ERUPTION OF USU VOLCANO MINAKAMI ET AL. 



267 



After the early eruptive outbreaks of July, moderate and violent 

 paroxysmal eruptions occurred successively for more than 3 months 

 until October 31. The main explosions were dated as follows: July 

 11 and 15, August 1, 5, and 26, September 9, 23, and 27, October 1, 16, 

 and 31. On July 11, a new large crater (the third crater) was opened, 

 and thereafter the issuing vapor increased remarkably. The fourth 

 crater was formed on August 26, the fifth one on September 9, the 

 sixth and seventh ones in October. All the craters were arranged 

 in an arc opening to the north. The seven craters opened close to one 

 another; the earlier ones were buried under the detritus and ash of 

 the later explosions, and after September 1944 they hardly retained 



Figure 3. — Geologic map of the newly formed mountain Syowa Sinzan. (After K. Yagi 



and J. Z. Okamoto.) 



their original forms. No incandescent bombs and blocks were found, 

 but the blocks, ash, and mud ejected in later eruptions had a higher 

 temperature than the earlier ones of June and July. Several of the 

 craters were dry, indicating a temperature higher than boiling water ; 

 others when active always contained hot water. 



During August and September, when paroxysmal eruptions oc- 

 curred, blasts loaded with heavy ash occasionally descended on Nisi- 

 kohan village, 1 kilometer north of the craters, breaking windowpanes 

 and even occasionally blowing off the roofs of houses. Fortunately, 

 these ash avalanches were not sufficiently hot to ignite wooden mate- 

 rials, but only in this respect did they differ from a "nuee ardente" 

 (hot avalanche) . It seems that the genesis of the hot avalanches is 

 mechanically related to the existence of the "belonite" of the "crypto- 



