DALY. — THE NATURE OF VOLCANIC ACTION. tV 



Since experiment shows that the viscosity of a liquid rises rapidly 

 with pressure, it is instructive to assume higher values of v for the 

 greater pressures. If v be taken again arbitrarily as 500 and 10,000 

 times that of water for magma under the pressures of 1000 atmospheres 

 and 2000 atmospheres, respectively, we have for x these values : 



Pressure vianr^aifu- ' Terminal Velocity (i) . 



(ats.). viscosiiy. Cm. per Second. Meters per Hour. 



200 1.15 0.47 16.9 



1000 5.75 .034 1.2 



2000 115.00 .0012 .04 



In all cases smaller bubbles would rise more slowly, x varying 

 directly as the sc^uare of the radius. 



Two important conclusions may be drawn from these computations. 

 Gas bubbles of the " standard " mass or of smaller mass must rise from 

 the deeper levels of an abyssal injection with extreme slowness. In 

 view of the high magmatic viscosity and great pressure in depth, it is 

 conceivable that it may take thousands of years for a " standard " 

 bubble to rise from a depth of, say, ten kilometers to the earth's 

 surface. This suggests one reason why gaseous emanation is so pro- 

 longed at central vents. 



Secondly, from the slowness with which bubbles rise, it is clear that 

 a swarm of bubbles, which for any reason have been aggregated locally 

 in .'special abundance, would be dispersed into the surrounding, less 

 vesiculated magma with great slowness. The local mass of magma 

 thus specially vesiculated would be less dense than the average magma 

 and, as a unit, would rise toward the crater. It now remains to indicate 

 that a very moderate amount of extra vesiculation must cause such a 

 two-phase mass to rise with comparatively great velocity. 



Of course, this case has not been investigated experiment^illy ; an 

 indirect method must be used in its discussion and the result can at 

 present hardly be other than (jualitative. 



( )nce again to make the mental picture clearer, it is well to assume 

 certain conditions arbitrarily. As an example, let the swarm-tilled 

 mass be spherical ; let the reigning pressures and nuigniatic viscosity bo 

 as in the foregoing cases ; let the surrounding magma have a density of 

 2.75 ; and let the extra vesiculation be to the extent of 50 " stundurd " 

 bubbles per cubic centimeter on the average. The corresponding den- 

 sities of the sphere are .shown in the second column of the following 

 table : 



