414 MOUNTAINS, EARTHQUAKES, VOLCANOES, GEYSERS 



since the beginning of the volcanic activity less than two 

 years before. 



How volcanic cones form. In the beginning, a volcano is 

 simply a weak place or vent in the earth's surface through 

 which hot matter is ejected. With each eruption the land 

 around the vent or crater builds up and takes the form of a 

 mound or cone. The crater rises higher and higher above its 

 original position and finally becomes the summit of a high 



FIG. 281. Lava streams. 



mound or a high hill, or even a mountain. The crater of Mt. 

 Vesuvius has been built up until it is now about 4000 feet high. 

 From the crater an opening runs down into the earth and affords 

 a channel for the escape of the hot lava. In some of the large 

 volcanoes the craters are more than a mile wide, in the smaller 

 ones the craters are only a few hundred feet in size. From these 

 craters enormous masses of matter are emitted and build <ID 

 massive volcanic cones. 





