HEAT — VOLCANOES. 695 



also is the centre of a tufa-cone. This latter kind of tufa-cone has a saucer- 

 shaped crater and the inward and outward slope of the layers represented in 

 fig. 971 ; while the preceding may fail mostly of the inward slope. 



(b.) Forces causing eruptions of lava. — The forces causing eruption pre 

 as follow : — 



A. Hydrostatic pressure in the lavas against the sides of the 

 mountain. An increase of 500 feet in the depth of the lavas is 

 an increase of 625 lbs. of pressure to the square inch. Such a 

 pressure tends to produce fractures for the escape of the lavas. 



B. Pressure of vapors. Vapors rising out of the lavas into any 

 confined space may bring pressure to bear against the sides of the 

 mountain, and, if suddenly evolved, the effect may cause fracture. 



Water may come in contact with hot lavas and enter the sphe- 

 roidal state (the state in which a drop of water is when it dances 

 about on a red-hot stove), and, when so, it will suddenly and explo- 

 sively pass into a state of vapor on cooling. This is one cause of 

 explosion in steam-boilers ; and with the apparatus of a volcanic 

 mountain the results may rend the mountain. 



C. Pressure from the slow contraction from cooling going on in 

 the earth's crust, producing in some regions a subsidence of the 

 crust and a pressure upward of the liquid rock in or beneath it. 

 Contraction may also have the reverse effect ; that is, it may make 

 internal cavities in the crust, which may receive such liquids and 

 draw off the lavas from open vents. It is uncertain whether the 

 cause acts in either way. 



The action of pressure alone is quiet ; of vapors gradually evolved, 

 quiet ; of vapors suddenly evolved, either directly or through the 

 spheroidal state, violent, with earthquakes. 



Three eruptions of Kilauea were consequent upon the rise of the lavas to 

 a height of 400 or 500 feet in the crater, and were attended with no violence. 



Fig. 973. 



Tufa-hills, Nanawale. 



When ready for eruption, there was active ebullition in most parts of the im- 

 mense crater, and occasional detonations were heard, but there was no subter- 

 ranean shaking. 



The eruption in 1840 was without earthquake ; and the first sign of the out- 



