VULCANISM 387 



The discharge of the gases is often spasmodic, and usually 

 consists -of a succession of distinct explosions. In some cases the 

 explosions follow one another at rather constant and frequent 

 intervals, as in Stromboli, where they occur at intervals of three 

 to ten or more minutes. In others the spasms are distant and 

 irregular. 



Kinds of gases. Steam is the chief volcanic gas. Free hydrogen 

 and oxygen are present also, and are perhaps the result of the dis- 

 sociation of steam at the very high temperature of the lava. Carbon 

 dioxide is probably next in abundance, and carbon monoxide is 

 present. Sulphur gases (sulphuretted hydrogen, sulphurous acid, 

 and perhaps sublimated sulphur) are common accompaniments of 

 volcanic eruptions. All of the sulphurous gases are liable to pass 

 into sulphuric acid by oxidation and hydration. Chlorine and 

 hydrochloric gases are also common, particularly at high tempera- 

 tures. Certain gases, such as hydrogen, chlorine, hydrochloric 

 acid, and some of the sulphurous gases, are especially associated 

 with high temperatures, and are perhaps dependent on them. Sul- 

 phuretted hydrogen, on the other hand, is commoner at lower 

 temperatures. Oxygen, nitrogen, and probably carbon dioxide 

 or carbon monoxide are present throughout all ranges of tempera- 

 tures. The gases mentioned above are the more abundant ones 

 in lavas, but the list is not exhaustive. 



Gases in volcanic rocks. Igneous rocks contain gases, often in 

 large quantities. 1 When the lavas lodge underground 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 freely exposed at the surface. At any rate, deep in- 

 trusive 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. 



Source of the gases. 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. 



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



