THE COMPOSITION OF THE EARTH 



47 



edges of the continents and in the sea, though some are far in- 

 land. With some notable exceptions, the active and recently 



FIG. 21. Lava flow (in right foreground) and cinder cones near Flagstaff, 

 Ariz. (R. T. Chamberlin.) 



extinct volcanoes are arranged in lines or belts where the earth's 

 crust has recently undergone severe movement. 



Lava plateaus. When lavas issue from long cracks (fis- 

 sures} in the surface, or from numerous more restricted vents, 

 they may spread widely over the surrounding country before 

 solidifying. The distance to which the lava flows depends 

 upon how fluid it is, upon the amount, and upon the character 



Flavna Loa 



FIG. 22. Profile of the cone of Mauna Loa, Hawaii. Vertical scale same as 

 horizontal. (U.S. Geol. Sun.) 



of the surface over which it moves. (What combination of 

 these conditions would enable the lava to flow farthest ?) The 

 lava congeals first at the top, forming a crust whose surface 

 is comparatively smooth (Fig. 23), unless it is repeatedly 

 broken by the continued movement of the still liquid mass 

 beneath. In the latter case the surface is extremely jagged 



