236 VULCANISM 



As lava flows, its upper surface may cool so much as to become 

 hard while the interior is still fluid. The fluid part may then 

 break out at the side or end of the hardened shell and flow away, 

 leaving a hollow crust of solidified lava. On further cooling, the 

 shell contracts and cracks, and perhaps caves in. The hardened 

 surface of a lava-flow may be broken by the movement of the fluid 

 lava below, and the solid fragments be displaced and upturned 

 so as to give the surface a jagged appearance. 



3. Gases and vapors. The gases and vapors which issue from 

 volcanoes are of many kinds. Among the commoner ones are those 

 of water (H 2 O), carbon dioxide (CC^), carbon monoxide (CO), 

 chlorine (Cl), hydrochloric acid (HC1), sulphur dioxide (802), and 

 hydrogen sulphide (H 2 S) ; but with these more important ones there 

 are many others. Oxygen and hydrogen are generally present, per- 

 haps produced by the dissociation of the elements of water. Some 

 of the gases are poisonous, and, as in the case of Pelee, their tem- 

 perature is in some cases so high as to be destructive to life. 



Formation of lava cones. Lava usually flows away from a vent 

 in streams which solidify before running far. As the lava-streams 

 flow in different directions at different times, the total effect is a low 

 cone formed of radiating tongues of lava. The streams may congeal 

 before they reach beyond the base of the cone, and not rarely while 

 yet on its slope. The volcanic cones formed of lava have low slopes, 

 since the fluidity of the lava prevents the development of high 

 gradients. It is, however, the exception rather than the rule, that 

 the cone is made up mainly of lava-streams, though the great Hawai- 

 ian volcanoes are of this class. The form of the cone, when com- 

 posed chiefly of lava, is also affected by the mass of the outflow and 

 by the fluidity of the lava. Other things being equal, the larger 

 the outflow at a given time, the more widely it distributes itself, 

 and the flatter the cone. 



Cinder-cones. The larger portion of the lava blown into the air 

 by expanding gas-bubbles falls back in the immediate vicinity of 

 the vent and builds up cinder-cones. This fragmental matter may 

 be disposed more or less symmetrically, making a cone with 

 steep slopes (Fig. 224). 



Minor cones. Small or temporary vents formed as offshoots 

 from the main vents may give rise to secondary or "parasitic" 

 cones. These may be numerous, as in the case of Etna, and so 

 important that a volcanic mountain becomes a compound cone. 



