270 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



Owing to the severe explosions, thick tuffaceous debris, clayey ma- 

 terial, and other ejecta accumulated on the surface of the upheaved 

 area covering the greater part of the dome's surface. Where lava was 

 exposed the color of its incandescence indicated a temperature of at 

 least 1,000° C. 



Numerous severe earthquakes in the pre-eruption stage took place at 

 depths ranging from 3 to 7 kilometers under the southern side of the 

 Usu volcano, but with the development of the eruption, the depth of 

 origin of the earthquakes came progressively nearer the earth's sur- 

 face at the small area where the craterlets opened and the lava dome 

 formed. 



THE NEW MOUNTAIN 



This newly formed mountain is now composed of two distinct parts : 

 a nearly circular platform, called the "roof mountain," and a conical 

 rock projection above it, called the "dome." 



The roof mountain is a nearly circular platform about 800 to 1,000 

 meters in diameter, surrounded, except for its western side, by slopes 

 of 20° to 40° inclination. Its surface, corresponding to the original 

 terrain, is undulating and covered by numerous boulders and blocks 

 of lava. The roof mountain is chiefly composed of the old somma lavas 

 of Usu, and it is interesting to note that Takinoue lava, which forms 

 the lowermost basement of Usu Volcano, is exposed in the highest 

 part of the roof mountain. The eastern flank of the roof mountain is 

 surrounded by an elevated terrace (or "front mountain") about 70 

 meters above the old alluvial plain of the Osaru and Sobetu Rivers. 

 Toward the middle of July 1944, when the paroxysmal eruption was at 

 its height, it was noticed that the ground around the base of the new 

 mountain began to rise abnormally. It developed rapidly and the hori- 

 zontal displacements in the radial direction were accelerated as the 

 lava dome grew. In May 1945, a hill in the form of a ring or a half- 

 ring, the "front mountain," was formed at the margin of the roof 

 mountain. Judged from morphological investigations, the "front 

 mountain" is built up more completely at its eastern part, which in- 

 cludes the former ricefields of soft material, than at the other part, 

 which is formed of firm materials such as the Usu lava flows and the 

 Takinoue lava. Obviously, the hill of ring form was built by pres- 

 sure acting laterally from the center of disturbance, the intrusion and 

 protrusion of viscous lava, and resulted in an abnormal upheaval 

 nearly 20 meters high, but still retaining the original surface of the 

 ground. At gullies formed in the front mountain, we can observe the 

 warping, folding, and the thrusts of small size caused in this manner. 



The dome of extrusive lava is situated a little west of the center of 

 the roof mountain and is about 150 meters higher, its summit being 

 405 meters above sea level. Its eastern side is an abrupt precipice with 



