410 



F. A. Ferret — Volcanic Vortex Rings. 



some years ago,* to the water in the lower portions of a geyser 

 or to the water in a boiler under tension. This, by reason of the 

 pressure upon it, is actually in liquid form, but it is potentially 

 a vapor because of its temperature, and if the pressure is 

 removed it will flash into the gaseous condition. Similarly, the 

 lower zones of lava in the volcanic conduit are potentially in a 

 condition of explosive extension and are maintained in a 

 liquid or semi-liquid state only by the pressure from above. 

 If now we imagine a relief of this pressure — say by a lateral 

 outflow of lava sufficiently rapid and copious to materially 



Fig. 5. 



reduce the height of the lava column — a powerful gaseous 

 expansion will be initiated in the magma below and this will 

 extend progressively downward with progressive relief of 

 pressure from above. This gaseous development will be 

 distributed, si&divisional, intermolecidar, the magmatic mass 

 expanding to a froth and being finally ejected as a cloud of gas 

 and ash — it is impossible that it should appear in a liquid form 

 under these conditions. 



Note that this ash will have been formed under very different 

 conditions from that which is blown off from the surface of 

 the lava in the crater during the early part of the eruption. 

 This latter is formed in contact with atmospheric air and is 

 quickly cooled ; it is, therefore, of a vitreous nature with 

 surface oxidation. But the ash from the depths will have 



* " Some Conditions affecting Volcanic Eruptions." Science, Aug. 38, 1908. 



