ERUPTION OF USU VOLCANO — MINAKAMI ET AL. 271 



huge slabs of lava surrounding a central core, like the scales of a lily 

 corm about the central bulb. The thick layers covering the surface 

 of the dome have numerous concentric cracks, emphasizing its shell- 

 like structure. The surface shows many parallel striations caused 

 by the intense friction experienced during the upheaval of the dome. 



The greater part of the dome is covered by thick layers of clayey 

 materials, which are mostly burnt into a reddish-brown bricklike sub- 

 tance by the heat emanating from the lava. Tuffaceous beds are also 

 common. The presence of gravels on many parts of the dome deserves 

 special mention ; even the very summit is crowned with beds of gravels 

 and sands 2 meters thick, so that the surface exposures of new lava 

 are greatly restricted. 



It is remarkable that the new lava is everywhere broken into irregu- 

 lar fragments by numerous cracks, suggesting that it did not solidify 

 in situ but had consolidated during its rise from the depths, and was 

 fragmented during the upward movement of the mass. On the surface 

 of the dome are many elliptical or irregular crevices several meters in 

 diameter, with incandescent interiors. These crevices emit vapors and 

 gases vigorously and deposit beautiful bluish-green, yellowish-green, 

 purple, and dark-gray sublimation products on the surrounding 

 surfaces. 



By the end of 1945 the roof mountain and the dome ceased growing 

 and were completely formed. Then, too, the earthquakes and the de- 

 formations subsided. The eruption of Syowa Sinzan, which had en- 

 dured for 22 months, essentially came to an end. 



Since the completion of the new mountain, no further remarkable 

 change has been experienced. A small explosion occurred on May 2, 

 1948, forming a new craterlet. The smoke that issued from the new 

 mountain diminished gradually, and in August 1946 we could climb 

 up on the dome. Sulf ataras around the dome became less active, but 

 several fumaroles with vapor temperature of 95.5° were formed at 

 the south side of the roof mountain. Temperature of the new lava, 

 however, seems to be not much lower than at its first appearance. 

 Ishikawa measured the temperature of the dull red lava exposed at 

 the west foot of the dome in November 1950 at 780° C. 



Small depressions were gradually formed at several places on the 

 roof mountain. Loose materials on steep slopes of the roof mountain 

 sometimes fell down to its foot. And after the cessation of volcanic 

 activity, water began to well out from the side of the front mountain, 

 near the site of the old springs at Hukaba. Its temperature measured 

 about 15° C. in November 1950. A pond that formed in a depression 

 near Matumotoyama in June 1944 has evaporated. 



No noticeable earthquake in the area has been felt since October 1945, 

 but weak tremors were recorded 29 times in 1951 on a seismograph 

 placed at Sobetu village by the Sapporo Meteorological Observatory. 



