116 EARTH FEATURES AND THEIR MEANING 



welling out to overflow the frozen surface ; from others, steam is 

 ejected in puffs as though from the stack of a locomotive. Within 

 others lava is seen heaving up and down in violent ebullition, and 

 at intervals a great bubble of steam is ejected with explosive vio- 

 lence, carrying up with it a considerable quantity of the still 

 molten lava, together with its scumlike surface, to fall outside the 

 crater and rattle down the mountain's slope into the sea. Fol- 

 lowing this explosion the lava surface in the pool is lowered and 

 the agitation is renewed, to culminate after the further lapse of a 

 few minutes in a second explosion of the same nature. The rise 



of the lava which 

 precedes the ejection 

 appears at night as a 

 brighter reflection or 

 glow from the over- 

 hanging steam cloud 

 the flash seen by 

 the mariner from his 

 vessel. 



What is going on 

 within the crater of 

 Stromboli we may 

 perhaps best illus- 

 trate by the boiling 

 of a stiff porridge 



over a hot fire. Any one who has made corn mush over a hot 

 camp fire is fully aware that in proportion as the mush becomes 

 thicker by the addition of the meal, it is necessary to stir the 

 mass with redoubled vigor if anything is to be retained within the 

 kettle. The thickening of the mush increases its viscosity to such 

 an extent that the steam which is generated within it is unable to 

 make its escape unless aided by openings continually made for it 

 by the stirring spoon. If the stirring motion be stopped for a 

 moment, the steam expands to form great bubbles which soon 

 eject the pasty mass from the kettle. 



For the crater of Stromboli this process is illustrated by the 

 series of diagrams in Fig. 112. As the lava rises toward the 

 surface, presumably as a result of convectional currents within 

 the chimney of the volcano, the contained steam is relieved from 



FIG. 111. The volcano of Stromboli, showing the 

 excentric position of the crater (after a sketch by 

 Judd). 



