380 



GEOLOGY 



The form of the cone, when composed chiefly of lava, is also 

 affected by the mass of the outflow and by its fluidity. Other 

 things being equal, the larger the outflow at a given time, the more 

 widely it distributes itself, and the flatter the cone. As a rule, 

 basic lava cones are flatter than the cones of acidic lavas. 



Cinder-cones. The larger portion of the lava blown into the 

 air by the expanding gas-bubbles falls back in the immediate 



Fig. 297. Spatter-cone and cavern. Kilauea, Hawaii. (Photo, by Libbey.) 



vicinity of the vent and builds up a cinder-cone. From the nature 

 of the case, this fragmental matter is often disposed symmetrical ly. 

 making a cone with steep slopes (Fig. 291). 



Small or temporary vents formed as offshoots from the main 

 vents often give rise to secondary or " parasitic" cones. These 

 are sometimes numerous, as in the case of Etna, and they may be 

 so important that a volcanic mountain becomes a compound cone. 

 A still more subordinate variety consists of "spatter-cones" formed 

 about small vents that eject little dabs of lava which form chimneys, 

 cones, domes, etc. Spatter-cones (Fig. 297) often arise from the 

 surface of the lava-flows themselves. 



