76 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Trro-phase Convection. — However, there is a different and very 

 powerful kind of convection constantly illustrated in Halemaumau 

 when that lake is in full activity. (Figure 5.) The persistent stream- 

 ing of the lava into the caves, characteristically developed at the shore 

 cliffs of the lake, is evidently due to surface gradients. In general, 

 the "scum " stands higher in the central part of the lake than it does 

 in the caves and in the channels leading to the caves. The " scum " 

 or thin crust of the lake prevents or retards the escape of the mag- 

 matic gases, which accumulate beneath it and form a kind of froth, or 

 emulsion of lava and gas, of relatively low density. The tendency is, 

 thus, to raise the crust in one or more areas. In each cave, because of 

 reflection from its roof, and perhaps also because of special heating 

 through actual combustion of sulphur, hydrogen, and other gases, the 

 crust is rapidly and completely fused. The escape of the gases is 

 there facilitated and the surface of the lava is correspondingly lowered. 

 The surface slopes are, therefore, steepest in the channels leading to 

 the caves, and streaming at the rate of two to five kilometers an hour 

 may be observed in the channels. Elsewhere the surface slopes are 

 lower and streaming is less rapid. The caves are not outletting tun- 

 nels, as so often stated, but each is closed at a distance of a few meters 

 from its entrance. The lava which has streamed into the cave must 

 return to the main part of the lake. Only one way of return is possi- 

 ble, that by a backward sub-surface current. Having lost its dilating 

 gas and grown rapidly denser, the heavy lava sinks and flows toward 

 the center of the lake. Similarly, the ever-changing surface slopes in 

 other parts of the lake compel vertical currents and vortices of the 

 most complex design. (Figure 6.) This type of magmatic movement 

 may be called " two-phase convection." ^o It depends on the presence 

 of a liquid " phase " and a gas " phase " in the lava. 



If vesiculation of the liquid magma is possible in great depth, two- 

 phase convection may cause a relatively speedy transfer of hot magma 

 to the surface. How effective this process can be is worthy of some- 

 what detailed statement. The imposing change in magmatic density, 

 which is effected by very slight increase in vesiculation, will first be 

 indicated. The speed at which individual bubbles rise will then be 

 estimated, and, finally, a rough quantitative idea of the convection 

 enforced by the development of gas bubbles in depth will be obtained. 



The specimens of Hawaiian pahoehoe lava collected by the writer 



'° The term "two-phase," so convenient in describing this type of convec- 

 tion, was suggested to the writer by Dr. W. C. Bray, of the Massachusetts 

 Institute of Technology. 



