752 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



duction into the crust, the lower basic part of which may be thus melted. An 

 unknown but possibly considerable fraction of the total superheat may remain 

 in the original substratum, and this amount of superheat would characterize 

 the basalt when rapidly injected into the crust. 



If, as generally believed, the earth's acid shell is specially radioactive, its 

 evolving heat must tend to be retained beneath a geosynclinal blanket and local 

 superheat in the substratum developed. Perhaps this is the principal cause for 

 the enormous excess of thermal energy in batholithic magmas. 



Another source of superheat is found in the conversion into heat of the 

 mechanical energy necessary for injecting a viscous melt into an opening 

 cavity. 



These sources of superheat would alone furnish enough thermal energy to 

 raise the injected basaltic magma from 1140° C. to some temperature short of 

 1500° C. or 1600° C. 



The piling up of 10,000 metres of lava over a large area would have an 

 analogous superheating effect on the substratum. This conclusion enables us to 

 give some explanation of the fact that the lavas of Kilauea and Mauna Loa 

 seem to be the hottest known in any volcanic vent. The vast Hawaiian lava 

 plateau has, apparently, been built up by the comparatively rapid effusion of 

 basaltic flows from Pacific depths averaging 6,000 metres to heights above sea of 

 about 4,000 metres. The unique lava fountains of Mokuaweoweo, while showing 

 obvious evidence of considerable superfusion, are described as glowing with 

 ' white heat.'* If a correct description, this implies a temperature of 1300° C. 

 or possibly 1400° C.f Such temperature must be a minimum for the sub- 

 stratum which feeds these vents, where there is continuous loss of heat in the 

 convectively stirred lava. 



Speculative argument and limited observations in nature agree, then, in 

 fixing some such temperature as 1300° C. as a minimum for the basaltic mass 

 injected into the crust-rock below a great mountain range. A batholithic body 

 of this magma is thrust into rocks which have already been abnormally heated 

 in the crush of mountain-building. 



Capacity of Superheated, Plutonic Magma for Melting and Dissolving 

 Xenoliths. — Basalt must have a thermal capacity much like that of diabase at the 

 same temperature. Barus's experiments show that the average specific heat of 

 diabase for the interval 1300-1140° C. is -350.^ The heat-energy contained in 

 the substratum, if it be superheated 160° C. above its melting point (1140° C), is 

 in excess of that contained in the substratum just above its melting point by 

 (160 x -350=) 55 + gram-calories. 



* J. D. Dana, Characteristics of Volcanoes; New York, 1891, p. 200. 



t LeChatelier and Boudouard's High Temperature Measurements ; New York, 1904, 

 p. 246. 



JC. Barus, Bull. 103, U.S. Geol. Survey, 1893, p. 53. For the interval 100-20°C, 

 the mean specific heat is about '185. There is, in fact, a steady increase in the mean 

 value as the temperature of any silicate or silicate mixture rises. This fact goes far 

 to explain the prolonged liquidity of assimilating magmas. Cf. J. H. L. Vogt in 

 Christiania Videnskabs-Selskabets Skrifter, math, naturv. Klasse, 1904, No. 1, p. 40. 



