THE EXTRUSIVE PROCESSES. 617 



VOLCANIC GASES. 



The most distinctive feature of volcanoes is the explosive action 

 arising from the gases and vapors pent up in the lava. There is not 

 a Httle explosive action of a secondar^^ character arising from the mere 

 outer contact of surface-waters with lavas or with the hot rocks of the 

 crater walls, or with the hot ashes and rocks thrown out; but these are 

 incidental; not essential, features. 



The precise nature of the occlusion or absorption of gases and vapors 

 has not yet been determined. It is thought that lava spontaneously 

 absorbs such gases when at high temperatures, and especially when 

 the gases are under great pressure, and that as the pressure is relieved 

 and the lava is cooled and solidified, the larger part of the gases escapes. 

 In those cases in which the eruption is quiet, the escape of the gases is 

 but partial while the lava is in the crater, and much gas remains to be 

 given out from the molten material after it has been extruded and is 

 about to congeal. The gases are then given off with relative slo^vness 

 and quietness. If, however, the lavas are surcharged with gases, and 

 if these are restrained from free escape by the viscosity of the lavas, 

 the gases gather in large vesicles in the lava in the throat of the volcano, 

 and on coming to the surface explode, hurling the enveloping lava up- 

 wards and outwards, often to great distances. The violence of pro- 

 jection reduces a portion of the lava to a finely divided state consti- 

 tuting the ''ash" and ''smoke" of the volcano. Other portions less 

 divided are inflated by the gases disseminated through them, and form 

 "pumice" and "scoria,'^ according to the degree of inflation, while 

 masses of lava that have already solidified into more or less rounded 

 masses in the crater are hurled forth as "bombs"; not infrequently 

 portions of the walls of the crater or of the duct below are also dis- 

 rupted and shot forth. 



Differences in gas action. — The causes of the differences of gas action 

 in different volcanoes are undetermined, but the following suggestions 

 may point to a part of the truth: (1) Doubtless some lavas contain 

 more gases than others, and hence are predisposed to be more explo- 

 sive; (2) some are more viscous than others and hence hold the gases 

 more tenaciously until they accumulate and acquire explosive force, 

 w^hile the more liquid lavas allow their gases to escape more freely and 

 easily ; (3) some are hotter than others, and hence hold their gases until 



