452 



FIRST YEAR SCIENCE 



Some volcanoes, like Stromboli, are in constant eruption; 

 others, like Etna, vary their eruptions with irregular 

 periods of rest, while still others remain quiet for very 

 long periods and then suddenly break forth with terrific 

 force, as did Vesuvius in 79. As a rule, but not always, 



CROSS SECTION OF A LAVA FLOW. 



eruptions are preceded and accompanied by earthquakes. 

 Just why volcanoes erupt is unknown. 



After a volcanic cone has come into being it is subject 

 to the action of the erosive forces, and unless its material 

 is renewed by fresh outpourings it will in time be worn 

 down. Unlike other kinds of mountains it is also liable 

 to disruption by explosions from within. 



216. Distribution of Volcanoes. The number of active 

 volcanoes on the earth is about 300. Most of them are 

 situated on the borders of the continents, on islands near 

 the continents, or else they form islands in the deep sea. 

 Soundings show that there are many peaks in the sea 



