ERUPTION OF USU VOLCANO — MTNAKAMI ET AL. 



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precursor earthquake, the lava at last appeared, extruding as a solid 

 from the earth's surface. It grew more and more as shown in figure 

 4, its development accompanied by numerous earthquakes and marked 

 deformation of the topography. According to seismometric observa- 

 tions these earthquakes differed greatly from those previously ob- 

 served. According to observations by Mimatsu, postmaster of Sobetu 

 village, a black conical tower, about 25 meters in height, could first 

 be distinctly made out on January 10, 1945, during the few days when 

 it stood revealed from the enveloping white smoke. Its top part in- 

 clined to the north at the end of January but soon tilted to the south. 



Figure 4.— Profile of the new mountain, Syowa Sinzan. 1, Fresh lava; 2 burnt clayish 

 rocks; 3, volcanic ejecta; 4, mud; 5, volcanic ashes; 6, somma lava of Usu; 7, clay; 

 [ 8, gravel beds; 9, tuff; 10, Takinoue lava. 



Glowing lava could be first observed in the crack opened at its 

 northern foot on February 11. As the cone grew larger, spots of 

 red-hot lava could be seen. Especially at night several bright red 

 spots on the sides of the dome gave a wonderful sight to inhabitants 

 of Sobetu village. 



Thus the solid lava continued to rise about a meter a day, with 

 marked trepidation during the early stage of development. In May 

 a branch lava mass appeared at its west side. At the southwest side 

 of the dome, a reddish glovelike mass of rock about 50 meters high 

 stood separated from the main body by a fissure at a place where many 

 solfataras vigorously ejected gases. It was called "Sangoiwa" 

 (meaning coral rock) from its form and color. The dome did not 

 grow uniformly as a single mass, but differentially or independently 

 in divided bodies. The division of its top part may have resulted 

 from fissure planes opened in the cap rocks by heavy friction after it 

 was solidified. This new dome is within the arc of the seven craters 

 with the periphery of the dome in contact with the craters. 



