VOLCANOES AND IGNEOUS INTRUSIONS 



299 



flow until it drains out, leaving a tunnel which may be several miles 

 long (p. 309). A lava tunnel on Mt. Shasta, California, which is 60 

 to 80 feet high and 20 to 70 feet broad, has been explored nearly a mile 

 without its end being reached. 



Effect of Composition on Fluidity. — Lava varies greatly in com- 

 position and fluidity. Some lava streams have flowed 20 to 30 miles 

 or more, while others have solidified as soon as they issued from their 

 craters ; some have flowed several miles, while others, with an equally 

 high temperature and even greater volume, have moved a much 

 shorter distance on an equal slope. This difference in the fluidity of 

 lavas is due largely to their chemical composition and to their tem- 

 perature. The basic, dark-colored lavas (p. 329) fuse at a lower 

 temperature and are consequently more likely to flow long distances. 

 The acid, usually light-colored lavas (p. 329) melt at a higher tem- 

 perature and consequently become solid while still hot. They are 

 therefore likely to solidify quickly. It is evident, however, that if a 

 basic lava has a temperature which is but slightly above the melting 

 point, it will be as stiff" (viscous) as an acid lava at a high temperature. 



Temperature. — The temperature of lava when it issues from the 

 vent of a volcano is probably often greater than 2000 F. This 

 is shown by the fact that copper wire, whose melting point is 2200 , 

 was fused in a Vesuvian lava stream which had already lost some of 

 its heat (p. 273). The temperature of the lavas in Kilauea in July, 

 1911, was 1260 C. 

 (2300 F.) ; that of 

 Stromboli in March, 

 1901, was 1150 to 

 1176 C. (2102 to 

 2149 F.). 



Surface of Lava 

 Flows. — The sur- 

 faces of lava flows 

 vary greatly, some 

 being so rough as to 

 make walking danger- 

 ous and difficult, 

 while others are com- 

 paratively smooth. 

 A fluid lava will con- p IG# 292- — Pahoehoe type of lava surface in the crater 

 solidate with smooth of Kilauea, Hawaii. (U. S. Geol. Surv.) 



