CHAPTER VII 



VULCANISM 



VOLCANOES 



A volcano is a vent in the earth's crust out of which hot rock 

 comes. The hot rock may flow out in a liquid form (called lava), 

 or it may be thrown out violently. It is generally built up into a 

 cone (Fig. 163), which may become a mound, a high hill, or even 



Fig. 163. The cone of Vesuvius in the background, and the ruins of Pompeii. 



a high mountain. Though they are the products of volcanoes, the 

 cones are themselves often called volcanoes. The volcano from 

 which lava flows makes a cone with gentle slopes, while the vent 

 from which solid matter is thrown makes a cone with steeper slopes. 

 Some volcanoes send out both liquid and solid rock. Both may 

 be issuing at about the same time, or lava may flow out at one time 

 and solid rock be thrown out at another. Quantities of gases and 



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