抄録
Many rootless fumaroles are found in the upper unwelded part of the Noboribetsu Pumice Flow Deposit-I (dacitic in composition; about 40,000 years old), which is distributed around the Kuttara Caldera, southwest Hokkaido. Alteration due to the fuma-rolic action extends into the Noboribetsu Base Surge Deposit overlying the Pomice Flow Deposit-I. The number and size of the rootless fumarole are larger, and also the degree of alteration is stronger in Kaminoboribetsu, about 4 km west of the Caldera, whereas the size of the rootless crater pits tend to increase in Satsunai, about 6 km southwest of the Caldera. The alteration part is usually reddish but white or dark reddish brown to almost black in strongly altered part. The characteristic alteration minerals determined by X-ray diffraction are: (1) kaolinite from the Noboribetsu Base Surge Deposit, (2) montmorillonite from the white part of the Noboribetsu Pumice Flow Deposit-I, and (3) hematite, natroalunite, and 10 A halloysite from the various parts of both the Base Surge Deposit and the Pumice Flow Deposit-I. The dark reddish brown color resulted from hematite.
