VULCANISM 



175 



those named above are so high that snow-fields and glaciers are 

 found upon their slopes. The number of volcanic cones is far greater 

 than the number of volcanoes, for the cones of many extinct vol- 

 canoes still remain. 



Many small islands, and some large ones, such as Iceland, are 

 due chiefly or wholly to the building up of volcanic cones which 

 have their foundations on the ocean bottom. The Aleutian Islands 

 and many of the islands of Australasia were formed in the same way. 



Destruction of volcanic cones. Volcanic mountains, like all 

 other elevations on the land, are subject to change and destruction. 

 They may be partially destroyed by violent explosions, as in the 

 cases of Krakatoa and Vesuvius, already cited. Again, the entire 

 summit of a volcanic mountain may sink, leaving a great depres- 



Fig. 173. Western border of Crater Lake. 



i 



sion, or caldera, where it was. Crater Lake, Oregon (Fig. 173), 

 occupies a caldera five or six miles in diameter, and 4,000 feet deep. 

 This lake, which with its surroundings has been made a National 

 Park, is encircled by nearly vertical walls of rock 900 to 2,200 feet 

 high. Since the sinking in of the top, a small cone, which now rises 

 as an island in the lake, has been built up. 



Volcanic cones are also destroyed by the slow processes of 

 weathering and erosion. Wind and rain attack volcanic cones as 

 soon as they are formed, but their results are not conspicuous until 

 the volcano is extinct and the cone stops growing. Cones com- 



