RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 117 



pressure, so that at some depth below the surface it begins to 

 separate out in minute vesicles or bubbles, which, expanding as 

 they rise, acquire a rapidly accelerating velocity. Soon they flow 

 together with a quite sudden increase of their expansive energy, 

 and now shooting upward with further accelerated velocity, a 

 layer of liquid lava with its cover of scum is raised on the surface 

 of a gigantic bubble and thrown high into the air. Cooled during 

 their flight, the quickly congealed lava masses become the tuff or 

 volcanic ash which is the material of the cinder cone. 



a b c 



FIG. 112. Diagrams to illustrate the nature of eruptions within the crater of 



Stromboli. 



Grander volcanic eruptions of cinder cones. Most cinder and 

 composite cones, in the intervals between their grander eruptions, 

 if not entirely quiescent, lapse into a period of light activity 

 during which their crater eruptions appear to be in all essential 

 respects like the habitual explosions within the Strombolian 

 crater. This phase of activity is, therefore, described as Strom- 

 bolian. By contrast, the occasional grander eruptions which have 

 punctuated the history of all larger volcanoes are described in 

 the language of Mercalli as Vulcanian eruptions, from the best 

 studied example. 



Just what it is that at intervals brings on the grander Vul- 

 canian outburst within a volcano is not known with certainty; 

 but it is important to note that there is an approach to periodicity 



