LIFE HISTORY OF A VOLCANO 451 



probable condition of Vesuvius before the eruption of 

 79 A.D. 



A HAWAIIAN CRATER. 



215. Life History of a Volcano. A volcano is simply a 

 place in the earth's surface where molten rock or frag- 

 mental material from within the earth is extruded. If 

 the extrusion of the lava is accompanied by gaseous ex- 

 plosions, it will be blown into fragments which will fall 

 around the vent and build up a steep-sided cone, like that 

 of Monte Nuovo. If the eruption is less violent, lava may 

 flow from the crater or pour from openings formed in its sides. 



As the same volcano usually ejects both the fragmental 

 and molten material, volcanic cones are generally complex 

 in their composition. Sometimes, however, cones are 

 found which are composed entirely of one sort of material. 

 Those which are largely or entirely formed of lava have 

 a much gentler slope than the others. Such are the great 

 Hawaiian cones. 



