抄録
The Towada volcano that formed Lake Towada was located in the upper reaches of the Iwaki River passing through the Tsugaru Plain, whose volcanic activities continued during the late Quaternary. The authors surveyed the formation processes of the southern Tsugaru Plain, laying stress on the influence of former activities of the Towada Volcano, with photo-interpretation and field surveys. The fan surfaces were divided older to younger into 4 from F·I to F·IV Younger fans were composed by sand and gravel produced from older fans. The younger fans have gentler slopes than the older ones. The lower part of the older fans has been covered by the deposits of the younger fans.
F·I consist of volcanics such as sand, silt, andesite rock etc. produced during the fromation of the Towada Caldera. Enormous amounts of volcanic products poured from Towada during the formation of F·I. Thereafter, volcanic activities of Towada decreased. Due to the decreasing of volcanics from the upper reaches, F·I was eroded by the streams and F·II was formed. F·III was formed by the materials from the upper reaches.
The authors correlated F·III with the lower terraces in the upper reaches and with the upper natural levees in the lower reaches. At last, F·IV and lower natural levees were formed.
The authors presume that F·I was formed during 12, 000 to 10, 000 years B. P. and F·III was formed 8, 000 to 4, 000 years B. P.
