DALY. — THE NATURE OF VOLCANIC ACTION. 71 



vent illustrates the close connection between volcanic and plutonic 

 geology. 



The original location of each first-rank vent is thus explained by the 

 roof- topography of the underlying magma chamber. Some one of the 

 cupola-like oll'shoots of the lluid magma, where it i)enetrates the solid 

 rock above, must become a place for the accumulation of the rising 

 gases. A vent once formed at the top of the cupola, it must tend to 

 persist as a vent throughout the period of magmatic tluidity. Other 

 vents from the same chamber may be opened, but must have shorter 

 lives, because of the drawing away of the juvenile gases toward the 

 more favored vent. (See Figure 7.) 



Continuance of Acti city at Central Vents: Analysis 

 of Conditions at Kilauea. 



Left to itself, the lava column of a vent must soon freeze and activ- 

 ity must cease. Long-continued activity is conditional on victory in 

 the struggle with cold. How is the victory attained ? How is the heat 

 of the underlying magma chamber transferred to the narrow vent? 

 Hawaiian vents supply data on this fundamental question. Though 

 Kilauea may be the vent of a satellitic injection (see p. 109), the mech- 

 anism is doubtless the same as for a vent over a main abyssal injection. 



Rate of Ihut-hi^.'i through Conduction into the IValls. — It is possible 

 to obtain a rough idea of the enormous rate at which heat is given out, 

 by conduction and radiation, at Kilauea. (Plate L) Actual calcula- 

 tion will show that radiation is much more responsible for the loss of 

 heat than is conduction into the wall-rocks of the vent. To make this 

 point clear it will be assumed that the cross-section of the conduit is 

 throughout as large as the area of the lava lake, though very probably 

 the lake represents a strongly flaring part of the lava column. (See 

 Figure 6.) The conditions of the lake in 190'.), when it was studied 

 by the writer, are assumed. The area of the lake (and therewith the 

 cross-section of the lava column) is considered as circular, with radius 

 of loo meters. This is more than the superficial extent of the lake in 

 1909 but less than its average extent since 1.S20. The cylindrical pipe 

 with the uniform cross-section is assumed to extend to a depth of two 

 kilometers, where it opens out into the great feeding chamber. 



Let the temperature of the magma be assumed as 1200° C. ; and 

 let the average original temperature of the rocks now forming the 

 conduit walls be assumed as 40''C. Two hundred and fifty years after 

 the conduit was first opened and henceforth occupied by lava at the 

 uniform temperature of 1200°C., the rate of flow of heat through the 

 wails would be nearly uniform; and 12 meters from the contact of 



