148 AET. 3. — B. KOTO : 



The ephemeral dome was then ruptured on its southwest side 

 through the expansion of lava and the pressure of the enclosed 

 gases, leaving behind the hoof- shaped carcass of the dome, as 

 may be seen in Plate XIV. on the left and the train of lava-flows 

 from the ruptured side on the right in Plate XV. It was in the 

 interior where Azuma observed the spattering eruption already 

 referred to (p. 146). After the lapse of a year the feature of the 

 Yuno-hira vent had entirely changed, the ephemeral dome had 

 disappeared, as may be seen in the foreground of the picture in 

 PL XII. Fi^. 3. 



In April, 1914, on his second trip the writer had a good 

 opportunity of observing the process in detail from an isolated 

 hill- top of Yebino-zuka near Kurokami on the east coast, when 

 the vent No. 5 made a forcible outburst without warning but with 

 tremendous detonation (Text-fig. 29), throwing up dark curdy 

 smoke and grains of juvenile lava- crust. When the act was ac- 

 complished, the lava gushed out down the slope with chnking as 

 if a viscous fluid was pumped out with one stroke, often from a 

 special lateral opening a Httle below the main vent, as may be 

 seen in some of the Nabé-yama vents. 



The process seems to be as follows : The pent-up gases 

 within the vent break open the plug or solidified skin of lava at 

 the upper end of the conduit, resulting in the release of pressure 

 in the column of lava. The occluded gases and steam then ex- 

 pand and cool the magma wliich in turn becomes viscous. The 

 liberation of gaseous bodies in the magma in consequence of relief 

 of pressure tends to let gases free from the viscous body, causing 

 thereby an increase of the volume of foaming, puffing and spatter- 

 ing lava, and at the same time forcing up the upper lava column. 



