74 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Mere thermal convection can hardly be regarded as an essential 

 factor in postponing the solidification of a lava column. In this 

 matter the analogy with water heated from below should be applied 

 only with due attention to quantitative values. The degree of super- 

 heat in the actual well-established vents is not indefinitely high ; it is 

 doubtless no more than 200° or 300^ C. If convection be lively enough 

 to keep the column molten, the maximum thermal-density differences 

 within the vent itself should certainly be less than those corresponding 

 to a difference of 100°. A temperature change of 100° C. means a 

 density change in magma of less than one half of one per cent.^s The 

 density change in water as it passes from 4° C. to 100° C, or vice 

 versa, is about 4.3 per cent. With a density difference about one 

 tenth that of water in the same temperature interval, and with that 

 difference distributed through kilometers of depth instead of through 

 decimeters, as in the ordinary convective experiment with water, the 

 convective potential in the lava column is evidently of a very low order. 

 Moreover, the speed of the convection depends on the viscosity of the 

 magma, which through chilling and through pressure is doubtless, on 

 the average, hundreds or thousands of times more viscous than water. 

 It follows that in resistance to be overcome, as in working potential, 

 heat convection must be incomparably less rapid in a volcanic conduit 

 than in artificially heated water. 



For example, let us suppose that at the depth of two kilometers the 

 conduit passes into the feeding magma chamber ; that there the tem- 

 perature is 1300° C, while the temperature at the surface is 1200° C, ; 

 that the average kinetic viscosity of the conduit lava is as low as that 

 of a liquid 100 times more viscous than water ; that the thermal con- 

 vection in the conduit is to be compared in rapidity with that obtain- 

 ing in water heated from 4° to 100° in a wide tube one meter high. 

 The maximum convective gradient for the water system may be ex- 

 pressed as 



4.3 (per cent expansion) _ , q 



1 (meter, thickness) 

 The gradient in the lava column is approximately 

 0.5 (per cent expansion) _ ^^^j,. 

 2000 (meters) 



The maximum speed of convection in the water of the imagined ex- 



/ 43 \ 



periment is, then, I - X 100= 11,720,000 or more times greater 



y^.UOU^O J 



than that of the lava in the conduit. 



" According to Barus, as quoted in Amer. Jour. Science, 26, 26 (1908). 



