[Abstract]

During the 20th century, two major eruptions of Hokkaido-Komagatake volcano occurred in 1929 and 1942. The 1929 eruption was one of the largest magmatic eruptions in Japan in the last 100 years, and has been investigated in detail. On the other hand, no systematic volcanological research has ever been done on the 1942 eruption. Since it has not been clarified whether the 1942 eruption was magmatic or phreatic, we decided to investigate the sequence and eruption mode of this event. At the summit, the 1942 eruptives overlie the 1929 pyroclastic fall deposits and are covered by ash-fall deposits of recent phreatic eruptions. Based on the photographs, documentary descriptions and sequence of the eruptive deposits, we were able to divide the eruption into three phases. Phase-1: Start of the eruption on November 16 at 8:00 (JST) that formed NW-SE trending fissure 1.6 km in length. This was followed by eruptions at the main crater (1929 crater) that produced the pyroclastic surges (~8:10). Phase-2: Eruption column had grown up from 8:18 - 8:20 and ash-fall started to spread towards the southeast portion of the 1929 crater. Phase-3: Decrease of the eruption activity at 8:40 and continuous ash emission until the evening.
We newly found fresh pumice and its fragments in the deposits associated with the Phase-1 event. This fresh pumice exhibits no alteration features and has similar petrological features to other historic pumice since 1640. This fresh pumice, however, could be distinguished from other historic pumice by the most silicic nature of its matrix glasses. Therefore, we concluded that the pumice was derived from the juvenile magma that caused the 1942 eruption. Considering the temporal evolution of the component of the eruptive materials, the 1942 eruption began with a phreatomagmatic eruption (Phase-1) followed by phreatic eruptions (Phase-2 and -3).
Before the two major eruptions in 1929 and 1942, small eruptions occurred intermittently from 1919 - 1924 and 1934 - 1938, respectively. Based on these observations, the preceding eruptions could be possible precursor activities of subsequent magmatic eruptions. Since 1996, small phreatic eruptions have intermittently occurred in Hokkaido-Komagatake until 2000. The present state of the volcano, as we would like to emphasize, has been quite similar to the conditions prior to the two previous magmatic eruptions.

Key words:

Hokkaido-Komagatake volcano, phreatomagmatic eruption, juvenile ejecta