北海道における第四紀火山砕屑物の分布とその編年

抄録

Hokkaido lsland has been a site of intense volcanic activity during Quaternary period through which 36 volcanoes have erupted along the inner zones of the Kurile arc and the Honshu arc. A number of Quaternary pyroclastic are widely distrbuted in this island, and vast pyroclastic plateaux are also developed around calderas. As shown in Fig. 1, most of the ash-fall deposits of Pleistocene as well as those of Holocene were accumulated on the eastern side of the volcanoes, which may give an important information on the atmospheric circulation in the past. Pyroclastic flow deposits around the calderas are so widely developed that thay are useful in Quaternary geology not only as important time-makers like ash-fall deposits but also as excellent indicators of old topographies. Tephrochronological studies on these pyroclatic deposits have been carried on by pedologists, geologists, archaeologists and geophysicists, since 1933. This paper is a summary of these studies. (Fig. 1 and Table 1)
In early Pleistocene, a tremendous amount of rhyolite pumice-flow were erupted from the central highland of Hokkaido where a major volcano-tectonic depression was formed through this violent activity. Then, a number of dacite pumice-flows accompanied by ash-falls was issued from several calderas. Most of them were erupted in glacial periods of middle Pleistocene.
During the last interglacial period, volcanic activities were rather quiescent. Then, in the last glacial period, violent activities of dacite pumice-eruptions took place again, and most of the calderas in Hokkaido completed their formation in this period. Some pyroclastic falls of this period embedded fossil forests composed of Picea jezoensis, and covered lower terrace deposits, from which Mammonteous primigenius primigenius was discovered. Some pumice-flow deposits swept over the middle and lower terraces and buried pre-existing valleys, which were formed by a glaclo-eustatic regression and reached scores of meters or more below the present sea level.
Volcanic activity in Holocene has been confined to small areas near the Pleistocene volcanoes, especially within calderas, and it was interrupted by two quiescent periods, 5000-2000 years B. P. and 1000-500 years B. P. respectively. Most of the pyroclastic materials of Holocene are of andesite. Owing to the Jomon transgression which reached about 10m high from the sea level, ash-falls of early Holocene were not accumulated as a primary deposit in alluvial plains near the sea coast. Most of the volcanoes constructed in Holocene erupted during the last 500 years. Detailed tephrochronology of the younger pyroclastic deposits may substitute for historic records of volcanic activity, because no reliable records older than 300 years B. P. are available in this island.