272 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



PETROLOGIC NOTE 



The new lava is a hypersthene dacite with sporadic phenocrysts of 

 andesine and hypersthene in a fine-grained groundmass. In spite of 

 a silica content as high as 69.7 percent, phenocrystic quartz is absent, 

 and its place is represented by the abundant presence of quartz and 

 cristobalite grains in the groundmass. 



It is noteworthy that the new lava closely resembles the lavas of the 

 central cones O-Usu and Ko-Usu, in mineralogical as well as chemical 

 composition, though there is a long time interval between the forma- 

 tion of these domes. These dome lavas are, however, entirely differ- 

 ent from the more mafic somma lava of Usu Volcano, a basaltic ande- 

 site with silica content as low as 53 percent. Lavas of intermediate 

 composition are lacking completely in this volcano. 



It is unknown how the felsitic dome lavas were derived from the 

 original mafic magma. It is evident that there has been no differ- 

 entiation within this felsitic portion of the magma reservoir through- 

 out this long period since the formation of Ko-Usu dome. This is 

 striking when compared with Paricutin Volcano lavas, which show 

 a noticeable change in chemical and mineralogical composition within 

 the course of only a few years. 



The preceding description shows that the recent eruption of Usu 

 has many striking features, such as series of severe earthquakes, 

 marked topographical deformation, violent explosions, and extrusion 

 of solidified mass of juvenile lava. Such remarkable characteristics, 

 quite unusual in many other volcanoes, are common in this volcano, 

 as is clear from the inspection of previous eruptions. 



Reprints of the various articles in this Report may be obtained, as long as 

 the supply lasts, on request addressed to the Division of Publications, Smith- 

 sonian Institution, Washington 25, D. C. 



