268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



dome" of solid lava, as yet unrevealed, and that the phenomenon de- 

 pended on the direction initally taken by the ejected materials. A 

 plug or cover closing a crater vent prevents juvenile gas from free 

 exit into the air. Accumulated gas sometimes tries to escape from the 

 plug's marginal foot, and accordingly ejected materials may be shot 

 out obliquely. Or the gas may move the plug or the cover by its 

 strong pressure, and squeeze through some newly formed weak zone 

 along the marginal side of the plug. In such cases the initial direc- 

 tion of ejected materials is not vertical, but oblique. There was such 

 an inclined vent in the Ansei crater of Komagatake Volcano, where 

 ash avalanches occurred in 1942. In other cases, as in the 1902 

 eruption of Pelee, the hot avalanche (nuee ardente) was caused by 

 the abrupt exposure of the incandescent interior of the newly formed 

 lava dome, and its destruction. As a result, lava blocks of larger 

 size would roll down along the steep slope in the form of a landslide, 

 accelerated by gravity, and the escaped vapor, containing dense, fine 

 materials, both at high temperature, would rise into the air by 

 adiabatic expansion. 



During the paroxysmal eruptions from July to October, extremely 

 shallow earthquakes took place at Hukaba, immediately below the 

 rising area. The epicentral areas, including the newly formed craters, 

 continued to rise without cease. Toward the end of October, the 

 cornfields, which formerly were 130 to 180 meters above sea level, 

 were now nearly 100 meters higher and formed a flat dome-shaped 

 mountain about half a kilometer square in area. 



By the end of October the dwelling houses of Hukaba village and 

 the roads passing through the village were entirely devastated, more 

 by the severe rise of the area than by the deposit of ash and other 

 ejecta. The railway running along the east foot of the newly formed 

 mountain was destroyed, first by the cracks and the upheaval of the 

 ground, then later by a lateral movement from the active center. 

 As a result, the roads and railways were compelled to move 60 meters 

 and 20 meters toward the east. The Sobetu River, running parallel 

 to the railway, was dammed by the upheaval of the river bed to form 

 a temporary lake 1 kilometer long. Owing to the changes in the 

 courses of the running water on and around the rising area, mud 

 streams carrying ash, soil, and detritus, especially after a heavy rain, 

 came down from unexpected directions and ravaged the downstream 

 area. 



BIRTH AND DEVELOPMENT OF THE LAVA DOME 



Early in November the paroxysmal eruptions ceased, and soon 

 one could see from Nisikohan a black mass close to the site 

 of the craters. By the middle of December this appeared from 

 Sobetu like a peak of pyramidal shape. Ten months after the first 



