NATURE OF LAVA 241 



pel us to place the beginning of movement of lava at least as low as the bottom of the 

 fracture zone, but they probably offer no sufficient ground for limiting tin- lava's 

 origin to this or any other specific depth. 



Volcanic gases. One of the most distinctive features of volcanoes is the 

 explosive ait ion arising from the gases and vapors pent up in the lava. Lavas 

 in the interior, under high pressure, contain much gas, and as they rise and the 

 pressure is relieved, some of these gases escape from the hot liquid. In those 

 ca>cs 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. The violence of the explo- 

 sion reduces a portion of the lava to the fineness of dust, the "ash" and 

 "smoke" of the volcano. 



The causes of the differences of gas action in different volcanoes are undeter- 

 mined, but the following suggestions may point to a part of the truth: (i) Some 

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

 plosive; (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 explosions lies in the fact that the lavas have begun to crystallize while 

 yet in the volcano. When crystals form in the magma (lava), 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 extraordinary eruptions as those of Krakatoa 

 and I'elee contain many small crystals which had formed before the explosion took 

 place. Incipient crystallization does not, however, appear to be a universal 

 accompaniment of explosive action. 



Igneous rocks contain gases in large quantities. 1 When the lavas lodge under- 

 ground without free communication with the surface, there is reason to think that 

 they retain a larger percentage of their original gases than the lavas which are 

 exposed freely at the surface. At any rate, deep intrusive rocks contain notable 

 quantities of gases. Recent surface lavas also contain gases of similar kinds, but 

 not in equal amount, so far as available analyses show. 



One of the outstanding problems of geology is to determine (a) how far the 

 material of the gases had the same origin as the material of the lavas, and (b) how 

 far the material for the gases penetrated from the surface. The peculiar propor- 

 tions of the rock-gases, among which hydrogen and carbon dio.vde greatly pre- 

 ponderate, seem to imply that they are not derived chiefly from surface waters or 

 the atmosphere; they appear to be original constituents of the rocks in the main, 

 and when given forth they appear to constitute real additions to the atmosphere. 



THE CAUSE OF VULCANISM 



The fundamental explanation of volcanic phenomena is wrapped 

 up in the origin of the earth, for the conditions which the earth in- 

 1 Rollin T. Chamberlin, Gases in Rocks, Carnegie Institution, 1908. 



