262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



months this protrusion grew into a massive dome rising above the 

 previously elevated ground. 



The final result of this new activity of Usu was a double volcanic 

 dome. The first dome, formed by the upheaval of the old ground, 

 is about 200 meters wide and 1,000 meters in diameter and has ter- 

 racelike forehills. The second dome, about 150 meters above the 

 first dome and 300 meters in diameter at its base, is formed by the 

 protrusion of a column of primitive viscous lava. 



Ever since the discovery of laccoliths by G. K. Gilbert in the 

 Henry Mountains in the United States, the possibility of a local 

 upheaval of the earth's surface has been suggested. Several cases of 

 remarkable upheavals of land about a crater of eruption have actually 

 been observed. Such a case was the eruption of Usu in 1910, when a 

 land mass 3 kilometers long and three- fourths kilometer wide at the 

 northern foot of the volcano, on the shore of Lake Toy a, was raised 

 a maxium of 155 meters. The present activity of the same volcano 

 supplies another and more striking instance, proving that a land mass 

 can be domed by the force of the slow rise of an underlying viscous 

 volcanic magma. 



The renewed activity of Usu offered a good opportunity for the 

 study of the mechanism of the birth of volcanoes in which a felsic 

 magma (dacite) participated. Because the country was in the midst 

 of war, the volcanic activity was not investigated by a large, well- 

 organized expedition, but its main events were observed at intervals 

 by persons from several research institutes, including the Earthquake 

 Research Institute and the Geophysical Institute, Tokyo University; 

 the Central Meteorological Observatory; the Geophysical Institute 

 and Petrological Institute, Tohoku University; and the Geological 

 Institute, Hokkaido University. 



PREVIOUS ERUPTIONS 



Usu Volcano is situated in the southern part of Hokkaido, nearly at 

 the north end of the Nasu volcanic zone, which traverses the central 

 part of the main island, Honshu, to the middle of the north island, 

 Hokkaido. 



According to ancient chronicles, Usu erupted four times prior to 

 1854— in July 1662, December 1768, November 1822, and March 1853. 

 All these eruptions took place on the upper part of the southern slope 

 of the somma, or large depression crater on the summit of the volcano. 

 A later striking eruption took place at the northern foot of Usu in 

 July-August 1910. No other information concerning the volcanic or 

 seismic activities of Usu Volcano is available for the years since then. 



But Usu Volcano had a long history previous to the events recorded 

 in the chronicles. On the basis of studies by Z. Harada and S. Sasaki, 



