RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 135 



what process this chimney 

 building goes on is not well 

 understood, though some light 

 is thrown upon it by the post- 

 eruption stage of Mont Pele" in 

 1902-1903 (see below). 



Both the older and newer 

 sections of this plug or chimney 

 are furnished some support 

 against the outward pressure 

 of the contained lava by the 

 surrounding wall of tuff; and 

 they are, therefore, in a condi- 

 tion not unlike that of the 

 inner barrel of a great gun over 

 which sleeves of metal have 

 been shrunk so as to give sup- 

 port against bursting pressures. 

 On the other hand, when not 

 sustaining the hydrostatic pres- 

 sure of the liquid lava within, 

 the chimney would tend to be 

 crushed in by the pressure 

 of the surrounding tuff. Its 

 strength to withstand bursting 

 pressures is dependent not 

 alone upon the thickness of its 

 rock walls, but also upon its 

 internal diameter or caliber. 

 A steam cylinder of given 

 thickness of wall, as is well 

 known, can resist bursting 

 pressures in proportion as its 

 internal diameter is small. So 

 in the volcanic chimney, any 

 tendency to remelt from within 

 the chimney walls must weaken 

 them in a twofold ratio. 



We are yet without accurate 



a 



FIG. 141. Three diagrams to illustrate 

 the sequence of events within the crater 

 of a composite cone during the cone- 

 building and crater-producing periods. 

 a and b, two successive stages of the 

 cone building or Strombolian period ; 

 c, enlargement of the crater, truncation 

 of the cone, and destruction of the upper 

 chimney during the relatively brief 

 crater-producing or Vulcanian period. 



