DALY. — THE NATURE OF VOLCANIC ACTION. 105 



1. Average analysis of twenty olivine basalts from Hawaii, repre- 

 senting closely the composition of the basement of Mauna Kea. 



2. Andesitic basalt, flow at ll,()()()-foot contour, Manna Kea. 



3. Trachydolerite, flow at 13,000-foot contour, Mauna Kea. 

 Analyses 2 and 3 by G. Steiger in the laboratory of the United 



States Geological Survey. 



Progress in Explosiveness at the Greater Vents. 



The explosive effect at central vents is a function of the magmatic 

 viscosity and of gas tension, which means gas concentration. 



Though the presence of much gas tends to lower the viscosity, 

 temperature is obviously in dominant control over that property of 

 magma. The initial store of heat in the abyssal injection is normally 

 lost through radiation in the crater, through conduction at the roof 

 and walls of the whole magma chamber, through assimilation of 

 country-rock, and possibly through the absorption of vadose water. As 

 the whole mass cools, the juvenile gas emanates with ever lowering 

 temperature and the lava of the volcanic conduit must have a slow 

 decrease of average temperature. 



Magmatic diflerentiation must tend to affect the viscosity of the 

 upper zone of lava, the exploding zone, in the same sense. The more 

 acid differentiate usually rises toward the top of the vent. Though 

 differentiation may be roughly cyclical, the successive splitting tends 

 to make a secular increase of acidity in the upper zone of the conduit 

 magma. Hence, irrespective of temperature, there is an increase of 

 viscosity in the magma zone where explosions originate. The case of 

 ^launa Kea, just described, is an example of the partial control by 

 magmatic differentiation over explosiveness. As the viscosity rises, 

 the escape of magmatic gases is more diflicult ; the resulting tension is 

 periodically relieved by explosions. Here also the action is cyclical, 

 but there is, on the average, a slow increase in the amount of gas 

 trapped before each explosion. 



Again, the amount of volatile matter entering the magma column, 

 either through assimilation of sediments or through the direct absorp- 

 tion of meteoric water, tends to increase with process of time. And, 

 with the gi-owth of a great, generally porous cone, the chance fir 

 phreatic explosion at or near the crater is favored. 



All of these factors vork fot/ctl/rr to produce maxinnim explosivenc-s 

 at central vents which are long-lived i)ecause fed from great abyssal 

 injections. The maximum normally appears in an advanced stage in 

 the evolution of a first-class volcanic cone, though necessarily some 



