7J2 ' DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



lit is much stronger than the mass of tuffs surrounding it. Without a prelimi- 

 nary thinning of the plug we should expect explosion to open a vent on the side 

 of the cone rather than at the old crater. In point of fact, the symmetry of 

 great cones like Etna, Fuji-yama, or May on, together with the known history 

 ©f many cones, shows that the greater activity is normally renewed at the 

 original vent. 



This behaviour is intelligible if it be granted that magmatic gases continue 

 to rise from the depths and collect under the plug. The temperature of the 

 lava column slowly rises because of the exothermic chemical reactions and 

 because of the compression of the accumulating gas, which steadily increases 

 In tension until it reaches a certain maximum value. The plug is thereby 

 slowly melted at the bottom. After sufficient melting has occurred, the magma, 

 with its newly acquired tension, becomes capable of bursting the plug with one 

 or more major explosions. A new period of activity is initiated; it will last 

 antil the special accumulation of gas at the top of the general magma chamber 

 is largely exhausted. 



This rhythmic action is, of course, subject to the complicating influence of 

 the solution of foreign matter by the magma, in the conduit or in the feeding 

 chamber, or in both. The material absorbed may be volatile, e.g., vadose or 

 resurgent water, and will therefore increase the gas-tension in the vent. Whole- 

 sale evisceration of the volcanic pile may occur, so that a Somma cone becomes 

 a caldera floor, later to be surmounted by a Vesuvius cone. 



In general, each central vent increases in explosiveness toward the end of 

 its life. Whether juvenile or resurgent, the gases have increasing difficulty 

 in escaping into the air. This means, of course, increase in the magmatic 

 viscosity, which is conditioned on several factors, the chief of which are tem- 

 perature and chemical constitution. As the temperature of the main magma 

 chamber falls (for several obvious reasons), the body passes through a stage 

 where differentiation of the magma is specially liable to take place. That magma 

 may be either the primary basalt or a syntectic. In either case gravity causes 

 the more acid, and generally more alkaline, lighter pole of the differentiation 

 to rise to the surface, where already radiation of heat specially heightens the 

 viscosity. With increase of silica and alkalies at the top of the lava column and 

 decrease of the iron oxides, magnesia, and lime, the viscosity must there rise. 



Finally, even in this brief section, some reference should be made to the 

 advisability of distinguishing two chief classes of central eruptions. So far, the 

 feeding magma chamber has been assumed to be a main abyssal injection. Yet 

 at is to be expected that vents may occasionally be opened in the roofs of lac- 

 coliths, thick sheets, and other satellitic injections, which have lost thermal 

 and hydrostatic connection with their own parent abyssal injections. For con- 

 venience, central vents which are fed directly from the main injections, may be 

 called ( principal ' ; those fed from satellitic chambers may be called ' subordi- 

 nate.' 



Living and extinct vents, probably belonging to the ' subordinate ' class will 

 be described in the forthcoming publication on the nature of volcanic action. 

 That paper will present grounds for the belief that Kilauea in Hawaii is a 



