104 



EARTH FEATURES AND THEIR MEANING 



reason, also, basaltic lava flows more freely and can spread much 

 farther before it has cooled sufficiently to consolidate. This is 

 equivalent to saying that its surface will assume a flatter angle of 

 slope, which in the case of basaltic lava seldom exceeds ten degrees 

 and may be less than one degree (Fig. 97). 



Siliceous lavas, on the other hand, are, when consolidated, rela- 

 tively light both in color and weight and melt at relatively high 

 temperatures. They are, therefore, usually but partly fused and 

 of a viscous consistency when they arrive at the earth's surface. 

 Because of this viscosity they offer much resistance to the libera- 

 tion of the contained water, which therefore is released only to 

 the accompaniment of more or less violent explosions. The lava 

 is blown into the air and usually falls as consolidated fragments 

 of various degrees of coarseness. 



It must not, however, be assumed that the temperature of lava 

 is always the same when it arrives at the surface, and hence it 

 may happen that a siliceous lava is exuded 

 at so high a temperature that it behaves 

 like a normal basaltic lava. On the other 

 hand, basaltic lavas may be extruded at 

 unusually low temperatures, in which case 

 their behavior may resemble that of the 

 normal siliceous lavas. If, however, as is 

 generally the case, the energy of explosion 

 of a basaltic lava is relatively small, any 

 ejected portions of the liquid lava travel 

 to a moderate height only in the air, so that on falling they are 

 still sufficiently pasty to 

 adhere to rock surfaces 

 and thus build up the 

 remarkably steep cones 

 and spines known as 

 " spatter cones 



or 



" driblet cones " (Fig. 

 98) . When, on the other 

 hand, the energy of ex- 

 plosion is great, as is nor- 

 mally the case with sili- 

 ceous lavas, the portions 



Fio. 98. A driblet cone 

 (after J. D. Dana). 





il^^S^-, 

 &##& 



'ff ^^ 



FIG. 99. View of Leffingwell crater, a cinder 

 cone in the Owens valley, California (after an 

 unpublished photograph by W. D. Johnson). 



