ERUPTION OF USU VOLCANO — MINAKAMI ET AL. 



263 



the succession of events in the geological history of the area about Usu 

 may be summarized as follows: (1) Effusion of rhyolitic lavas from 

 volcanoes in Miocene and pre-Miocene times; (2) deposition of the 

 Toyoura formation, consisting mainly of green tuff and tuff breccia 

 in Miocene times ; (3) effusion of andesite lavas during Pliocene times ; 

 (4) deposition of the Rusutu formation (gravel beds, and volcanic ash 

 and pumice) in early Diluvial; (5) uplift of the land in Diluvial 

 times; (6) depression of the Toy a caldera in late Diluvial times; (7) 

 formation of the two volcanoes Toya-Nakazima and Usu since the end 

 of the Diluvial, or early Eecent. 



xAxxk Explosion produeta 

 ***«! and uplifted area 

 * In 1910 



Tatelwa oaid flow 



v Bunael raid flow 



t' + '+l Mlnamlbyobuya 

 + +-t mud flow 



Central cone lava 

 (Hypersthene 



andesite) 



Somma laval Pyro- 

 xene andesite with 

 or without olivine) 



Terrace deposits 

 and aluvial depo- 

 sits 



Ruautsu formation 

 (Diluvium) 



Toyoura formation 

 (Miocene) 



Figure 1. — Geologic map of the Usu Volcano prior to the recent activity. Villages: Sobetu 

 (Sobetsu), Hukaba (Fukaba), Yanagihara (Yanagiwara), Kami-Osaru (Kamiosaru), Usu, 

 and Toy a Hot Springs (Toyako Spa). 



Usu Volcano itself may be considered to have been formed in the 

 following sequence of events: (1) Formation of the main cone and 

 the somma of augite hypersthene andesite lavas and fragmental ejecta ; 

 (2) growth of two central cones of hypersthene dacite lava, O-Usu 

 and Ko-Usu, in the somma ; (3) an explosion at the foot of O-Usu, ac- 

 companied by a mud flow down the southern flank of the main cone ; 

 (4) an explosion at the foot of Ko-Usu in 1822, with a mud stream 

 flowing down the southwestern side of the mountain ; (5) an explosion 

 at the base of O-Usu in 1853, in which part of the dome was blasted 

 away; (6) a remarkable eruption at the north foot of the mountain in 



236639—53- 



-18 



