300 



PHYSICAL GEOLOGY 



F 













n 







*■ - .- *> * 







%&]&&*<% 



^\*w'& 



■ 



V%«< 









'.-..'■ 







%%T, 



•* w¥^\ W^m 















. 



Fig. 293. — The rough aa surface of a lava flow on the 

 volcano Colima. Mexico. 



and ropy surfaces, while a viscous one will become very scoriaceous. 

 This latter condition is partially due to the gas in the lava, which 



instead of escaping 

 freely to the air forms 

 bubbles in the sur- 

 face of the lava, just 

 as air blown into 

 soapy water forms a 

 frothy surface ; the 

 crust may also be 

 broken to some ex- 

 tent by the continued 

 movement of the 

 more liquid mass 

 below, causing an ex- 

 tremely rough sur- 

 face when the mass 

 hardens. The Ha- 

 waiian word pahoehoe is used to designate the smooth type of 

 lava, with the gently rounded, ropy surface which is characteristic 

 of fluid lavas (Fig. 292) ; while another Hawaiian term aa is used 

 for the rough, cindery surface (Fig. 293). 



Velocity of Lava Flows. — The rate of flow of lava depends upon 

 its fluidity and upon the slope over which it moves. In Iceland lava 

 streams have flowed over surfaces which appear flat to the naked eye, 

 while elsewhere they have consolidated on slopes which were almost 

 vertical. A lava stream on Mauna Loa flowed fifteen miles in two 

 hours, and the main stream from Vesuvius in 1906 descended the 

 first steep slopes with a velocity of about two miles an hour. Such 

 rates as the above are, however, rather unusual. The rate of flow is 

 gradually reduced as the stream cools and as the slope diminishes. 

 Lava often continues moving for a long time after the eruption ceases. 

 A lava stream which began to move on Vesuvius in 1895 was found 

 to be still in motion four years afterward. 



Nature of Lavas. — The slag formed in an iron furnace is really an 

 artificial lava, and from it much concerning the nature and behavior 

 of lavas can be learned. When lava is spoken of as a molten rock it 

 should be understood that, since rocks are composed of minerals 

 varying in fusibility and solubility, it is really a liquid rock in which 

 some mineral matter is dissolved in other mineral matter; i.e.. it is a 



