MAGMATIC DIFFERENTIATION IN HAWAII 311 



inhibiting convection which is so powerful in highly fluid columns 

 like those of the active Mokuaweoweo or Kilauea. In a paper 

 published in the current volume of the Proceedings of the American 

 Academy of Arts and Sciences, the writer has indicated the prob- 

 able cause of the very energetic stirring visible in the Kilauea lava 

 column. The action is there called "two-phase convection," as 

 it depends on vesiculation of the lava in depth. The gas-bubble 

 phase is formed through supersaturation of the liquid with juvenile 

 volatile matter. A small amount of vesiculation in depth must 

 lend much buoyancy to the magma, which rushes up the conduit 

 in periodic gushes; its place is taken by sinking magma that has 

 been rendered more dense by the escape of its included gas at 

 the surface. This type of stirring — incomparably more effective 

 than thermal convection can be in such a column — keeps the vent 

 open by transferring the abyssal heat to the zone of radiation; 

 and as well, tends to prevent differentiation because of the con- 

 tinuous, thorough mixing of components in the magmatic column. 

 A dormant state is introduced when the special supply of gas is 

 largely exhausted. Then two-phase convection is slowed down, 

 and if the other conditions permit, gravitative differentiation may 

 affect the column more or less. Revival of activity is the result 

 of a renewed concentration of juvenile gases rising into the con- 

 duit from the feeding magma chamber. The consequent strength- 

 ening of two-phase convection means a speedy remixing of the 

 products of differentiation in the column. Hence, in such hot 

 vents as those at Kilauea, Mokuaweoweo, or Matavanu, outflows 

 of highly differentiated lavas are not to be expected. 



When Mauna Kea was approaching extinction, its magmatic 

 column or columns, characterized by increasing viscosity and 

 perhaps less charged with juvenile gases, were less stirred by 

 two-phase convection. In relative quiet they differentiated to 

 a slight extent, giving a trachydoleritic type as the upper, salic 

 pole. The gases became dissipated at the craters, and the 

 effluent lavas of this latest phase of the volcanic pile are 

 "clinkstones" because comparatively free of gas-pores. The 

 explosions which formed numerous cinder-cones at the summit 

 may have been due to the inhibition and superheating of meteoric 



