PRODUCTS OF VOLCANOES 



Fitf. 2jS. The edge of ;iu old stream of l;iv;i, showing (i) its broken character 

 due to movement alter the outside had hardened, and (2) the steep slope of the 

 stream of stiffened lava. Near Flagstaff, An/.. (Fairbanks.) 



but the temporary quiet very likely means an exhaustion of the 

 supply of gas or lava, or both. 



Products of volcanoes, i . Pyroclastic material. The f ragmen tal 

 matt-rials which are blown out of a volcano are, as a rule, portions 

 of lava which solidified before ejection, or during their flight in the 

 air. From masses of rock tons in weight, the fragments grade down 

 to particles of dust. The dust particles (often called ash) are thrown 

 high into the air in some cases, and, caught by the winds, are shifted 

 incredible distances (p. 13). In some cases, beds of volcanic ash 

 many feet in thickness (as those of Nebraska) are found far from any 

 known volcanic center. The extremely fine ash from the great 

 explosion of Krakatoa floated several times around the earth in the 

 equatorial belt, and spread northward into the temperate zones. 



Liquid rock, lava. The term lava is applied to all kinds of liquid 

 rock, and also to the solid rock formed when fluid rock congeals. 

 The various phases assumed by lava, on solidification, have been 

 noted in connection with igneous rocks. Lava never flows so freely 

 as water, and is, in many cases, very stiff or viscous. The distance 

 to which it flows depends on its liquidity, its amount, and the slope 

 of the surface on which it is poured out. 



