CHAPTER IX 



THE RISE OF MOLTEN ROCK TO THE EARTH'S 

 SURFACE 



VOLCANIC MOUNTAINS OF EXUDATION 



Prevalent misconceptions about volcanoes. The more or less 

 common impression that a volcano is a " burning mountain " 

 or a " smoking mountain " has been much fostered by the school 

 texts in physical geography in use during an earlier period. The 

 best introduction to a discussion of volcanoes is, therefore, a disil- 

 lusionment from this notion. Far from being burning or smoking, 

 there is normally no combustion whatever in connection with a 

 volcanic eruption. The unsophisticated tourist who, looking out 

 from Naples, sees the steam cap which overhangs the Vesuvian 

 crater tinged with brown, easily receives the impression that the 

 material of the cloud is smoke. Even more at night, when a bright 

 glow is reflected to his eye and soon fades away, only to again glow 

 brightly after a few moments have passed, is it difficult to remove 

 the impression that one is watching an intermittent combustion 

 within the crater. The cloud which floats away from the crest of 

 the mountain is in reality composed of steam with which is ad- 

 mixed a larger or smaller proportion of fine rock powder which 

 gives to the cloud its brownish tone. The glow observed at night 

 is only a reflection from molten lava within the crater, and the 

 variation of its brightness is explained by the alternating rise and 

 fall of the lava surface by a process presently to be explained. 



Not only is there no combustion in connection with volcanic 

 eruptions, but so far as the volcano is a mountain it is a product 

 of its own action. The grandest of volcanic eruptions have pro- 

 duced no mountains whatever, but only vast plains or plateaus 

 of consolidated molten rock, and every volcanic mountain at 

 some time in its history has risen out of a relatively level surface. 



When the traditional notions about volcanoes grew up, it was 

 supposed that the solid earth was merely a " crust " enveloping 

 still molten material. As has already been pointed out in an ear- 



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