386 GEOLOGY 



been thought that lava spontaneously absorbs gases, especially when 

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

 relieved and the lava cooled and solidified, the larger part of the 

 gases escape. 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 off after the lava has been extruded 

 and is about to congeal. The gases are then given off slowly and 

 quietly. If, however, the lava is surcharged with gases, and if 

 their escape is retarded by the viscosity of the lava, they gather 

 in large vesicles or bubbles in the lava in the throat of the volcano, 

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

 upwards and outwards, often to great distances. The violence of 

 the explosion reduces a portion of the lava to the fineness of dust, 

 the "ash" and "smoke" of the volcano. Portions of the lava 

 may be inflated by the gases without being blown to bits, forming 

 pumice and scoria, as already noted. Masses of lava that have 

 solidified into more or less rounded masses in the crater are hurled 

 out as bombs, and not infrequently portions of the walls of the crater 

 or of the duct below are broken off 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) Some lavas 

 contain more gases than others, and hence are predisposed to be 

 more explosive; (2) some are more viscous than others and hence 

 hold the gases more tenaciously until they accumulate and acquire 

 explosive force, while the more liquid lavas allow their gases to 

 escape more freely; (3) probably a main occasion of violent ex- 

 plosions lies in the fact that the lavas have begun to crystallize 

 while yet in the volcano. When the crystals form in the magma, 

 they exclude the gases which were in the substance from which 

 they are developed, and this excluded gas overcharges the remainder 

 of the lava. This view is supported by the fact that the pumice and 

 ash of such extraordinarily explosive eruptions as those of Krakai <>a 

 and Pele contain many small crystals which had certainly formed 

 before the explosion took place. Incipient crystallization docs 

 not, however, appear to be a universal accompaniment of explosive 

 action. 



