230 



VULCANISM 



though not now the dominant type. It has been thought that the 

 volcanic type of extrusion followed fissure eruptions as a phase 

 of decline; but this view has not been substantiated. 



Volcanoes 



A volcano is a cir- 

 cumscribed vent in the 

 earth's crust, out of which 

 hot rock, gases, and va- 

 pors issue. The ejected 

 material is generally built 

 up into mounds or cones 

 (Figs. 2 24-2 2 5), which are 

 often called volcanoes, 

 though they are really 

 the products of volcanoes. 



Fig. 223. Lava-flows of the northwestern So long as a volcano is 

 part of the United States. active there is likdy to 



be a depression, or crater (Fig. 226), in the top of its cone. The 

 crater connects downward with the source of lava at unknown 



Fig. 224. Cinder cone forming the summit of Mt. Vesuvius. 



depths. Craters may be a mile or more across, but most of them are 

 smaller, some much smaller. After sufficient erosion, extinct vol- 



