43 



VOLCANOES 



(i) Lava. — A lava is a more or less completely melted rock; 

 the degree of fluidity varies greatly in different lavas, but is rarely, 

 if ever, perfect. Instead of being a true liquid, a lava ordinarily 

 consists of larger and smaller crystals, embedded in a pasty mass, 

 which is saturated with steam and gases. The degree of fluidity 

 depends upon several factors, the most obvious of which is tempera- 



FlG. 10. 



Edge of Hale-mau-mau, showing the ropy forms of the highly fluid 

 lava, when cooling. (Photograph by Libbey.) 



ture ; the more highly heated the mass is, the more perfectly will 

 it be melted. The quantity of imprisoned gases and vapours 

 present has also an important effect, and some lavas appear to owe 

 nearly all their mobility to these vapours. A third and most sig- 

 nificant factor is the chemical composition. Those lavas which 

 contain high percentages of silica (Si0 2 ), the acid lavas, are much 

 less readily fusible than the basic lavas, in winch the percentage of 

 silica is lower. The difference in the proportion of silica present 



