VULCANISM 171 



of Pelee, their temperature is sometimes so high as to be destructive 

 to life. 



Number, Distribution, etc. 



Number. The number of volcanoes is not easily determined, 

 and is not definitely known. There are various reasons for this 

 uncertainty. Thus it is often impossible to say whether a quiet 

 volcano is dormant or extinct. For this and other reasons, the 

 number of active volcanoes cannot be stated exactly, but it is com- 

 monly estimated as between 300 and 400. Something like two- 

 thirds of them are on islands, and the remainder on the continents. 

 There may be others in the sea which are not known, for feeble 

 volcanoes in the deep sea would make little show at the surface. 



Distribution. The general distribution of active volcanoes is 

 shown in Fig. 171. Many of them are arranged in belts, within 

 which they are sometimes in lines. The most marked belt nearly 

 encircles the Pacific Ocean, as with a girdle of steaming vents. 

 This belt may be said to begin with the volcanic islands south of 

 South America, and includes the numerous vents in the Andes, 

 and in the mountains of Central America and Mexico. It widens 

 in the western part of the United States, where the volcanoes are 

 extinct, but narrows again in Alaska and the Aleutian Islands. 

 On the western side of the Pacific, the belt includes the many active 

 vents of Kamchatka, Corea, Japan, the Philippine Islands, New 

 Guinea, New Hebrides, and New Zealand, and an off-shoot from 

 it includes the volcanoes in the islands of Java and Sumatra. The 

 volcanoes of the West Indies are sometimes considered as an east- 

 ern branch of the same belt. Outside this belt, volcanoes are 

 numerous in and about the Mediterranean Sea, and there are many 

 others which cannot be connected with any well-marked system. 



Most volcanoes are in the sea or near it. Not a few of them 

 are in the mountain regions, though they do not occur in all moun- 

 tains. Many are on ridges or swells on the sea bottom, or on ridges 

 or swells which rise above the sea. Such, for example, are the West 

 Indian volcanoes. The volcanoes on the continents are mostly 

 near the shores, but many shore lands are without them, and there 

 are a few volcanoes far from the sea. Thus there is an active 

 volcano in Africa 700 miles from the sea. and there are fresh cones 



