study examines the factors causing this difference, especially focusing on the temporal variations in the vesicularity of the juvenile pyroclasts from eruptive episode A of Towada Volcano, Northeast Japan. This examination was conducted through four analyses: density measurements of pumice lapilli, thin section texture classification of pumice lapilli, classification of glass shards, and surface texture classification of pumice lapilli. Further, pumice lapilli from plinian eruptions have a low density, and those from phreatoplinian eruptions are characterized by high density. The density of the pumice lapilli depends on the eruption style and is hence determined after the eruption. A progressive increase in the amount of large bubbles is observed in glass shards ejected during an eruptive magmatic to phreatomagmatic sequence. Because it does not hinge on the eruptive style, it is assumed that the vesicularity of the glass shards is kept from the conduit before contact with water, especially on fragmentation by magma vesiculation in the conduit. The surfaces of the pumice lapilli show a similar increase in vesicularity with time as glass shards. However, this increase is not successive throughout, but decreases temporarily at the phreatomagmatic stage of the eruption, as in the case of density. This indicates that the successive bubble growth continues within the pumice, and additional vesiculation is superposed when the magmatic eruption comes into contact with water. Because of this, different juvenile clasts exhibit different vesicularities upon cooling. Interestingly, magma vesicularity increases before coming into contact with water, i.e., before transitioning to the phreatomagmatic stage. In fact, the magma–water ratio aside, this might be an important factor causing phreatomagmatic eruptions.