Abstract
Kuttara volcano overlies Tertiary sediments and in part, early to middle Quaternary andesitic volcanic rocks. The history of the volcano is divided into three stages: somma, caldera, and post caldera stages. Somma stage: A large stratovolcano, 6 km basal diameter and 1 km relative height was formed and made by olivine basaltic and pyroxene andestic lavas and pyroclastics. Caldera stage: Eruptive material of this stage is subdivided into three units which are in ascending order, the Rampoge pumiceous deposit, the Noboribetsu pumiceous deposit and the Noboribetsu base surge deposit. Rampoge pumiceous deposit (pyroxene andesitic) is composed mainly of four sheets of pumice flow deposits and three sheets of pumice fall deposits. Among them pumice fall deposits IIa and lib have a possibility of areal tephras. Noboribetsu pumiceous deposit (dacitic) is composed mainly of two sheets of welded pumice flow deposits, accompanied by pumice fall deposits. A fossil forest was found in the upper part of the deposit II (COLLABORATIVE RESEARCH GROUP for IBURI, 1983). Many fossil rootless fumaroles are developed in the upper unwelded part of the Noboribetsu pumiceous deposit I. 14C-age of the Noboribetsu pumiceous deposit II is 40,190 y. B. P. (Gak-10195). Karurusu clay bed was deposited in small lakes formed by the Noboribetsu pumiceous deposit. Noboribetsu base surge deposit overlies conformably the Noboribetsu pumiceous deposit I, and underwent fumarolic alteration as above mentioned. A caldera of Crater Lake type was formed immediately after the base surge deposit was formed. Post caldera stage: Last events of this area are the extrusion of Tachibanaike lava flow (pyroxene andesitic), and Hiyoriyama lava dome (dacitic), and the formation of explosion deposits of Oyunuma and Jigokudani. The total volume of Kuttara volcano exceeds 18 km3.
