1988 Volume 33 Issue 4 Pages 305-320
Andesitic volcano groups were emplaced during middle Pleistocene around the Waitasan area in the center of the Hohi volcanic zone, Kyushu, Japan. These andesitic volcanoes formed on the buried Shishimuta caldera, after eruption of the Yabakei pyroclastic flow in an extensional stress field. Stratigraphic successions, radiometric ages, volumes and compositions of the volcanic rocks in the Waitasan area are investigated in order to clarify the evolution of the post-Shishimuta caldera activity. The characteristics of volcanic activity, which took place during 1.0-0.3 Ma, changed at about 0.7 Ma from predominance of large-volume pyroxene andesite lavas to that of small-volume hornblende andesite lavas, suggesting that the magma beneath Shishimuta caldera decreased its temperature. At about 0.7 Ma, the dominant volcanic edifice changed from lava plateau to lava domes accompanied by a few stratovolcanoes. The appearance of stratovolcanoes can be accounted for by the occurrence of local compressional stress field which initiated at about 0.7 Ma and started to overlap the previously dominant N-S extensional stress field.
