Temporal variations in vesicularity of erupted products at Unzen Volcano, Kyushu, Japan

The Unzen Scientific Drilling Project (USDP) commenced in April 1999 with the aim to drill to the volcano's conduit, still hot from the 1990-'95 eruption, in addition to revealing much about Unzen's eruptive history. Recent activity of the volcano has consisted largely of emplacement of viscous domes, which produce block and ash flows. This almost wholly effusive activity is in contrast to the evidently highly volatile nature of the stored magma. An important question that may provide a clue to this paradox is how eruptive behavior has evolved with time. By examining samples from the cores from boreholes drilled into the flank of the volcano, we were able to characterize more than 500 ka of activity. Vesicularity of magma reflects its volatile content and decompression history, and plays a role in producing effusive or explosive behavior. To quantify variations in vesicularity as a function of depth and hence time, we measured bulk density, connected void space, and void-space-to-area ratios for samples from USDP-1 and USDP-2. We found a negative correlation between density and depth, reflecting increasing vesicularity, despite consistent bulk composition. We also found decreasing crystallinity with increasing depth.