3H 



PHYSICAL GEOLOGY 



Fig. 308. 



Volcano Colima and a secondary cone 

 on the left. 



stiff that it solidified as soon as it reached the surface. The profile 

 of a cone depends also, to a greater or less degree, upon the force 

 and direction of the wind during eruptions, upon the position of the 



crater, and upon the 

 amount of erosion 

 which it has suffered. 

 Shape of Craters. 

 — The shape of the 

 crater of a volcano 

 depends both upon 



(1) the violence of 

 the explosions, the 

 diameter of the 

 crater of an explosive 

 volcano being, in 

 general, proportional 

 to the violence of the 

 eruption; and upon 



(2) the character of 

 the materials. A crater has steep, rugged inner walls when lava 

 and coarse cinders are ejected (Fig. 309 A), but a much less steep 

 slope when dust and fine ash are thrown out and fall back into it 

 (Fig. 309 B). 



Erosion of Volcanic Cones. — Up to this point we have discussed 

 the phenomena of an eruption, the shape of cones and craters, and 

 other features connected with 

 recent volcanoes, but aside from 

 these observations, we have 

 learned little of the internal 

 structure of volcanic cones. The 

 structure is, however, revealed to 

 us by an examination of ancient 

 volcanoes which have been deeply 

 eroded by atmospheric agencies 

 or by the sea (Fig. 310). Great 

 explosions also, as we have seen 

 in the case of Krakatao, expose 

 the internal structure to some extent, and it is also brought to light 

 when the top of the volcano sinks in, as in the case of Crater Lake, 

 Oregon (p. 309). 



B 



Fig. 309. — A y a cone formed of coarse 

 fragments; B, a. cone formed of ash. 



(After Maug.) 



