296 proceedings: geological society 



shale shown by Winchester. This jelly-Uke material moves with the 

 movement of the water. Storms move the water-weeds and the hght 

 floating jelly-hke stuff may be moved at the same time, and rumpled. 



REGULAR PROGRAM 



Robert B. Sosman: Note on volcanic explosions. The usual con- 

 ception of a volcanic explosion is that of the release of a store of pent- 

 up energy which has been held in a confined space by external pressure. 

 The idea takes two forms: (i) The "boiler explosion," in which the 

 pressure has been raised by heat to a value which exceeds the break- 

 ing strength of the containing rocks; or, (2) the "geyser eruption," 

 in which a metastable configuration of the materials of the volcano 

 has been disturbed and a violent reaction has begun. In either case, 

 the conception is that of a system in rapid reaction in an effort to reach 

 equilibrium, following some change in external conditions. 



An experiment by the speaker, made several years ago, showed that 

 finely divided alumina, which is well known to be very hygroscopic, 

 could be superheated in an open electric furnace and then be "ex- 

 ploded" by a mechanical disturbance. The phenomenon is strikingly 

 analogous to the dust explosions of Lassen Peak and Mont Pelee. 



The steam boiler and the geyser may be called "explosive systems." 

 Another class of -explosions results from the initiation of chemical re- 

 actions in "explosive mixtures," such as gunpowder, or a mixture of 

 sulfur and potassium chlorate. It seems unlikely that many volcanic 

 explosions can be of this character, on account of the difficulty of ac- 

 cumulating the necessarily large quantities of substances capable of 

 reacting (such as oxygen and hydrogen), without the dissipation of the 

 energy by continuous quiet reaction, as at Kilauea. 



In a third class are the "explosive substances," of which nitroglycerin 

 is the most powerful common representative. These explode by in- 

 ternal disintegration and recombination, and can be "detonated" 

 by methods other than simple rise of temperature. Organic dust 

 (flour, coal) with oxygen and moisture adsorbed on the surfaces of its 

 grains is, to all intents and purposes, a chemical compound and an 

 explosive substance, though of a milder type than the nitro-compounds; 

 and even inorganic dust with adsorbed moisture, such as the alumina 

 in the experiment cited, is, analogously, an explosive. It is suggested 

 that dust explosions of the Peleean type, which are often plainly super- 

 ficial and not deep-seated, are true explosions, possibly set off by me- 

 chanical disturbances. The same may be true of Vulcanian and Plinian 

 explosions, the explosive in these cases being a metastable liquid silicate 

 or a mixture of liquid and solid silicates, brought into its metastable 

 condition by a gradual rise of temperature or by the gradual accumula- 

 tion of water or magmatic gases, either through distillation or through 

 fractional crystallization. The specific character of the shock necessary 

 to detonate an explosive substance; the limited range of propagation 

 of the explosive wave in a powdered material, as contrasted with the 

 more complete detonation which can be brought about in a continuous 



