138 EARTH FEATURES AND THEIR MEANING 



grew upward from the 'chimney of the main crater until it had 

 reached an elevation of more then a thousand feet above its base, 

 a figure of the same order of magnitude as the probable height of 

 the upper section of the Vesuvian chimney previous to the erup- 

 tion of 1906. The Pele spine (Fig. 142) did not grow at a uniform 

 rate, but was subject to smaller or larger truncations, but for a 

 period of 18 days the upward growth was at the rate of about 41 

 feet per day. Later, the mass split upon a vertical plane revealing 

 a concave inner surface, and was somewhat rapidly reduced in 

 altitude to 600 feet (Fig. 143), only to rise again to its full height 

 of about 1000 feet some three months later. 



While apparently unique as an observed phenomenon, and not 

 free from uncertainty as to its interpretation, the growth of this 

 obelisk has at least shown us that a mass of rock can push its way 

 up above the chimney of an active volcano even when there are no 

 walls of tuff about it to sustain its outward pressures. 



The aftermath of mud flows. When the late Vulcanian ex- 

 plosions of Vesuvius had come to an end, all slopes of the moun- 

 tain, but especially 



the 



higher ones, 

 were buried in 

 thick deposits of 

 the cocoa-colored 

 ash, included in 

 which were larger 

 and smaller pro- 

 jectiles. As this 

 material is ex- 

 tremely porous, it 

 greedily sucks up 

 the water which falls during the first succeeding rains. When 

 nearly saturated, it begins to descend the slopes of the mountain 

 and soon develops a velocity quite in contrast with that of the 

 slow-moving lava. The upper slopes are thus denuded, while 

 the fields and even the houses about the base are invaded by these 

 torrents of mud (lava d' acqua). Inasmuch as these mud flows are 

 the inevitable aftermath of all grander explosive eruptions, the 

 Italian government has of late spent large sums of money in the 

 construction of dikes intended to arrest their progress in the future. 



Fio. 144. Corrugated surface of the Vesuvian cone 

 after the mud flows which followed the eruption in 1906 

 (after Johnston-La vis). 





