The 1944 Eruption of Usu. 

 in Hokkaido, Japan 1 



By Takeshi Minakami 



Earthquake Research Institute, University of Tokyo 



TOSHIO ISHIKAWA 



Geological Institute, University of Hokkaido 



Kenzo Yagi 



Institute of Earth Science, University of Tohoku 



[With 6 plates] 



Usu Volcano, which had been in a dormant state since 1910, again 

 became active at the end of 1943 and in the following 2 years gave 

 birth to a new composite volcanic dome, or mountain, now called 

 Syowa Sinzan. This dome is one of the rare examples of a volcano 

 whose growth and history have been traced by eyewitnesses from the 

 very beginning of its birth. It is, moreover, unique in the mode of 

 its formation, quite different from Paricutin Volcano in Mexico, for 

 example, also born in 1943, which is an ordinary basaltic cone with 

 lava flows. 



The seat of the new mountain was once, like Paricutin, a cornfield, 

 called Kumantubo. It is now covered by numerous boulders and 

 blocks of lava and presents a dismal scene. An upheaval of the 

 ground fissured the cultivated lands at the eastern foot of Mount 

 Usu in the early months of 1944. This was followed by more rapid 

 uplift in the gently sloping and partly cultivated ground adjoining 

 this area on the north early in April. Late in June, while this up- 

 heaval was still in progress, volcanic explosions began in the central 

 part of the rising mass which, during the succeeding three months, 

 formed successively seven small craterlets by the ejection of small 

 quantities of pyroclastic materials. About the middle of November 

 the ground in and about the craterlets began to be pushed up en masse 

 by the protrusion of a column of red-hot viscous lava to form several 

 huge lava masses separated by narrow crevices. In the ensuing 10 



1 Revised from a report that originally appeared in Bulletin Volcanologique, ser. 2 

 vol. 11, 1951. 



261 



