THE ERUPTIOX OF USA-SAX AXD THK FORMATIOX 

 OF A XFW MOrXTAIX IX lAFAX. 



Bv CHARLES D.WISOX. Sc.l).. I".(;.S. 



The eruption of Usii-san in Japan, which took 

 place in the sutnnier and autumn of 1910, was 

 in no way remarkable for its violence or duration. 

 Nor was it attended by any loss of life, for the 

 police wisely took precautions in time and removed 

 all the inhabitants of the disturbed region beyond 

 the reach of danger. But few eruptions have con- 

 tributed more to our knowledge of the volcanic 

 mechanism : and the formation of a new mountain, 

 not by the accumulation of ashes, nor by the over- 

 flow of lava, but by the actual upheaval of tlie 

 ground, is a phenomenon that is 

 certainly rare in our annals (^f 

 volcanic action. Fortunatelv, the 

 different phases of the eruption 

 were witnessed by an observer of 

 wide experience. Professor Omori, 

 the director of the Seismological 

 Institute in the Imperial Universitv 

 of Tok\-o, paid two visits to the 

 district, one during the course of 

 the eruption, and the other three 

 months later, and his first report, 

 which has recentl\- been issued, 

 contains so much that is of 

 interest that a summarv of it mav 

 be useful. 



The mountain of Usu lies in 

 the south-western portion of 

 Hokkaido, the northern island of 

 the Japanese empire, a little more 

 than fift\- miles to the north of 

 Hakodate, and on the north-east coast of Volcano 

 Hay (Figure 200). In the same district there 

 are three other volcanoes, Makkari-nopli, twent\- 

 miles north of Usu-san, Tarumai-san, thirt\' miles 

 to the north-east, and Komaga-take fifty-four miles 

 to the south. Between these volcanoes there is 

 evidently a close connexion. In 1874, there was 

 a great eruption of Tarumai-san, after which, for 

 nearly thirty-two years, no outburst of any import- 

 ance took place in Hokkaido. In August, 1905, the 

 recent period of volcanic activity began with an 

 eruption of Komaga-take, which lasted a fortnight. 

 This was followed in January, 1909, by a remarkable 

 eruption of Tarumai-san, which lasted more than 

 tliree months, during which a lava-dome four 

 hundred and forty feet in height was formed, and in 

 July. 1910, by the eruption of Usu-san, here 

 described. 



There appears also to be an intimate relation 

 between the volcanic and seismic phenomena of the 

 district, ("ommon as earthquakes are in certain 

 parts of Japan, the}- are comparatively infrequent 



FiGL'KE 200. 



Map of part of the Island of Hokkaido 



visitors in Hokkaido. In April and .Ma\-, 1908, how- 

 ever, about a year before the eruption of Tarumai- 

 san, the Island of Rebun, close to the northern 

 point of Hokkaido, was disturbed b\' numerous 

 earthquakes and earth-sounds, the origin of which 

 was under the sea to the south-west of the island (at 

 A, Figure 200). On June 15th, 1910, or about six 

 weeks before the beginning of the Usu-san eruption, 

 a violent local earthquake, also with a submarine 

 origin (at B, Figure 200), caused some damage near 

 Rumoe. This was followed during the eruption, on 

 September 8th, by another earth- 

 quake, somewhat less strong, in 

 the same district. Before these 

 two, the last strong earthquake 

 felt at Rumoe occurred thirt}-six 

 years earlier, on Februarv 28th, 

 1S74, shorth' after the great 

 eruption o( Tarumai-san. The 

 origins of the earthquakes of 1908 

 and 1910, with those of earlier 

 shocks in 18i4 and 1856 (at C 

 and D, Figure 200), lie along a 

 band (indicated by the broken 

 line, bigure 200), which runs 

 nearly north and south, passing 

 through or close to Tarumai-san. 

 Taking into account the rarity 

 of earthquakes in Hokkaido, it 

 is difficult to resist Professor 

 Omori's conclusion that the 

 relation between the earthquakes 

 and eruptions of the island is not accidental. 



Usu-san is a comparatively small volcano. It 

 rises from an irregularly circular base, about 

 fourteen square miles in area, bounded on the 

 north by the Lake of To\a. It is a flat plateau-like 

 mass with a central crater, the longer and shorter 

 diameters of which are about two thousand three 

 hundred and one thousand eight hundred and sixty 

 \ards in length. The north and south rims of the 

 crater are one thousand seven hundred and seventy 

 one feet above the sea. but on the east and west 

 sides there rise two lava domes (O-Usu and Ko-Usu) 

 to heights of two thousand two hundred and seventy 

 and two thousand two himdred and fourteen feet, 

 respectiveh'. The surface of the Toya lake stands 

 at two hundred and seventy-nine feet above sea 

 level. Before the last eruption, the northern slope 

 of the mountain dipped into the lake at a small 

 angle, so that along the southern coast of the lake 

 there was a band of fiat ground three miles long and 

 less than half a mile in width. At the east and west 

 ends of this band are two hills, East Maru-yama 



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